Package reverse cycle (heat pump) systems

Packaged DX heat pumps in EnergyPlus represent the all-in-one rooftop or ground-level units you see bolted to concrete pads outside buildings. Everything lives in one box: compressor, indoor coil, outdoor coil, fan, controls, and usually some electric resistance heat for backup. Constant volume means the fan runs at full speed when it's on, which is straightforward to model but comes with the usual comfort and efficiency compromises you get from on-off cycling.

Object Structure

The primary object is AirLoopHVAC:UnitaryHeatPump:AirToAir, which bundles all the components into one controlled system. Inside this wrapper you're specifying a heating coil (Coil:Heating:DX:SingleSpeed), a cooling coil (Coil:Cooling:DX:SingleSpeed), a supply fan (Fan:ConstantVolume or Fan:OnOff), and a supplementary electric heating coil (Coil:Heating:Electric).

This matches how packaged equipment actually arrives: prewired, with factory controls that manage the interaction between heating, cooling, fan operation, and backup heat. You're not cobbling together individual components and hoping they play nicely. The manufacturer already sorted that out, and EnergyPlus mirrors that approach by handling the sequencing internally once you've defined the pieces.

The constant volume fan runs at one speed whenever it's operating. Some units cycle the fan with the compressor (fan off when no heating or cooling is needed), others run the fan continuously during occupied hours. Both strategies have tradeoffs. Cycling saves fan energy but creates temperature swings. Continuous operation improves comfort and air distribution but burns power moving air even when you're not actively conditioning it.

DX Heating Coil Configuration

The heating coil needs rated capacity, rated COP, and a collection of performance curves that modify those ratings based on actual operating conditions. This is where generic assumptions give way to real equipment behaviour, and where your simulation results start to matter or become wishful thinking.

The key curves are heating capacity as a function of temperature (biquadratic, using indoor and outdoor dry-bulb temperatures), heating capacity as a function of airflow (quadratic or cubic), energy input ratio as a function of temperature, energy input ratio as a function of airflow, and part-load fraction correlation for cycling losses.

Default curves exist in various libraries and example files, but they're approximations. Real heat pump performance varies between manufacturers and even between models from the same manufacturer. If you're trying to model actual energy consumption rather than just satisfy a code requirement, you'll want manufacturer-specific performance data, which is sometimes available from AHRI directories and sometimes requires pestering the manufacturer's technical support until they send you something useful.

The temperature-dependent capacity curve is particularly important because heat pump output drops as outdoor temperature falls. A unit rated for 10 kW at 8°C outdoor might only deliver 6 kW at negative temperatures. EnergyPlus accounts for this through the capacity modifier curve, and if that curve doesn't match your equipment's actual performance, your heating energy predictions will be optimistic in cold climates.

Defrost Strategy

Heat pumps collect frost on the outdoor coil when it's cold and humid. This happens. The system has to deal with it or efficiency degrades until the outdoor coil is effectively insulated by ice and nothing works properly anymore.

EnergyPlus offers reverse-cycle defrost and resistive defrost. Reverse-cycle temporarily runs the system in cooling mode to warm the outdoor coil and melt frost. This works but you're briefly cooling the building whilst heating the outdoors, which is exactly as efficient as it sounds (not at all). Most systems include a resistive heater in the indoor airstream during defrost to prevent blowing cold air on occupants, which adds to the energy penalty.

Resistive defrost uses electric heaters at the outdoor coil to melt frost. Simpler control logic, still terrible efficiency because you're converting electricity to heat at 100% efficiency to fix a problem caused by trying to extract heat from cold outdoor air.

Either way, defrost cycles hurt your heating COP. The model needs defrost strategy, maximum outdoor temperature for defrost operation (usually around 5°C), defrost time period fraction, and resistive defrost heater capacity if you're using that strategy. If you skip these inputs or leave them at defaults without thinking, your simulation will show better performance than you'll measure in reality during winter, and then you'll spend time trying to figure out why the model doesn't match the utility bills.

Electric Auxiliary Heat

The supplementary electric coil provides backup when the heat pump can't keep up. You define capacity in watts and efficiency, which is 1.0 because it's resistance heat and thermodynamics doesn't allow you to do better than that without a heat pump.

Control strategy matters. You can set a lockout temperature below which the compressor shuts off entirely and auxiliary heat takes over. Some systems lockout at 0°C, some keep running to negative temperatures, some are adjustable in the field. This affects energy consumption substantially because resistance heat at COP 1.0 is expensive compared to heat pump operation at COP 2 to 3.

Alternatively, auxiliary heat can supplement the heat pump when outdoor conditions reduce capacity below what's needed to meet load. The compressor keeps running, electric heat makes up the difference. This is more efficient than early lockout but requires controls that can stage the two heat sources properly.

There's also a maximum outdoor temperature for auxiliary heater operation, which prevents backup heat from firing during mild weather even if the heat pump is struggling. This stops the system from defaulting to expensive resistance heat when the real problem is an unrealistic setpoint or inadequate equipment sizing.

The interaction between heat pump and auxiliary heat shows up clearly in utility data. Buildings with aggressive lockout strategies or frequent auxiliary heat operation have surprisingly high heating costs for systems that are supposedly efficient. Your EnergyPlus model should reflect this if that's how the equipment actually operates.

Fan Control and Energy

The fan moves air at constant volume whenever it's running. How often it runs depends on control strategy: cycling with heating/cooling demand, or continuous operation during occupied hours.

Cycling fan operation turns on with the coils and off when neither heating nor cooling is active. This saves energy but creates temperature swings in the space as the airstream temperature drops when the compressor cycles off. Occupants notice this, particularly in heating mode when you alternate between warm air and neutral-temperature air blowing across them.

Continuous fan operation keeps air moving regardless of whether the compressor is running. Better comfort and air distribution, higher fan energy consumption, and in heating mode you're potentially cooling occupants with airflow when the heating coil isn't active.

The fan object needs pressure rise (in Pascals), total efficiency, and motor efficiency. Fan power shows up in HVAC energy consumption and also adds heat to the airstream. This heat contribution is constant for constant volume systems, which reduces heating load slightly and increases cooling load. It's not huge but it's not negligible either, particularly if you've specified an undersized fan with high static pressure.

Performance Curves and Reality

Getting the performance curves right determines whether your model predicts reality or fantasy. The curves modify rated capacity and efficiency based on temperatures and airflow, and the difference between generic curves and equipment-specific data becomes obvious when you compare simulation to measured consumption.

Heating capacity drops with outdoor temperature. Energy input rises with outdoor temperature (you're working against a bigger lift). Part-load operation introduces cycling losses. All of this gets captured in the curves, but only if the curves actually represent your equipment.

AHRI certification data gives you a few operating points. Manufacturer cut sheets provide rated performance at standard conditions. What you need is performance across the full operating envelope: temperatures from design heating conditions up through mild weather, part-load ratios from minimum to maximum, and defrost penalties during frosting conditions. Sometimes you can get this data. Sometimes you're curve-fitting from three data points and hoping the interpolation is reasonable.

Default curves from example files or equipment libraries are better than nothing, but they're generic. Two different heat pumps with the same rated capacity and COP can have meaningfully different performance curves, which means different energy consumption in your specific climate with your specific operating schedule.

Sizing and Autosizing

EnergyPlus can autosize the heat pump and auxiliary heat based on design day conditions. This means sizing to meet peak heating load at design outdoor temperature, which sounds straightforward but introduces questions about how much capacity should come from the heat pump versus auxiliary heat.

If you autosize both based on full heating load, you end up with massive overcapacity at mild conditions (heat pump plus full auxiliary capacity). This doesn't match how equipment actually gets selected, where the heat pump is often sized for cooling load and auxiliary heat covers the gap during peak heating conditions.

Manual sizing lets you specify heat pump capacity for some fraction of peak heating load, with auxiliary heat making up the difference. This better represents real installations but requires you to think about balance point temperature and how often the building will rely on expensive resistance heat versus the heat pump.

The sizing affects both equipment cost (in reality) and seasonal energy consumption (in your simulation). An undersized heat pump runs auxiliary heat frequently, which tanks your heating efficiency. An oversized heat pump cycles excessively at part load, which introduces cycling losses and comfort issues. Getting sizing roughly right matters for both energy and comfort predictions.

Control Sequences

The unitary system object handles sequencing between heating, cooling, and fan operation based on zone thermostat calls. In heating mode the typical sequence is: thermostat calls for heat, fan starts (if cycling), heat pump compressor runs, if heat pump can't meet load then auxiliary heat energises, when satisfied everything shuts off or the fan continues (if continuous operation).

This happens internally in the unitary system object. You don't explicitly programme the sequence, you just configure the components and control settings and EnergyPlus figures out the logic. This works well for straightforward systems but can obscure what's actually happening if you're trying to troubleshoot why your model behaves unexpectedly.

You can add complexity with economiser controls, night setback strategies, or demand limiting, but the basic packaged heat pump is relatively simple: one zone thermostat, one piece of equipment, fan either on or off, heating or cooling but not both simultaneously.

Typical Performance

A properly configured packaged heat pump model should show seasonal heating COPs between 1.5 and 3.0 depending on climate and how often auxiliary heat operates. Colder climates with frequent defrost cycles and auxiliary heat operation trend toward the low end. Mild climates where the heat pump rarely needs backup trend toward the high end.

If you're seeing COPs outside this range, either your climate is unusual, your performance curves are wrong, your defrost settings are unrealistic, or something in the control configuration doesn't reflect actual operation. COPs above 3.5 suggest your curves are too optimistic or you're not accounting for defrost and auxiliary heat properly. COPs below 1.5 mean you're running on electric resistance most of the time, which defeats the purpose of having a heat pump.

The constant volume operation means comfort issues at part load are somewhat inevitable. Airstream temperature drops when the compressor cycles off, particularly noticeable in heating mode. Variable speed equipment or multi-stage systems address this but that's a different model configuration entirely. If you're truly modelling constant volume single-speed equipment, this is the expected behaviour and trying to make the simulation show perfect comfort at all hours is misrepresenting how these systems actually perform.