Package/Split DX reverse cycle (heat pump) systems

EnergyPlus models constant volume direct expansion heat pumps through a set of objects that reflect how these systems actually get installed and controlled. Whether you're dealing with a packaged rooftop unit or a split system with separate indoor and outdoor components, the modelling approach is similar. The constant volume part means the supply fan runs at full speed whenever the system is on, and the electric auxiliary heat kicks in when the heat pump can't keep up or outdoor conditions make it inefficient to try.

Object Structure

The core component is a unitary system object, typically AirLoopHVAC:UnitaryHeatPump:AirToAir, which bundles the DX heating coil, DX cooling coil, supply fan, and supplementary electric heating coil into one controlled package. This matches how packaged equipment arrives or how a split system gets controlled in practice: everything operates as a coordinated unit rather than independent components.

Inside that wrapper you'll specify a Coil:Heating:DX:SingleSpeed (or one of the newer unified coil objects), a Coil:Cooling:DX:SingleSpeed for the cooling season, a Fan:ConstantVolume or Fan:OnOff, and a Coil:Heating:Electric for auxiliary heat.

The fan runs at constant volume during operation. For many systems this means the fan is either on at full flow or off completely, though EnergyPlus also supports continuous fan operation with the coils cycling.

DX Heating Coil Setup

The heating coil needs rated capacity, rated COP, and a collection of performance curves. The curves modify capacity and efficiency based on actual operating conditions versus rated conditions. You're looking at heating capacity as a function of temperature (usually biquadratic, taking indoor dry-bulb and outdoor dry-bulb as inputs), heating capacity as a function of flow fraction (typically quadratic or cubic), heating energy input ratio functions for both temperature and flow, and a part load fraction correlation curve that accounts for cycling losses.

Getting these curves right matters more than most other inputs. Default curves exist in various libraries, but they're generic. Real equipment performance varies, and heating-dominated climates will expose any optimistic assumptions in your curves pretty quickly.

Defrost Strategy

Heat pumps collect frost on the outdoor coil when it's cold and humid outside. The system has to periodically clear this frost or performance degrades to the point of uselessness. EnergyPlus offers two defrost strategies.

Reverse cycle defrost temporarily switches the refrigerant flow so the outdoor coil becomes the condenser, melting the frost. This works but you're heating the outdoors whilst cooling your building, which is about as efficient as it sounds. You need to specify a defrost time period fraction and usually a resistive heater to keep the indoor airstream from blowing cold during defrost.

Resistive defrost uses electric heaters at the outdoor unit to melt frost. Simpler but you're directly converting electricity to heat at the outdoor coil, which also isn't great from an efficiency standpoint.

Either way, defrost cycles penalise your heating COP. The model needs a defrost strategy, maximum outdoor temperature for defrost operation, and a fractional defrost time. If you're in a climate where frost matters (basically anywhere cold enough for heat pumps to struggle), don't skip the defrost inputs or your simulation will show better performance than you'll ever see in reality.

Electric Auxiliary Heat

The supplementary electric coil provides backup heating. You define its capacity (in watts) and efficiency (which is 1.0 because it's resistance heat). The control logic determines when it fires.

You can set a maximum outdoor temperature for compressor operation. Below this temperature, the heat pump shuts off entirely and auxiliary heat takes over. This is sometimes called lockout temperature. Some systems lockout at 0°C, others keep the compressor running down to negative temperatures depending on the equipment.

Alternatively, auxiliary heat can supplement the heat pump when outdoor conditions reduce heat pump capacity below what's needed to meet load. The heat pump runs, auxiliary heat makes up the difference. This is generally more efficient than early lockout but requires compatible controls.

EnergyPlus also lets you set a maximum outdoor temperature for auxiliary heater operation, which prevents the backup heat from running during mild conditions even if the heat pump is struggling to meet an aggressive setpoint. This avoids wasting expensive resistance heat when the real problem is an unrealistic thermostat setting.

The interaction between heat pump operation and auxiliary heat matters for both energy consumption and demand. Resistance heat is your COP dropping to 1.0, which destroys any efficiency benefit you got from the heat pump. Systems that aggressively lockout or supplement with electric heat can show surprisingly high heating energy consumption in real buildings, and your model should reflect that if it's set up to operate that way.

Fan Control

Constant volume means the fan moves the same airflow whenever it's on, but you still have choices about when it runs.

Cycling fan turns on with the heating or cooling coil and off when neither is needed. This saves fan energy but can create temperature swings in the space.

Continuous fan runs all the time during occupied periods regardless of whether heating or cooling is active. This improves comfort and air distribution but increases fan energy consumption, and in heating mode you're potentially blowing cooler air across occupants when the coil isn't firing.

The fan object itself is straightforward: you specify pressure rise (Pascals), total efficiency, and motor efficiency. Fan power shows up in your HVAC energy and also contributes heat to the airstream, which slightly reduces heating load and increases cooling load. For constant volume systems this heat contribution is constant, which makes it easy to account for, at least.

Packaged vs Split Configuration

From EnergyPlus's perspective, the difference between packaged and split systems is mostly about where the outdoor unit sits and how you model its ambient conditions.

For a packaged unit sitting on the roof or beside the building, the outdoor coil experiences outdoor ambient conditions directly. The entire system is exposed to outdoor temperature, which affects heat pump performance through the temperature-dependent performance curves.

For a split system, the outdoor condensing unit is outside (obviously) whilst the indoor air handler is in a mechanical room or plenum. If your mechanical room isn't directly conditioned, it might operate at temperatures different from the occupied space, which affects the indoor side entering air temperature to the heating coil. EnergyPlus handles this through zone connections: you can place the air handler in a zone or return plenum and the model will use that zone's conditions.

The actual refrigerant piping between indoor and outdoor units doesn't get explicitly modelled. Performance impacts from long line sets or elevation differences get lumped into the performance curves if you account for them at all, which mostly you don't unless you have specific manufacturer data for that installation condition.

Control Sequences

The unitary system object manages sequencing between heating, cooling, and fan operation. You specify a controlling zone thermostat, and the system responds to heating or cooling calls.

In heating mode at low outdoor temperatures, the typical sequence is: thermostat calls for heat, fan turns on (if cycling), heat pump DX coil operates, if heat pump can't meet load and auxiliary heat is enabled then the electric coil energises, when thermostat is satisfied the coils turn off, and the fan turns off (if cycling) or continues (if continuous).

You can add complexity with economiser controls, dehumidification modes, or staging, but for a basic constant volume heat pump system this is the core sequence. EnergyPlus handles the logic internally once you've defined the components and control settings.

Sizing Considerations

Autosizing in EnergyPlus calculates required capacities based on design day conditions. For heat pumps with auxiliary heat, you have options.

You can autosize both the heat pump and auxiliary heat based on the full heating load, which means you have massive overheating capacity (heat pump plus full auxiliary) at mild conditions. This wastes equipment cost in reality but might represent what gets installed if the heat pump is primarily sized for cooling and auxiliary heat covers the heating shortfall.

Alternatively, you can manually size the heat pump for some fraction of peak heating load and let auxiliary heat cover the rest. This better represents actual installations where the heat pump provides most heating duty and electric backup covers the coldest days.

The balance point matters here. If your heat pump is adequately sized for your climate's typical winter conditions, auxiliary heat rarely runs and you get decent seasonal efficiency. If your heat pump is undersized or you're in a climate with extended cold periods, auxiliary heat becomes a significant energy consumer and your heating costs reflect that.

Performance Reality

A properly configured constant volume DX heat pump model in EnergyPlus should show seasonal heating COPs somewhere between 1.5 and 3.0 for typical climates, degrading towards the lower end as you move into colder regions where defrost cycles and auxiliary heat operation become frequent. If you're seeing COPs outside this range, either you're in an unusual climate, your performance curves are wrong, or something in the model setup isn't reflecting realistic operation.

The constant volume part means you're moving the same amount of air regardless of heating or cooling load, which can lead to comfort issues at part load conditions. The airstream temperature drops when the coil is cycling off, and occupants notice. Real systems sometimes address this with variable speed fans or multi-stage operation, but if you're modelling a true constant volume system, this is the tradeoff: simpler controls and equipment at the cost of some comfort and cycling losses.