0. How to read this document

Three layers, in this order.

• Part 1 — Fundamentals. The mental model that makes everything else readable. Five stages, two sides, four water temperature regimes. Ten pages that save months of confusion.

• Part 2 — Component catalogue. Each building block on its own: the air handler, the coil, the fan, the chiller, the pump. A short physical description, then what you can actually set on it in OpenStudio.

• Parts 3 and 4 — System fiches. The complete assemblies, ordered by increasing complexity. Every fiche answers the same questions in the same order, including "where it is used and why" and "does it use more or less energy than the reference".

Parts 5 and 6 cover controls and reference appendices.

The structure of a system fiche

On the energy indices. They are indicative and exist to build intuition, not to predict. The reference of 100 is a VAV system with terminal reheat, chiller and boiler, set up conventionally and not optimised. The real gap depends on climate, internal loads and above all on controls: two identical VAV systems can differ by 40% on box minimums and resets alone.

1. Fundamentals

1.1 The five stages

Every HVAC system on earth is some arrangement of these five stages. When you look at an unfamiliar system, identify them one by one and it becomes readable.

Two orthogonal questions then define almost any system:

• What is the transport medium? All-air, all-water, air-water, or refrigerant.

• Is ventilation coupled to or decoupled from thermal conditioning? A VAV system does both with the same air stream (coupled). A DOAS plus fan coils separates them (decoupled). This is the great dividing line between older systems and high-performance ones.

If you can answer those two questions about a system, you already understand most of it.

1.2 Supply side and demand side

This is the most important structural idea in HVAC modelling, and it is shared between the way engineers talk and the internal architecture of EnergyPlus.

Note the recursion: a water-cooled chiller is supply-side equipment on the chilled water loop and demand-side equipment on the condenser loop. That single sentence explains most of the confusion beginners have with plant modelling.

1.3 Water temperature regimes

The whole hydronic story is told in temperatures. Every drop in regime makes one technology possible and rules another out. It is the backbone of design, and it is the first thing to ask when someone describes an installation to you.

1.4 Units and the three constants

The three constants every American engineer assumes you know:

Q_sensible (Btu/h) = 1.08 x cfm x deltaT (F)

Q_total (Btu/h) = 4.5 x cfm x delta_enthalpy (Btu/lb)

Q_water (Btu/h) = 500 x gpm x deltaT (F)

The 1.08 / 4.5 / 500 coefficients bundle air density and specific heat in IP units. They are used from memory, every day, in every meeting.

2. The component catalogue

Each building block on its own. The goal of this part is that you can name and locate any object in a plant room, and know what it becomes in the model.

2.1 Air side — inside the box

AHU — Air Handling Unit

In the real world A sheet-metal cabinet, anywhere from the size of a refrigerator to the size of a shipping container, in a plant room or on the roof. Air enters one end, gets filtered, heated or cooled, and is blown out the other end into ductwork. Sound: a continuous low rumble. It is the most common piece of equipment in commercial buildings.

Job Condition and move air. Everything else on the air side follows from it.

In OpenStudio An AHU is not one object. It is an AirLoopHVAC whose supply side contains a sequence of components in a precise order: outdoor air system, then coils, then fan. You build it piece by piece.

Mixing box and outdoor air system

In the real world The section where return air from the building meets outdoor air, controlled by two or three motorised dampers. From the roof, this is the intake louvre and the relief hood.

Job Blend recirculated and fresh air to meet the ventilation code at the lowest energy cost. A huge share of all energy savings lives here.

In OpenStudio AirLoopHVAC:OutdoorAirSystem, containing OutdoorAir:Mixer (the physical mixing) and Controller:OutdoorAir (the logic, including economizer operation).

Damper

In the real world Pivoting metal blades inside a duct, driven by an actuator. Parallel blade: poor throttling, good for two-position service. Opposed blade: acceptable linear control, which is what you use for modulation.

Job Control how much air goes where.

In OpenStudio Almost never an explicit object. Dampers are implicit inside Controller:OutdoorAir and inside the terminal unit objects. You model their effect — an airflow fraction — not the hardware.

Filter

In the real world A rack of pleated media. Rated by MERV from 1 to 16; MERV 13 became the commercial baseline after COVID, MERV 8 was the old default. HEPA is a separate class, labs and healthcare.

Job Particulate removal.

In OpenStudio No object. A filter only exists energetically through the pressure drop it adds, which you bury in the fan static. A dirty MERV 13 adds 0.5 to 1.0 in. w.c., which is very real.

Coil

In the real world A matrix of copper tubes with aluminium fins sitting in the air stream, with water, refrigerant or steam inside. Vocabulary: rows (4, 6, 8), fins per inch (8 to 14), face velocity (400 to 550 fpm — above that, condensed water blows off the coil into the duct).

Job Transfer heat between the air and the fluid. A cooling coil does sensible and latent work because its surface is below dew point; a heating coil is sensible only.

In OpenStudio One object per technology. Placement in the sequence matters as much as the values.

Fan

In the real world An impeller in a housing. Centrifugal fans dominate AHUs, axial fans dominate exhaust and condensers. Modern AHUs use housingless plug fans, sometimes as an array of several small fans (fan wall).

Job Create the pressure difference that pushes air through ducts, coils and filters.

In OpenStudio Fan:SystemModel is the modern unified object and what you should use for new work. The legacy objects (Fan:ConstantVolume, Fan:VariableVolume, Fan:OnOff) live on in older files.

Economizer (air side)

In the real world Not a device — a control strategy implemented with the dampers you already have.

Job When outdoor air is cooler or drier than return air, open beyond the ventilation minimum and cool for free instead of running the chiller. In a marine climate this covers most of the cooling season.

In OpenStudio Fields on Controller:OutdoorAir, not a separate object.

Energy recovery (ERV / HRV)

In the real world A device that transfers heat between exhaust and intake. Enthalpy wheel (sensible and latent, 70–80%, turns slowly, slight cross-leakage), sensible wheel, fixed plate exchanger (no mixing, mandatory where exhaust is contaminated), heat pipe, runaround glycol loop (45%, but the two streams can be far apart).

Job Collapse the energy cost of ventilation air. Required above certain airflow and operating-hour thresholds.

In OpenStudio HeatExchanger:AirToAir:SensibleAndLatent, placed in the equipment list of the AirLoopHVAC:OutdoorAirSystem.

Humidification and dehumidification

In the real world Steam grid, evaporative media or ultrasonic atomiser to humidify. To dehumidify: overcool then reheat, or a desiccant wheel.

Job Hold relative humidity. Museums, labs, healthcare, data centres, print shops. In most offices in a marine climate, nobody humidifies.

In OpenStudio Humidifier:Steam:Electric with SetpointManager:SingleZone:Humidity:Minimum. For dehumidification, a setpoint manager on the cooling coil plus reheat, or Dehumidifier:Desiccant:System.

2.2 Air side — out in the building

Ductwork

In the real world Galvanised sheet metal, round or rectangular, sometimes internally lined, with flex duct on the last few feet. Vocabulary: main, branch, runout, takeoff, transition, elbow, turning vanes, balancing damper, fire and smoke damper, shaft, riser.

Job Distribute air. And in practice, determine whether your fan static assumption was realistic.

In OpenStudio Ducts are not modelled geometrically. Their effect shows up in fan pressure rise, and optionally as thermal loss through AirflowNetwork.

Return plenum

In the real world Instead of ducting the return, air is dumped into the space above the ceiling tiles, which acts as a large return duct back to the shaft.

Job Save the return duct network. And capture part of the heat from recessed lighting before it reaches the space, which reduces cooling load.

In OpenStudio AirLoopHVAC:ReturnPlenum, with the plenum defined as its own ThermalZone. It interacts with the Return Air Fraction field on the Lights object.

Diffusers, grilles, registers

In the real world The visible fixture at the room boundary. Diffuser = supply, shaped to induce room air and avoid drafts. Grille = return, fixed blades. Types: square ceiling, linear slot, swirl, displacement (low velocity at floor level), underfloor swirl outlet.

Job Deliver air into the occupied zone at an acceptable velocity and throw.

In OpenStudio Normally not modelled at all — the zone is assumed well mixed. For displacement ventilation or underfloor air, you switch the zone air model: RoomAirSettings:ThreeNodeDisplacementVentilation or RoomAirSettings:UnderFloorAirDistributionInterior.

VAV box (terminal unit)

In the real world A box about the size of a carry-on suitcase, above the ceiling near each zone. Contains a damper, an airflow sensor, a controller and often a small reheat coil. Failure symptom: the whistling in a meeting room is a damper hitting its stop.

Job Throttle the airflow to a single zone from its thermostat, while one central AHU supplies cold air to dozens of boxes at once.

In OpenStudio One AirTerminal object per type. Choosing the type is a design decision, not a detail.

2.3 Water side — the central plant

Chiller

In the real world A machine producing chilled water, typically at 44 °F. Compressor, evaporator, condenser, expansion device. Scroll up to 200 tons, screw 100 to 500, centrifugal 200 to 3000+. A 500-ton centrifugal weighs about five tons and does not fit through a standard door, which is why plant rooms have knockout panels.

Job Make the building’s cooling. Air-cooled (simple, 1.0–1.2 kW/ton) or water-cooled with a tower (0.5–0.6 kW/ton, but tower, pumps and water treatment).

In OpenStudio Chiller:Electric:EIR for most cases. Chiller:Electric:ReformulatedEIR is better for a centrifugal machine because its curves use leaving condenser water temperature, the way manufacturers publish data.

Boiler

In the real world A vessel where fuel burns and heats water. Non-condensing: hot flue gas, 80–84%, needs return water above 140 °F or it corrodes. Condensing: stainless or aluminium heat exchanger designed to condense the flue gas, 88–98%, but only if the return is cold.

Job Produce heating hot water.

In OpenStudio Boiler:HotWater. The Normalized Boiler Efficiency Curve field lets efficiency depend on part-load ratio and inlet water temperature — that is how you correctly model condensing behaviour.

Heat pump (building scale)

In the real world A chiller that can run backwards, or a machine purpose-built to make hot water. ASHP from ambient air (capacity drops when it gets cold, plus defrost cycles). WSHP/GSHP from a water loop or a borefield (stable source, stable performance). Air-to-water: the core of most commercial electrification projects today.

Job Produce heat at a COP of 2 to 4 instead of an efficiency of 0.95.

In OpenStudio HeatPump:PlantLoop:EIR:Heating and :Cooling, which can be declared as a companion pair. GroundHeatExchanger:System for a borefield.

Cooling tower

In the real world A large box on the roof where condenser water is sprayed over fill while a fan pulls air through. Some water evaporates and carries the heat away. Sound: falling water plus a rush of air. Visible plume in cold weather.

Job Reject heat to the atmosphere, getting below ambient dry bulb through evaporation. That is why a water-cooled chiller beats an air-cooled one.

In OpenStudio CoolingTower:SingleSpeed / :TwoSpeed / :VariableSpeed, on the supply side of a PlantLoop declared as Condenser type.

Pumps

In the real world Centrifugal pumps circulating water. Vocabulary: head (in feet of water), flow (gpm), pump curve versus system curve, decoupler bridge, NPSH.

Job Circulate. And consume far more than people expect once the real delta-T collapses.

In OpenStudio Pump:VariableSpeed, Pump:ConstantSpeed, HeaderedPumps:* for a bank of pumps in parallel.

Waterside economizer

In the real world A plate-and-frame heat exchanger between the condenser water loop and the chilled water loop.

Job When it is cold and dry outside, the tower alone makes water cold enough and the chiller shuts off entirely. Enormous in marine climates and data centres.

In OpenStudio HeatExchanger:FluidToFluid bridging the two loops, with a control type such as CoolingSetpointOnOffWithComponentOverride.

Domestic hot water

In the real world Separate from space heating. Gas or electric storage tanks, tankless heaters, or heat pump water heaters.

Job Showers and sinks. Minor in a high-performance office, dominant in a hotel, multifamily or hospital, and the hardest thing to electrify.

In OpenStudio WaterHeater:Mixed or :Stratified, with WaterUse:Equipment and WaterUse:Connections for the draws. WaterHeater:HeatPump:PumpedCondenser for a HPWH — and note it cools and dehumidifies the room it sits in, which E+ captures if you place it in a zone.

2.4 The things you meet on every project that nobody explains

This section exists because these terms come up constantly in drawings, meetings and submittals, and none of them are in a textbook chapter of their own. They are configurations and auxiliary equipment rather than headline systems, which is exactly why they get skipped.

Buffer tank

In the real world An insulated steel tank, anywhere from 50 to several thousand gallons, sitting next to the boilers or heat pumps. It looks like a plain vertical cylinder with four or six connections and a thermometer. There is nothing inside it but water.

Job Add water volume to a hydronic loop. Modern piping is small and modern machines hold almost no water, so a system can contain very little water in total. When the load falls below the machine’s minimum output, the machine satisfies it in a minute, shuts off, and restarts a minute later. That is short cycling, and it wrecks compressors, ruins efficiency and destroys comfort. The tank gives the loop enough thermal inertia that the machine can complete a decent run.

Why it became a big topic. Air-to-water heat pumps have a minimum run time, a minimum flow rate and very little internal volume, and manufacturers make the warranty conditional on a minimum system volume. Modular boilers and small chillers have the same issue. A buffer tank is now a standard part of almost every electrified hydronic design, which is why it suddenly appears everywhere.

Two-pipe or four-pipe. A two-pipe tank sits in series and every gallon passes through it: it adds volume, nothing more. A four-pipe tank has separate source and load connections, so it also acts as a hydraulic separator — the machine can run at its required flow while the building runs at whatever flow it needs. Note that this is a completely different use of "two-pipe" and "four-pipe" than the one in the next entry. The words are reused, and that is a genuine source of confusion.

Sizing rule. V (gallons) = Q (Btu/h) x t (minutes) / (500 x deltaT). With a 100 MBH minimum output, a 10-minute minimum run time and a 10 °F swing, you need 200 gallons. Manufacturers also quote a blunter rule of a few gallons per ton of machine capacity.

Two-pipe, three-pipe and four-pipe systems

What it means. The count is literally the number of pipes running to each terminal unit. A two-pipe fan coil has one coil fed by one supply and one return, and that water is either hot or chilled depending on what the central plant is making that season. A four-pipe unit has separate heating and cooling coils with their own supply and return, so any zone can heat while any other cools.

Why two-pipe still exists. It is roughly half the pipework, half the insulation, half the valves and much less shaft space. In a hotel or an apartment building with hundreds of identical rooms, that is a very large amount of money. It works acceptably where the outdoor climate genuinely decides the mode for everyone at once.

Where it fails. Shoulder season. On a sunny day in April, the south side needs cooling and the north side needs heating, and a two-pipe system can only offer one of them. Changeover between modes typically takes hours or days, so occupants are simply uncomfortable in between. This is the single reason four-pipe dominates in offices.

Three-pipe. Separate hot and chilled supply, but a shared common return. It saves one pipe and mixes hot and chilled return water together, which is thermodynamic vandalism. Energy codes effectively rule it out. You will only meet it in old buildings, and when you do, it explains a lot about their bills.

The VRF version of the same idea. Refrigerant systems use the same counting. A two-pipe VRF is a heat pump: all indoor units must be in the same mode. A three-pipe VRF, or a two-pipe VRF with a branch controller box depending on the manufacturer, is a heat recovery system that moves heat between zones. When someone says "is it two-pipe or three-pipe VRF", they are asking whether it can do simultaneous heating and cooling.

The AHU family — which box does what

Every one of these is a fan in a cabinet, and on drawings they all look similar. The difference is what fraction of outdoor air they handle and what job they do in the balance above.

Relief versus exhaust. They are not the same thing and the words are not interchangeable. Relief air is clean return air being dumped because the economizer brought in more outdoor air than the building needs; it only happens during economizer operation. Exhaust air is dirty air being deliberately removed from a specific space, and it runs on its own schedule regardless of what the AHU is doing.

Transfer air. Air moving from one space into another through a door undercut or a wall grille, with no duct and no fan. The classic case is a toilet room: the exhaust fan pulls air out, and the makeup comes from the corridor rather than from a supply duct. It is free ventilation reuse, and it is a real design strategy, not an accident.

Return fan or relief fan. A design decision with real consequences. A relief fan only runs during economizer operation and costs nothing the rest of the year, but it gives you less control of building pressure. A return fan runs whenever the system runs, costs energy all year, and gives firm pressure control. On anything except very large or very leaky buildings, a relief fan is usually the better answer.

Heat recovery — the whole family, not just the wheel

"Heat recovery" is used loosely for at least six different things. When someone says a project has heat recovery, the useful follow-up question is which of these they mean.

Cooling tower variants and the water vocabulary

The vocabulary that goes with any evaporative device, because it will come up in a meeting:

• Makeup water. What you add to replace what evaporates, drifts away and is drained.

• Blowdown. Water drained deliberately to stop dissolved solids concentrating as pure water evaporates away.

• Cycles of concentration. How many times the dissolved solids are allowed to concentrate before blowdown. Higher cycles save water and stress the treatment chemistry.

• Drift. Droplets carried out of the tower by the air stream. Drift eliminators are the baffles that catch them, and they matter for both water use and Legionella.

• Basin heater. An electric heater that stops the sump freezing in winter. A real and frequently forgotten energy end use in cold climates.

• Plume abatement. Mixing warm dry air with the saturated exhaust so the visible white plume does not alarm the neighbours.

• ASHRAE 188. The Legionella risk management standard. It is why open towers get scrutinised in healthcare and why some owners refuse them entirely.

3. The system fiches

Ordered by increasing complexity. Each level adds something to the one before, and the order is not decorative: it is roughly the historical order in which these systems appeared, each answering a limitation of the previous one.

The whole ladder on one page

Level 0 — Ideal Loads Air System

The load reference — not a real system

What you would actually see

Nothing. This does not exist in a building. It is a zone magically supplied with exactly the air it needs, at exactly the temperature it needs, with no capacity limit whatsoever.

The problem it solves

Separate the building load from system inefficiency. When you compare two envelopes, two insulation levels or two window-to-wall ratios, you do not want the answer polluted by VAV behaviour. Ideal Loads gives you pure demand, in kBtu, zone by zone.

Where it is used, and why

Relative energy use: not applicable (reference = VAV with reheat, non-optimised, Level 6). No system energy. It is a denominator, not a numerator.

Strengths and trade-offs

Use it first on every project — then climb the ladder.

Building it in OpenStudio

• Create the ThermalZones and assign Spaces with loads and schedules.

• Attach a ZoneHVAC:IdealLoadsAirSystem to each zone — no AirLoopHVAC, no PlantLoop.

• Set heating and cooling limits to NoLimit for genuinely pure load.

• Enable ventilation and recovery on the object if you want to measure the impact of outdoor air.

What you can set

Classic mistakes on this system

• Comparing Ideal Loads to a real system and drawing a savings conclusion. The two do not measure the same thing.

• Leaving recovery on by default and understating the ventilation load.

• Forgetting that sizing from Ideal Loads is not equipment sizing.

What to check after the run

• Check that unmet hours are zero — if they are not, your schedules or setpoints are inconsistent.

• Look at the heating/cooling split by zone: it is the fastest envelope diagnostic that exists.

• Compare ventilation load to envelope load. On a modern well-insulated building, ventilation often dominates.

Level 1 — Heating and ventilation only

90.1 baseline systems 9 and 10 — unit heaters, radiators, exhaust

What you would actually see

A warehouse, a covered car park, an unconditioned gym. Unit heaters hanging from the structure blowing warm air downwards, or radiators along the walls. A roof exhaust fan. No cooling anywhere.

The problem it solves

The bare minimum: prevent freezing, change the air, hold a tolerable temperature in winter. It is accepted that it will be hot in summer.

Where it is used, and why

Relative energy use: 30 to 50 (reference = VAV with reheat, non-optimised, Level 6). Very low, but this is not performance: there is simply no cooling.

Strengths and trade-offs

Building it in OpenStudio

• ZoneHVAC:UnitHeater or ZoneHVAC:Baseboard:Convective:Water on each zone.

• If hot water: a PlantLoop with Boiler:HotWater and Pump:VariableSpeed.

• Ventilation: either ZoneHVAC:OutdoorAirUnit, a minimal 100% outdoor air AirLoopHVAC, or assumed infiltration.

• Fan:ZoneExhaust for exhaust, with its schedule.

What you can set

Classic mistakes on this system

• Modelling a 40 ft high warehouse as a perfectly mixed zone. Stratification is real and changes the heating load.

• Understating infiltration at loading docks and sectional doors.

• Forgetting code-mandated exhaust (car parks, workshops), which often drives consumption.

What to check after the run

• Unmet heating hours — if they spike, your setpoint or infiltration is inconsistent.

• Gas use per square foot, against a warehouse benchmark.

• Ventilation share of the heating load: it should dominate.

Level 2 — PTAC and PTHP

90.1 baseline systems 1 and 2 — the box under the window

What you would actually see

The unit under a hotel room window, with its façade grille and its dial. It goes right through the wall. Sound: a fan starting and stopping, and a compressor clunking on. Every room has its own, completely independent.

The problem it solves

Condition room by room with no network, no plant room, no coordination. Each unit takes its outdoor air directly from outside and rejects its heat directly outside.

Where it is used, and why

Relative energy use: 115 to 140 (reference = VAV with reheat, non-optimised, Level 6). Higher than the reference: small inefficient machines, no sharing, no economizer.

Strengths and trade-offs

Same zonal idea, done better: Level 9 (WSHP) and Level 10 (VRF).

Building it in OpenStudio

• ZoneHVAC:PackagedTerminalAirConditioner or :PackagedTerminalHeatPump on each zone.

• No AirLoopHVAC. Outdoor air is a field on the object itself.

• If hot water heating (PTAC, baseline system 1): a PlantLoop with Boiler:HotWater.

• Fan:OnOff and Coil:Cooling:DX:SingleSpeed are the expected sub-objects.

What you can set

Classic mistakes on this system

• Forgetting the electric backup coil on a PTHP: the model becomes unrealistic as soon as it freezes.

• Leaving the fan running continuously when in reality it cycles with the compressor.

• Not modelling outdoor air, when on this system it enters each room raw with no pretreatment at all — it is a brute-force load.

What to check after the run

• Electric backup consumption: if it is significant, the balance point is wrong or the climate does not suit.

• Compressor cycle count: excessive cycling signals oversizing.

• Unmet hours per zone, separating corner rooms from typical rooms.

Level 3 — PSZ-AC and PSZ-HP — the single-zone rooftop

90.1 baseline systems 3 and 4 — Packaged Single Zone

What you would actually see

The beige box on the roof of a supermarket, a school or a small office. Everything is inside: compressor, condenser, gas burner, fan, dampers. A short duct network drops down to a single thermal zone. Sound: the condenser on the roof. It is the best-selling HVAC system in North America.

The problem it solves

Add a duct network and an economizer to the previous level, while keeping one fully autonomous machine per zone. The economizer is the real jump: free cooling from outdoor air becomes possible.

Where it is used, and why

Relative energy use: 85 to 110 (reference = VAV with reheat, non-optimised, Level 6). Often better than the reference thanks to the economizer and the absence of reheat — provided the zoning is honest.

Strengths and trade-offs

Multi-zone from one unit is exactly what Levels 4–6 add.

Building it in OpenStudio

• One AirLoopHVAC per zone. That is the defining constraint of this system.

• Supply side: AirLoopHVAC:OutdoorAirSystem, then Coil:Cooling:DX:SingleSpeed, then Coil:Heating:Fuel (or DX for a heat pump), then Fan:OnOff or Fan:SystemModel.

• In practice use AirLoopHVAC:UnitarySystem, which wraps the coils and fan cleanly and handles cycling.

• Demand side: AirTerminal:SingleDuct:ConstantVolume:NoReheat to the single zone.

• SetpointManager:SingleZone:Reheat on the outlet node — this is what varies supply air temperature with the zone demand.

What you can set

Required setpoint managers

• SetpointManager:SingleZone:Reheat on the fan outlet node — mandatory, it is the basic single-zone control.

• SetpointManager:MixedAir upstream and downstream of the fan if you build the chain by hand, to account for fan heat.

Classic mistakes on this system

• Lumping several orientations into "the" zone of a PSZ. A rooftop sees one thermostat: merge south and north and your model is comfortable while the building is not.

• Forgetting the economizer, or leaving it at NoEconomizer. In a marine climate that is worth several EUI points.

• Modelling a constant-volume fan running continuously when the machine cycles.

• Using default DX curves while claiming a premium-efficiency unit: efficiency is a family of curves, not a number.

What to check after the run

• Economizer hours: they must be consistent with the climate. In 4C they should be numerous.

• Compressor cycles and part-load factor: oversizing shows up immediately.

• Heating and cooling on the same day in the same zone: legitimate here in shoulder season, but understand why.

Level 4 — CAV with terminal reheat

Constant Air Volume with reheat — the 1960s system

What you would actually see

A large AHU in a plant room, a fixed-flow duct network, and a reheat coil in each zone branch. You find it in pre-1975 buildings, in operating rooms, and in some laboratories where the flow must never vary.

The problem it solves

Serve several zones from one machine. The chosen solution: cool all the air down to what the most demanding zone needs, then reheat locally for all the others. It works, it is robust, and it is energetically absurd.

Where it is used, and why

Relative energy use: 130 to 170 (reference = VAV with reheat, non-optimised, Level 6). The worst on the list. Every Btu of reheat cancels a Btu of cooling you already paid for.

Strengths and trade-offs

Level 5 exists precisely to fix this system's fan and reheat waste.

Building it in OpenStudio

• A multi-zone AirLoopHVAC with Fan:ConstantVolume or Fan:SystemModel in constant mode.

• Supply side: OutdoorAirSystem, Coil:Cooling:Water, possibly a preheat coil, then the fan.

• Demand side: AirTerminal:SingleDuct:ConstantVolume:Reheat on each zone, with Coil:Heating:Water.

• Two PlantLoops: chilled water and hot water.

• SetpointManager:Scheduled at 55 °F on the supply node, or SetpointManager:Warmest to model a reset.

What you can set

Required setpoint managers

• SetpointManager:Scheduled or :Warmest on the supply node.

• SetpointManager:MixedAir on both sides of the fan.

Classic mistakes on this system

• Not modelling supply air temperature reset when it exists. On CAV it is the only savings measure available, and it is large.

• Defaulting to CAV for an existing building without checking whether it was converted to VAV, which happened en masse in the 1980s and 1990s.

• Forgetting that ventilation here is over-supplied by construction: outdoor airflow far exceeds the code minimum.

What to check after the run

• Terminal reheat energy against cooling energy: if both are high at once, that is normal on this system and exactly what you should be showing.

• Simultaneous heating and cooling, hour by hour, by zone.

• Compare against a VAV variant: it is the most legible demonstration there is for a client.

Level 5 — Packaged VAV with reheat

90.1 baseline systems 5 and 6 — VAV on direct expansion

What you would actually see

A large rooftop, but this time feeding a network serving ten to forty VAV boxes. Cooling is made by direct expansion inside the rooftop, not by a central chilled water plant. Very common in mid-size US offices.

The problem it solves

Vary the airflow instead of holding it constant. Fan power scales with the cube of flow, so halving the flow divides fan power by five or six. It is the single largest efficiency jump on this whole ladder.

Where it is used, and why

Relative energy use: 95 to 115 (reference = VAV with reheat, non-optimised, Level 6). Close to the reference. DX is less efficient than a chiller, but you save the pumps.

Strengths and trade-offs

Building it in OpenStudio

• One AirLoopHVAC with Fan:VariableVolume or Fan:SystemModel in variable mode.

• Supply side: OutdoorAirSystem, then AirLoopHVAC:UnitarySystem containing Coil:Cooling:DX:TwoSpeed or :VariableSpeed and a heating coil.

• Demand side: AirTerminal:SingleDuct:VAV:Reheat on perimeter zones, :VAV:NoReheat on interior zones.

• A hot water PlantLoop if reheat is hydronic (system 5); Coil:Heating:Electric if electric (system 6).

• SetpointManager:Warmest or :Scheduled on the supply node.

What you can set

Required setpoint managers

• SetpointManager:Warmest on the supply node to model the reset properly.

• SetpointManager:MixedAir on both sides of the fan — a classic omission that corrupts coil control.

Classic mistakes on this system

• Leaving box minimums at their default value. By far the most expensive mistake in VAV modelling.

• Modelling a VFD without changing the fan power curve: you declare the drive without getting its savings.

• Using ZoneSum instead of Standard62.1VentilationRateProcedure: on a multi-zone system this gives a wrong outdoor airflow.

• Putting reheat on interior zones, which never need it.

What to check after the run

• Terminal reheat energy: if it exceeds 15% of heating, your minimums are too high.

• Fan flow profile over the year: it should spend a lot of time near minimum.

• Simultaneous heating and cooling hours by zone.

• Delivered outdoor air against the 62.1 requirement.

Level 6 — VAV with reheat on a chilled water plant

90.1 baseline system 7 — THE REFERENCE for this document

What you would actually see

The large US office system. One or two big AHUs per floor or per vertical zone, a chilled water plant in the basement with two or three chillers, a tower on the roof, a boiler room. Dozens to hundreds of VAV boxes. This is what you will model most often, and it is what everything else gets compared to.

The problem it solves

Make cooling at scale more efficiently than direct expansion. A water-cooled centrifugal chiller runs at 0.5 kW/ton against 1.0 for air-cooled DX. Above a few hundred tons the gap pays for the plant, the tower, the pumps and the operator.

Where it is used, and why

Relative energy use: 100 — the reference (reference = VAV with reheat, non-optimised, Level 6). Two identical VAV systems can differ by 40% on box minimums and resets. The index describes a conventional setup, not an optimised one.

Strengths and trade-offs

Two identical Level 6 systems can differ by 40% on controls alone — see §5.

Building it in OpenStudio

• AirLoopHVAC: OutdoorAirSystem, then Coil:Cooling:Water, optionally a Coil:Heating:Water preheat, then a variable Fan:SystemModel.

• Demand side: AirTerminal:SingleDuct:VAV:Reheat (perimeter) and :VAV:NoReheat (interior).

• Chilled water PlantLoop: Pump:VariableSpeed, Chiller:Electric:EIR, SetpointManager:Scheduled at 44 °F. Demand side: the cooling coil.

• Condenser PlantLoop: pump, CoolingTower:VariableSpeed, SetpointManager:FollowOutdoorAirTemperature. Demand side: the chiller condenser.

• Hot water PlantLoop: Pump:VariableSpeed, Boiler:HotWater, SetpointManager:OutdoorAirReset. Demand side: the reheat coils.

• PlantEquipmentOperationSchemes to stage multiple chillers or boilers.

What you can set

Required setpoint managers

• SetpointManager:Warmest on the air loop supply node.

• SetpointManager:MixedAir upstream and downstream of the fan.

• SetpointManager:Scheduled or :OutdoorAirReset on the chilled water loop outlet.

• SetpointManager:OutdoorAirReset on the hot water loop outlet.

• SetpointManager:FollowOutdoorAirTemperature on the condenser loop.

Classic mistakes on this system

• Leaving Sizing:Plant at the default delta-T when the coils were designed differently. Water flow and pump energy end up wrong by a factor of two, silently.

• Omitting SetpointManager:MixedAir around the fan: the coils control to the wrong temperature and fan heat disappears.

• Declaring a condensing boiler on a 180/160 regime.

• Modelling one chiller where three are staged: part-load behaviour is completely different.

• Ignoring the waterside economizer when the climate justifies one.

What to check after the run

• Chiller part-load histogram over the year. If it runs at 15% all the time, your sizing is wrong.

• Actual loop delta-T against the Sizing:Plant delta-T.

• Terminal reheat share of total heating.

• Pumping energy against cooling generation energy — a ratio above 15% signals a problem.

• Air-side economizer hours, and waterside economizer hours if one exists.

• Unmet hours under 300, and above all where they fall: concentrated on a few zones is a sizing issue; spread everywhere is a controls issue.

Level 7 — VAV with fan-powered boxes

90.1 baseline systems 6 and 8 — series and parallel

What you would actually see

A VAV box with a small fan added that draws warm air from the ceiling plenum. In series, the fan runs continuously and the room gets constant airflow. In parallel, it only starts in heating.

The problem it solves

Two problems with plain VAV. First, when a perimeter zone drops to its minimum flow, air motion stops and comfort degrades. Second, reheating with a coil costs high-grade energy, while the plenum is full of free warm air from the lighting.

Where it is used, and why

Relative energy use: 95 to 120 (reference = VAV with reheat, non-optimised, Level 6). Parallel can beat the reference; series often exceeds it because of the permanent terminal fans.

Strengths and trade-offs

Building it in OpenStudio

• Identical to level 6 for the air loop and the plant loops.

• Demand side: AirTerminal:SingleDuct:SeriesPIU:Reheat or :ParallelPIU:Reheat.

• Each box contains its own fan and reheat coil.

• The plenum must be modelled as a real ThermalZone and declared as the Secondary Air Inlet Node of the box, otherwise the induced air comes from nowhere.

What you can set

Classic mistakes on this system

• Not connecting the secondary node to the plenum: the box then induces air at zone temperature and the entire benefit vanishes.

• Failing to count the cumulative electrical power of dozens of small fans, which is far from negligible.

• Modelling series while intending parallel, or the reverse. The two have opposite energy signatures.

• Leaving Return Air Fraction unset on the Lights object, which empties the plenum of its recoverable heat.

What to check after the run

• Total terminal fan electricity, isolated from the central fan.

• Plenum temperature over the year: if it sits close to zone temperature, the recovery is not working.

• Reheat energy compared against a plain VAV variant.

Level 8 — DOAS with four-pipe fan coils

Dedicated Outdoor Air System — decoupling

What you would actually see

A modest AHU that treats outdoor air only, with a recovery wheel and a small network of slim ducts. In each room, a fan coil in the ceiling void or under the window, fed by two pairs of pipes. Sound: a discreet, permanent hiss.

The problem it solves

VAV moves a lot of air only because air is its only way of carrying heat. Water carries roughly 3,500 times more energy per unit volume. Decoupling lets you size ventilation for ventilation and thermal for thermal. Ducts shrink dramatically, which buys floor-to-floor height.

Where it is used, and why

Relative energy use: 70 to 90 (reference = VAV with reheat, non-optimised, Level 6). A real and robust gain. Fan energy collapses and recovery works on 100% of the outdoor air.

Strengths and trade-offs

The DOAS pattern returns at Levels 10, 11 and 12.

Building it in OpenStudio

• An AirLoopHVAC whose Controller:OutdoorAir forces 100% outdoor air (minimum = maximum = design flow).

• Inside the OutdoorAirSystem: HeatExchanger:AirToAir:SensibleAndLatent, then the coils.

• DOAS demand side: AirTerminal:SingleDuct:ConstantVolume:NoReheat to each zone.

• ZoneHVAC:FourPipeFanCoil on each zone, connected to both PlantLoops, with its own outdoor air flow set to zero.

• ZoneHVAC:EquipmentList: the order matters. The DOAS is the ventilation equipment, the fan coil is the thermal equipment.

• Fan:ZoneExhaust or a dedicated exhaust system to feed the recovery wheel.

What you can set

Required setpoint managers

• SetpointManager:Scheduled on the DOAS supply — choosing between neutral air and cold air is a structural design decision.

• SetpointManager:MixedAir around the DOAS fan.

• The usual setpoint managers on both water loops.

Classic mistakes on this system

• Assigning outdoor air to the fan coil on top of the DOAS: ventilation then gets counted twice.

• Supplying neutral air in a humid climate, which leaves the whole latent load to fan coils that cannot handle it.

• Forgetting the exhaust that feeds the wheel, and therefore overstating recovery.

• Mis-ordering the ZoneHVAC:EquipmentList, which makes the equipment work in the wrong sequence.

What to check after the run

• Outdoor air delivered per zone against the 62.1 requirement.

• Zone relative humidity over the year — this is the system’s weak point.

• Recovered energy against the extra fan power it costs.

• Split between central fan and zone fans.

Level 9 — Water-source heat pump loop

WSHP loop — heat transfer between zones

What you would actually see

A small heat pump above the ceiling of each zone, all connected to one water loop circulating through the building. A boiler adds heat when the loop cools, a tower or fluid cooler removes it when the loop warms.

The problem it solves

In a real building, some zones are cold while others are hot, constantly. Every previous system treats those two needs separately and pays for both. Here, the heat pulled out of the south is literally delivered to the north by the loop. Central plant only pays the net balance.

Where it is used, and why

Relative energy use: 65 to 85 (reference = VAV with reheat, non-optimised, Level 6). Very good when simultaneity is real. On a building without simultaneity the advantage disappears.

Strengths and trade-offs

The same trade grid, refrigerant-side: Level 10. Ground-coupled: §4.5.

Building it in OpenStudio

• ZoneHVAC:WaterToAirHeatPump on each zone.

• A Condenser-type PlantLoop: Pump:VariableSpeed, Boiler:HotWater, and CoolingTower:* or EvaporativeFluidCooler:*.

• The zone heat pumps sit on the demand side of that loop.

• PlantEquipmentOperationSchemes with two bands: boiler below a low threshold, tower above a high threshold, and a wide deadband between them.

• Ventilation: a separate DOAS, or outdoor air on each heat pump depending on the design.

What you can set

Classic mistakes on this system

• Setting too narrow a deadband: the boiler and the tower fight each other and the entire transfer benefit disappears. This is the mistake that kills the system, in models and in reality alike.

• Modelling constant pumping when variable pumping is what makes the system interesting.

• Forgetting that ventilation is not included and must come from somewhere else.

• Demonstrating this system on a building with no interior zones: there is then nothing to transfer.

What to check after the run

• Loop temperature hour by hour: it should spend a lot of time floating freely, with neither boiler nor tower.

• Number of hours where boiler and tower both run on the same day — this should be rare.

• Share of load met by internal transfer against central generation.

• Pumping energy, which becomes a significant term here.

Level 10 — VRF with DOAS

Variable Refrigerant Flow — continuous modulation

What you would actually see

A compact outdoor unit on the roof or a plant terrace, linked by thin copper lines to dozens of indoor units that look like fan coils. No water anywhere. Sound: very quiet. Heat recovery versions can heat some zones while cooling others.

The problem it solves

The variable-speed compressor modulates continuously instead of cycling, which collapses part-load losses. And the inter-zone transfer of level 9 becomes possible with no water loop and no boiler.

Where it is used, and why

Relative energy use: 60 to 80 (reference = VAV with reheat, non-optimised, Level 6). Among the best, especially with heat recovery. The lack of an economizer limits the gain in mild climates.

Strengths and trade-offs

Building it in OpenStudio

• AirConditioner:VariableRefrigerantFlow as the outdoor unit (or a :FluidTemperatureControl variant).

• ZoneHVAC:TerminalUnit:VariableRefrigerantFlow on each zone.

• ZoneTerminalUnitList to link the indoor units to the outdoor unit.

• A separate AirLoopHVAC for the DOAS, exactly as in level 8.

• Watch the ZoneHVAC:EquipmentList: the DOAS and the VRF unit coexist on the same zone.

What you can set

Classic mistakes on this system

• Using default curves. VRF is the system where the gap between generic curves and manufacturer data is largest, because all its performance lives in the modulation.

• Forgetting capacity fall-off and defrost in cold climates.

• Ignoring the piping length correction factor on an extended installation.

• Counting an economizer that does not exist: VRF has none, only the DOAS can have one.

• Not modelling the DOAS and letting the VRF provide ventilation, which it does not.

What to check after the run

• Simultaneous heating and cooling hours, if heat recovery is declared.

• Part-load profile of the outdoor unit: it should be widely spread, which is the whole point.

• Electric backup consumption in winter.

• Comparison against a variant without heat recovery, to quantify what recovery actually delivers on this specific building.

Level 11 — Radiant with DOAS

Radiant floor or ceiling — comfort through surfaces

What you would actually see

Tubes embedded in the concrete slab, or metal panels in the ceiling. Nothing visible, nothing audible. The room is silent and there is almost no air movement. The floor is warm underfoot in winter.

The problem it solves

Air is a poor carrier: you must move a lot of it, with fans, in large ducts. A 1,000 ft² surface at a 4 °F difference carries as much as a substantial airflow, with no fan at all. And radiant comfort lets you shift the air setpoint by 2 to 4 °F, which is free.

Where it is used, and why

Relative energy use: 55 to 75 (reference = VAV with reheat, non-optimised, Level 6). Among the lowest, but heavily dependent on climate and on the quality of anticipatory control.

Strengths and trade-offs

Building it in OpenStudio

• ZoneHVAC:LowTemperatureRadiant:VariableFlow (or :ConstantFlow) on each zone.

• Define a ConstructionProperty:InternalHeatSource and assign it to the slab construction: the tubes live inside a layer of the construction.

• Choose the source layer and the exact tube position — this genuinely changes the result.

• Two PlantLoops: hot water at a low regime (say 95/85 °F) and chilled water at a high regime (say 55/60 °F).

• A separate DOAS carrying the entire latent load.

• A dew point control that cuts cooling if zone humidity rises.

What you can set

Classic mistakes on this system

• Simulating at 4 timesteps per hour: radiant converges poorly and results become unstable.

• Not enabling condensation control, which gives you a model that happily cools below dew point.

• Using an air temperature setpoint when radiant comfort is judged on operative temperature.

• Modelling intermittent occupancy on radiant without anticipatory control: the system always arrives late.

• Forgetting that an active slab rules out a raised floor or a thick insulating finish.

What to check after the run

• Slab surface temperature over the year, and its margin against zone dew point.

• Operative temperature against air temperature: the gap is the system’s value.

• Number of hours where condensation control cut the cooling.

• Check convergence: look for timestep warnings in the .err file.

Level 12 — Chilled beams with DOAS

Active and passive chilled beams

What you would actually see

A ceiling enclosure holding a finned coil. In the active version, primary air from the DOAS leaves through nozzles at high velocity and entrains room air through the coil. No fan, no motor, no moving parts to maintain. Complete silence.

The problem it solves

Get high cooling capacity without a terminal fan. Level 8 still has a fan in every fan coil; here induction does the work using energy already present in the primary air.

Where it is used, and why

Relative energy use: 60 to 80 (reference = VAV with reheat, non-optimised, Level 6). Comparable to VRF. The gain comes from eliminating terminal fans.

Strengths and trade-offs

Building it in OpenStudio

• AirTerminal:SingleDuct:ConstantVolume:FourPipeBeam for active beams — it is both the air terminal and the emitter.

• ZoneHVAC:CoolingPanel:RadiantConvective:Water for passive beams.

• Primary air comes from the DOAS, so an active beam sits on the demand side of the air loop AND of both water loops.

• High chilled water regime, 55/60 °F, which opens the door to a very effective waterside economizer.

• Dew point control is mandatory.

What you can set

Classic mistakes on this system

• Sizing primary air on the thermal load instead of the ventilation requirement. The point of the system is precisely that primary air stays minimal.

• Using a 44 °F chilled water regime: guaranteed condensation and the main advantage thrown away.

• Neglecting dew point control.

• Forgetting that a beam only heats effectively if it is at sill level or the room has little heating demand — a ceiling beam heats poorly, the warmth stays up high.

What to check after the run

• Zone relative humidity and dew point, hour by hour.

• Actual primary airflow against the 62.1 requirement.

• Waterside economizer hours, which should be numerous thanks to the high regime.

• Total fan power, which should be remarkably low: that is the proof the system is doing what you expect.

4. The plant loops

The fiches in part 3 describe what happens on the air side and at the zone. Water plants are designed separately, and the same air-side system can be served by very different central plants. It is also where the quietest mistakes hide.

4.1 Chilled water — primary variable

The current arrangement for any new project. One set of variable-speed pumps varies flow through the chillers themselves, with a bypass valve to guarantee each machine its minimum flow. Fewer pumps, less energy, less space.

4.2 Chilled water — primary secondary

The 1990s and 2000s arrangement, still very common in existing stock. Constant-flow primary pumps dedicated to each chiller, variable-flow secondary pumps for the building, and a decoupler that hydraulically separates the two. More robust, more consumptive.

In modelling, this is built either as two linked PlantLoops or with the EnergyPlus common pipe mechanism. If you are modelling an existing building, check which of the two arrangements is actually installed before choosing: the pumping energy difference is significant.

4.3 Hot water — the condensing question

Everything turns on the regime, as explained in part 1.3. The table below is the decision shortcut.

4.4 Condenser water loop

The loop linking the chiller condensers to the cooling tower. Three levers: the tower supply setpoint, the tower fan type, and the presence of a waterside economizer.

4.5 Ground loop

A vertical borefield replaces the tower and the boiler. Ground temperature is stable, so heat pump performance is stable too. Field sizing depends on the annual balance between heat injected and heat extracted: a badly unbalanced building drifts thermally over years, which is modellable and visible.

Objects: GroundHeatExchanger:System with a response model (g-functions), or GroundHeatExchanger:Vertical:* for a detailed definition. The governing parameter is total bore length, followed by soil conductivity and borehole spacing.

4.6 Domestic hot water

An independent loop, often neglected in models and sometimes dominant in reality. In a hotel or multifamily building it is the first gas end use.

5. Controls

This is the layer where models diverge most from reality, and where engineers spend most of their design effort. The same system can vary by 40% on controls alone.

5.1 Resets — the highest-value family

In E+, most of these use SetpointManager:OutdoorAirReset or SetpointManager:Warmest. Static pressure reset has no dedicated object and is modelled either with a variable setpoint, with EMS, or with a Python plugin.

5.2 Availability and intermittency

• Night cycle. The AHU restarts outside occupancy only if a zone drifts beyond a widened band. AvailabilityManager:NightCycle.

• Optimum start. The system starts just early enough to reach setpoint exactly at opening, instead of at a fixed hour. AvailabilityManager:OptimumStart.

• Morning warmup. Full recirculation with no outdoor air during the pre-occupancy ramp.

• Night setback. A widened setpoint band, set on the thermostat schedules rather than on a control object.

5.3 Ventilation

The ASHRAE 62.1 multi-zone calculation is not the sum of the zone requirements. It is driven by the critical zone and by the system ventilation efficiency. Using ZoneSum instead of Standard62.1VentilationRateProcedure gives a wrong outdoor airflow on any multi-zone system.

• DCV. CO₂ sensors modulate outdoor air to real occupancy. Required above a density threshold. Fields on Controller:MechanicalVentilation.

• Air distribution effectiveness Ez. DesignSpecification:ZoneAirDistribution. 1.0 for overhead cooling, 0.8 for overhead heating, 1.2 for displacement. Often left at default when it changes the code-required airflow.

5.4 ASHRAE Guideline 36

The published high-performance sequences of operation for VAV systems and their terminal units. It is the reference that translates control intent into unambiguous logic. Modelling it faithfully in E+ requires EMS or the Python API, which is exactly the kind of work where a developer profile has the advantage over a conventional modeller.

6. Appendices

6.1 System and climate zone matrix

Quick read. ++ very well suited, + suited, ~ possible with care, − poorly suited.

A note on zone 4C, the marine climate of the US Pacific Northwest: mild winters, dry summers, enormous economizer potential, very little latent load. Systems that can exploit free outdoor air win there, and those that cannot — VRF in particular — lose part of their theoretical advantage.

6.2 French–English terminology

For working between a French engineering background and a US practice.

6.3 The most expensive modelling mistakes, all categories

• VAV box minimums left at default. Massive phantom reheat. The first thing to check on any VAV model.

• Sizing:Plant left at the default delta-T. Flow and pumping energy wrong by a factor of two, with no error message.

• SetpointManager:MixedAir missing around the fan. Coils control to the wrong temperature and fan heat disappears.

• ZoneSum instead of Standard62.1VentilationRateProcedure. Wrong outdoor airflow on any multi-zone system.

• Recovery with no fan penalty and no defrost. Overstated savings.

• DX or VRF curves left at default. Efficiency is a family of curves, not a number.

• Condensing boiler on a 180/160 regime. The declared efficiency is never reached.

• Air-source heat pump with no capacity fall-off and no backup. A wrong and flattering model — the signature error of electrification studies.

• Ideal Loads used to compare systems. It does not measure the same thing.

• One chiller modelled where several are staged. Completely different part-load behaviour.

6.4 Acronyms

6.5 Still to be added

This document is built to grow. Identified gaps:

• Laboratory systems: variable air volume fume hoods, pressure cascades, 100% outdoor air, runaround glycol recovery.

• Healthcare systems: air change requirements, filtration, redundancy.

• Parasitic loads: toilet exhaust, smoke control, garage ventilation, elevators, electric trace heating.

• Thermal storage and grid-interactive operation.

• Commercial kitchen systems and their makeup air.

• A section on simulation convergence pitfalls, system by system.