How a machine is run

In a simulation a controller decides, hour by hour, what each machine does to meet what its rooms ask for. The vocabulary below is the control panel's — every term is a real thing a real machine does, and the defaults are the conservative reading of a machine nobody has described.

Stages

The controller tries the cheapest thing first: fans only (ventilation, and free cooling if the outside air is good enough), then evaporative devices, then compressors; for heating, the heat pump before the heater. The first stage that can reach the required supply temperature is the one that runs. You can declare your own stages — which components each one switches on — or let them be derived from the drawing.

Cycling and modulation

A stage that has more capacity than the hour needs unloads. A fixed-speed compressor cycles — runs flat out for part of the hour and stops — and its supply is the hour's average; an inverter modulates down to its minimum speed and cycles below it; a valve on a chilled-water coil modulates the water. Which one your machine does is declared on the compressor (variable speed, minimum and maximum speed) and on the strategy panel (modulate). Fixed speed is the default.

Constant volume and SZVAV

A constant-volume machine moves the same air every hour and changes how cold it is. SZVAV — single-zone variable air volume — is the other way round for a machine serving one room: the supply temperature is held at a setpoint (say 13 °C) and reset upward as the load falls, and the fan slows down so the room gets less air rather than warmer air. It saves fan energy, which is most of a light-load hour's electricity, and it is what ASHRAE 90.1 requires of larger single-zone units with variable-speed fans. Declare it on the strategy panel with the setpoint, the reset ceiling and the fan's minimum flow.

Multi-zone VAV and terminal boxes

One handler, several rooms: the handler makes a cold deck, and each room has a terminal box that meters its own share of that air between a minimum and a maximum, reheating at its minimum when the room wants less cooling than the minimum air brings. The deck temperature is reset from the neediest room. Declare boxes on the block's serve links in the building tab — maximum flow, minimum fraction, reheat kind and capacity — and give the handler an SZVAV strategy, which is where the deck setpoint and its ceiling live.

Economiser and outside air

An outside-air damper has a minimum position (the ventilation the rooms need) and, with an economiser, opens further on a cooling hour when the outside air is cooler — or, in enthalpy mode, carries less heat — than the return: free cooling. Demand-controlled ventilation lets the minimum follow how full the rooms are; a damper schedule shuts it outside occupied hours. These live on the outside-air damper; the merge with the return leg is read off the wiring.

Head-pressure control

A condenser fan bank can be told to hold a minimum condensing temperature by taking fans off on a cool day, which is what an expansion valve needs to keep working. It costs compressor power and saves fan power; it is a constraint, not an economy, and the simulation treats it as one.

Humidity: ceilings and floors

A room may declare a humidity ceiling (ASHRAE 55's 0.012 kg/kg is the usual one). To hold it the controller over-cools the coil to a lower dew point and warms the air back — with a heater downstream of the coil, or, on a machine drawn with face-and-bypass, by mixing untreated air past the coil, which costs nothing. A room may also declare a floor, which steers a steam humidifier. A machine with neither device reports the hours it could not hold.

Fan ON or AUTO, occupancy and setback

With the fan switch on ON the ventilation stage runs through occupied hours whether or not there is a call; on AUTO an hour with no call starts from off, fans included. Rooms carry an occupancy profile and may relax their setpoints when empty (setback), and a machine may start early to recover the room by opening time (optimum start). Unmet hours are counted only while a room is occupied.

Several machines on one room

Machines serving the same room take it in turn: the one declared with the higher priority is offered the whole load, the next is offered what the first did not deliver, and the same holds for moisture. Machines of equal priority share it equally among those that can work toward it: a heating call is not halved with a machine that cannot heat that hour, which would leave the room short with the other machine at half load. A dedicated outside-air unit is the usual lead.

Couplings between blocks

Blocks are joined at their open ports: a chiller's leaving water to a coil's inlet (and back), an exhaust unit's air to a recovery unit's intake, a condensing unit's refrigerant to an indoor coil. Each coupling carries last hour's state across, one hour behind, which is how real plants behave through their pipework; a water coupling may declare the loop's volume as a buffer.

A VRF: one outdoor unit, several indoor units. Put the same coupling on the outdoor unit's suction and liquid line once per indoor unit, and they share it — a manifold, the same rule that lets one chiller feed several fan coils. Each indoor unit serves its own room and modulates to it; the outdoor unit is asked for the sum of what they take, and its capacity is shared out among them in proportion to what each room is asking for that hour. So a machine that cannot cover every room at once falls short honestly, rather than behaving like one machine per indoor unit. Heating works: the whole system turns round together, indoor units condensing and the outdoor coil evaporating out of the weather. What is not modelled: heat recovery, which is indoor units heating and cooling at the same time; the derate for a long refrigerant pipe run is modelled, on the makers' own published tables — tell the compressor its equivalent length to the farthest indoor unit and it charges for it; the units' shared state carries the same one-hour lag every coupling does. There is no correction curve for the connected-capacity ratio either, and none is needed for its shape. Manufacturers who publish one agree on a two-regime law — capacity rises linearly with connected indoor capacity up to 100 %, and above that it saturates, gaining a few per cent at most — and this model produces both without a correction term: below full connection each indoor coil limits its own take, and above it the outdoor unit's own capacity is the ceiling. That last is the conservative end of what makers publish, which is the right direction for a tool you size equipment with. Two machines you can open and wire up this way are published as examples: a 56 kW (20 HP) outdoor unit and the 14 kW indoor unit that goes with it — four indoor units on one outdoor unit being the 100 % connection ratio the makers rate at. The pair is held to a maker's own published capacity grid: modulated to its rated duty it draws within a few per cent of the maker's power input at every outdoor temperature from 25 to 35 °C. How the numbers are checked.