Revision history [back]

There are 2 control methodologies in EnergyPlus. A constant flow system where a load or set point is the target, or a cycling system where a load is the target.

A constant flow system "outputs" an average SAT, where the compressor cycles on and off to meet the load or temperature set point (the fan never cycles off). These systems are the CoilSystem or VAV types (and now AirloopHVAC:UnitarySystem).

A cycling system "outputs" a compressor on SAT while averaging the flow rate (PLR*MaxFlow while RTF is used for power/energy calculations). These systems are the furnaces, heat pumps, and unitary systems.

In the Q = mCpdT calculation, a constant flow system has a constant flow (m) and the dT is the average (ToutPLR + Tin(1-PLR)). A cycling system has a constant dT while the flow is averaged. Similar calcs for Q = mdH.

Although real systems would cycle (cycling compressor) over a long interval, on the order of 10's of minutes, EnergyPlus simulates this cycling over shorter time intervals using a part-load degradation curve to account for cycling losses. This methodology allow EP to hit a zone thermostat temperature set point (since no thermostat hysteresis exists in EP).

The AirloopHVAC:UnitarySystem object just recently began to allow a cycling system to meet a set point temperature [which can be difficult with cycling systems since temperature is constant while flow is proportional to PLR...HINT: use constant fan with SP control for AirloopHVAC:UnitarySystem].

There are 2 control methodologies in EnergyPlus. A constant flow system where a load or set point is the target, or a cycling system where a load is the target.

A constant flow system "outputs" an average SAT, where the compressor cycles on and off to meet the load or temperature set point (the fan never cycles off). These systems are the CoilSystem or VAV types (and now AirloopHVAC:UnitarySystem).

A cycling system "outputs" a compressor on SAT while averaging the flow rate (PLR*MaxFlow while RTF is used for power/energy calculations). These systems are the furnaces, heat pumps, and unitary systems.

In the Q = mCpdT calculation, a constant flow system has a constant flow (m) and the dT is the average (ToutPLR + Tin(1-PLR)). A cycling system has a constant dT while the flow is averaged. Similar calcs for Q = mdH.

Although real systems would cycle (cycling compressor) over a long interval, on the order of 10's of minutes, EnergyPlus simulates this cycling over shorter time intervals using a part-load degradation curve to account for cycling losses. This methodology allow allows EP to hit a zone thermostat temperature set point (since no thermostat hysteresis exists in EP).

The AirloopHVAC:UnitarySystem object just recently began to allow a cycling system to meet a set point temperature [which can be difficult with cycling systems since temperature is constant while flow is proportional to PLR...HINT: use constant fan with SP control for AirloopHVAC:UnitarySystem].

There are 2 control methodologies in EnergyPlus. A constant flow system where a load or set point is the target, or a cycling system where a load is the target.

A constant flow system "outputs" an average SAT, where the compressor cycles on and off to meet the load or temperature set point (the fan never cycles off). These systems are the CoilSystem or VAV types (and now AirloopHVAC:UnitarySystem).AirloopHVAC:UnitarySystem). A water coil or DX coil can be used in these applications.

A cycling system "outputs" a compressor on SAT while averaging the flow rate (PLR*MaxFlow while RTF is used for power/energy calculations). These systems are the furnaces, heat pumps, and unitary systems.systems. These systems typically use a DX coil and refrigerant compressor but can also be applied to water coils.

In the Q = mCpdT calculation, a constant flow system has a constant air flow (m) and the dT is the average (ToutPLR (Tout(PLR) + Tin(1-PLR)). A cycling Tin(1-PLR)). A cycling DX system has a constant dT while the air flow is averaged. Similar calcs for Q = mdH.mdH. When water coils are used, there is no cycling involved and the water coil water-side flow rate is constant the entire time step (but will modulate to meet the load or temperature set point).

Although real systems would cycle (cycling compressor) compressor or water control valve opening and closing) over a long interval, on the order of 10's of minutes, EnergyPlus simulates this (DX) cycling over shorter time intervals using a part-load degradation curve to account for cycling losses. This methodology allows EP to hit a zone thermostat temperature set point (since no thermostat hysteresis exists in EP).EP) or coil outlet air temperature set point.

The AirloopHVAC:UnitarySystem object just recently began to allow a cycling system to meet a set point temperature [which can be difficult with cycling systems since temperature is constant while flow is proportional to PLR...HINT: use constant fan with SP control for AirloopHVAC:UnitarySystem].

There are 2 control methodologies in EnergyPlus. A constant flow system where a load or set point is the target, or a cycling system where a load is the target.

A constant flow system "outputs" an average SAT, where the compressor cycles on and off to meet the load or temperature set point (the fan never cycles off). These systems are the CoilSystem or VAV types (and now AirloopHVAC:UnitarySystem). A water coil or DX coil can be used in these applications.

A cycling system "outputs" a compressor on SAT while averaging the flow rate (PLR*MaxFlow while RTF is used for power/energy calculations). These systems are the furnaces, heat pumps, and unitary systems. These systems typically use a DX coil and refrigerant compressor but can also be applied to water coils.

In the Q = mCpdT calculation, a constant flow system has a constant air flow (m) and the dT is the average (Tout(PLR) + Tin(1-PLR)). A cycling DX system which uses a cycling fan has a constant dT while the air flow is averaged. Similar calcs for Q = mdH. When water coils are used, there is no rarely any cycling involved and the water coil water-side flow rate is constant the entire time step (but will modulate to meet the load or temperature set point).

Although real systems would cycle (cycling compressor or water control valve opening and closing) over a long interval, on the order of 10's of minutes, EnergyPlus simulates this (DX) cycling over shorter time intervals using a part-load degradation curve to account for cycling losses. This methodology allows EP to hit a zone thermostat temperature set point (since no thermostat hysteresis exists in EP) or coil outlet air temperature set point.

The AirloopHVAC:UnitarySystem object just recently began to allow a cycling system to meet a set point temperature [which can be difficult with cycling systems since temperature is constant while flow is proportional to PLR...HINT: use constant fan with SP control for AirloopHVAC:UnitarySystem].

Interpretation of results will therefore depend on the system type, either constant fan operation or cycling fan operation.