Water-to-Air Heat Pump

A Water-to-Air Heat Pump (WAHP) connects to a central water loop and uses a refrigerant cycle to transfer heat between the water and air systems. These units typically include a compressor, a fan coil, and heat exchangers, offering both heating and cooling functions depending on the system's mode. The WAHP modulates temperature in individual zones by drawing heat from or rejecting heat to the water loop, making it ideal for use in geothermal or water-loop heat pump systems.

Water to Air Heat Pump

System

Water Flow Control Mode

Specifies how water flow is managed through the heat pump. Common options include Constant (flow remains steady) or Cycling (flow adjusts based on demand).

Max. Cycling Rate

Defines the maximum number of compressor cycles per hour. A higher rate can improve responsiveness but may reduce equipment lifespan.

Fraction of On-cycle Power Use

Indicates the percentage of full power consumed when the compressor is cycling off. This accounts for energy use during standby periods.

Heat Pump Delay Time

Sets the delay time, in seconds, before the compressor restarts after shutting off. Helps avoid rapid cycling that can wear out components.

Heat Pump Time Constant

Represents the time, in seconds, over which the heat pump responds to a change in load. A longer time constant results in smoother, slower response behaviour.

Sizing

Sizing Method

Determines how air flow rates are calculated. Selecting Manual allows you to input custom values but requires advanced knowledge of HVAC systems. Inaccurate inputs may result in simulation errors.

Cooling Supply Air Flow Rate

Specifies the volume of air delivered to the space during cooling mode. This should match the cooling load requirements of the zone.

Heating Supply Air Flow Rate

Defines the airflow delivered to the space when the system is in heating mode. This value should be appropriate for the heating demands of the space.

No Load Supply Air Flow Rate

Represents the airflow when the heat pump is on but neither heating nor cooling. Typically a low or zero value.

Cooling Outdoor Air Flow Rate

Sets the amount of outdoor air introduced during the cooling cycle. Useful for ventilation and fresh air requirements during cooling.

Heating Outdoor Air Flow Rate

Defines the outdoor airflow brought in when the system is in heating mode, ensuring ventilation is maintained year-round.

No Load Outdoor Air Flow Rate

Specifies the outdoor air intake rate when there is no active heating or cooling. This can help maintain baseline ventilation even during standby operation.

Cooling Coil

System

Cooling Coil Rated Cooling Coefficient of Performance (CoP)

This value defines the efficiency of the cooling coil, measured as the ratio of cooling output (in watts) to electrical input (in watts). A typical CoP of 3.0 means the coil delivers three times more cooling energy than the energy it consumes.

Cooling Coil Rated Entering Water Temperature (°C)

Specifies the temperature of water entering the cooling coil during rated operation. A value of 30°C is standard for testing purposes and affects the coil's heat exchange performance.

Cooling Coil Rated Entering Drybulb Temperature (°C)

Indicates the temperature of the air entering the coil, measured without considering humidity. A typical drybulb temperature like 27°C is used for performance evaluation under standard cooling conditions.

Cooling Coil Rated Entering Wetbulb Temperature (°C)

Represents the humidity-adjusted temperature of the air entering the coil. A value of 19°C reflects typical indoor conditions with moderate moisture, impacting latent cooling capacity.

Sizing

Sizing Method

Specifies how cooling coil parameters are determined. Selecting Manual allows users to enter specific design values. This method is intended for advanced users, as incorrect values may lead to simulation errors or unrealistic results.

Cooling Coil Rated Air Flow Rate

Defines the volumetric airflow rate across the coil during peak cooling operation. This value influences the coil’s ability to deliver cooling and should align with the system’s design airflow.

Cooling Coil Rated Water Flow Rate

Specifies the volumetric flow rate of chilled water entering the coil. This impacts the coil’s heat exchange effectiveness and must be sized in line with the cooling load and water-side system design.

Cooling Coil Rated Total Cooling Capacity

Represents the total cooling output of the coil, including both sensible and latent loads. Entered in watts, this value should match the cooling demand of the space it serves.

Cooling Coil Rated Total Sensible Capacity

Indicates the portion of total cooling dedicated to lowering air temperature (excluding humidity control). This helps balance air temperature performance with latent load handling.

Heating Coil

System

Sizing Method

Determines how the cooling coil parameters are set. Selecting Manual allows users to input specific design values. This method is recommended for advanced users only, as inaccurate entries may lead to unrealistic results or simulation errors.

Cooling Coil Rated Air Flow Rate

The volumetric airflow rate delivered across the coil during peak cooling operation. This value directly affects cooling performance and should match the design airflow rate of the system.

Cooling Coil Rated Water Flow Rate

The rate at which chilled water flows into the coil. This setting influences the coil’s heat transfer efficiency and should be aligned with both the cooling load and the chilled water system capacity.

Cooling Coil Rated Total Cooling Capacity

The total cooling output of the coil, measured in watts. This includes both sensible (temperature) and latent (moisture) cooling and should reflect the peak cooling load of the space.

Cooling Coil Rated Total Sensible Capacity

The portion of the total cooling capacity dedicated to reducing air temperature. This value excludes latent (humidity-related) cooling and helps determine how well the coil handles sensible heat loads.

Sizing

Sizing Method

Determines how the heating coil parameters are set. Selecting Manual allows users to input specific design values. This method is recommended for advanced users only, as inaccurate entries may lead to unrealistic results or simulation errors.

Heating Coil Rated Coefficient of Performance

Indicates the efficiency of the heating coil, expressed as the ratio of heating output to electrical input. A value of 3.0 means the coil delivers three units of heat for every unit of electricity consumed.

Heating Coil Rated Entering Water Temperature

Defines the temperature of water entering the heating coil under rated conditions. A typical value like 20°C represents standard inlet temperature for performance testing.

Heating Coil Rated Entering Drybulb Temperature

Specifies the drybulb temperature of air entering the coil. This affects the heating load and the coil’s capacity to warm the air, with 20°C being a common default for moderate indoor conditions.

Heating Coil Ratio of Heating to Cooling Capacity

Sets the heating capacity as a percentage relative to the cooling capacity. A value of 100% means the heating coil is designed to match the full cooling capacity of the system, ensuring balanced thermal performance.

Heating Coil (Electricity)

Sizing Method

Specifies how the coil's performance is defined. When set to Manual, users must input fixed design values. This approach is best suited to experienced users, as incorrect values may result in simulation inconsistencies.

Coil Efficiency (%)

Indicates the efficiency of the electric heating coil. A value of 100% means all electrical energy is converted directly into heat. Lower values account for system losses or inefficiencies in real-world performance.

Capacity (W)

Defines the rated heating output of the coil, in watts. This value should match the design heating load of the space and ensures the coil can meet demand under peak conditions.

Supply Air Fan

System

Fan Speed Method

Determines how fan speed is modelled. The Discrete option represents fixed-speed operation (e.g. low, medium, high settings), suitable for staged fan control. Other methods may include continuous or variable speed options.

Fan Motor Efficiency

Specifies the efficiency of the fan motor. A value of 92% means that 92% of the electrical input is converted into mechanical energy to drive the fan, with the remaining 8% lost as heat or friction.

Fan Motor In Air Stream Fraction

Defines how much of the fan motor’s heat is added to the airstream. A value of 100% assumes all motor heat is transferred to the airflow, which is common for fans where the motor is located directly in the air path.

Sizing

Fan Design Power Sizing Method

Specifies how fan power is calculated. The option Total Efficiency and Pressure bases fan sizing on the total mechanical efficiency and the pressure rise across the fan, giving a more precise reflection of system performance.

Fan Total Efficiency

Represents the combined efficiency of the fan, motor and drive system. A value of 50% means that only half the input energy is converted into useful airflow, with the rest lost to mechanical and electrical inefficiencies.

Fan Design Pressure Rise

Defines the expected pressure increase provided by the fan, measured in pascals. A value of 250 Pa is common for typical ducted systems and reflects the resistance the fan must overcome to move air through the system.

Sizing Method

Determines how the fan capacity is defined. When set to Autosize, the system calculates optimal fan flow rates and power requirements based on building load and system performance criteria.