Air to Water Heat Pumps Explained


  • An air to water heat pump is a piece of mechanical equipment that extracts heat from the air and uses it to heat hot water - in Sefaira's case it heats hot water for heating the building.
  • In the waterside tab you can choose air-source heat pumps as a way of making heating hot water to heat the building.
  • If your air source heat pump is reverse cycle (ie it does cooling and heating) use the air-cooled chilled component in the chilled water card.
  • Air source heat pumps are a good design option because they are typically more efficient at converting a unit of energy to heat than a pure electric or gas boiler. In climates that have cool to mild winters they are especially popular.
  • EnergyPlus does not have any supported air source heat pump objects for making heating hot water but they do have a way for us to add our own components to the idf calculations. This is what we have done here and how we did it is described below.


What is an Air to Water Heat Pump?

“Air to Water Heat Pump” uses outdoor air to absorb (or reject) heat depending on the operating mode, heating or cooling.

The most commonly known "Air to water heat pumps" are air-cooled chillers (which reject heat into the air). In Sefaira "Air to water heat pump" refers to a unit that only makes heating hot water by absorbing the heat from the air.

Air to Water Heat Pumps are usually used for low temperature heating like radiant panel heating, radiators or sometimes fan coils. The efficiency of the heat pump is substantially affected by the temperature of the heating water it is trying to make, with the unit generally being quite inefficient at making very hot heating water.

When could I use an Air to Water Heat Pump?

Air to Water Heat Pumps are increasingly popular in some regions including Northern Europe and Australia. However there are some design constraints to be aware of with this heating component:

  1. Efficiency is very poor when outdoor air temperatures are below 0C. If your climate has lots of hours below 0C you might find the unit is not that much more efficient than a regular boiler.
  2. The Efficiency of the unit depends a lot on the supply water temperature. Make sure your heating system can cope with the low water temperatures. Good applications are radiators and radiant floors. VAV boxes may end up being substantially bigger with low heating water temperatures.
  3. Anecdotally the units may have a higher capital cost than regular boilers.

How we implemented the Air to Water Heat Pump component in Sefaira using EnergyPlus

This feature was new for Sefaira. We had a lot of customers who wanted this feature. However it has not been supported by EnergyPlus. We decided to make our own component. We welcome feedback on the process described below.

EnergyPlus doesn’t support “Air To Water Heat Pump” systems for heating . EnergyPlus has an object for type of “Air To Water Heat Pump” that serves Domestic Hot Water system which can not be used in heating systems. EnergyPlus also has air-cooled chillers which can be used for chilled water but not for heating.

To effectively simulate the “Air To Water Heat Pump” system we therefore applied the workaround below to the idf file.

  1. In the Idf we will use a Boiler object that is tagged as a “District Hot Water System”. The difference between the DHWS and the in-house Boiler is that DHWS is delivering heating energy to the building HVAC system to satisfy the actual heating demand where its efficiency and the input energy as the amount of fuel used is not tracked.
  2. We are using a dummy “Exterior:Lights” object that we call “AirToWaterHeatPump” and we create a meter for it to collect (hourly) the electric energy (kWh) used by the “Air To Water Heat Pump” boiler.
  3. We are using E+ Energy Management System (EMS) via small programming language called EnergyPlus Runtime Language (Erl) to calculate the electric consumption of the “Air To Water Heat Pump” boiler hourly when it is operating.
  4. In the EMS routine we are calculating the actual COP factor of the “Air To Water Heat Pump” boiler
  5. COP (Coefficient Of Performance) is a measure of energy efficiency of the “Air To Water Heat Pump” boiler. The higher the COP, the higher the efficiency is. Heating energy delivered to the HVAC system via hot water is a sum of heat that is extracted from the outside air (low temperature heat) and the electric energy used by a compressor to elevate (pump) that low temperature heat to the higher temperature heat that can be than used by our HVAC System - that’s why system is called Heat Pump. COP is calculated as (“heat extracted from OA” + “compressor energy”) / “compressor energy”.
  6. COP factor is not a constant all the time because COP is a function of:
  • Outside Air Temperature (OAT)
  • Supply Water Temperature (SWT)

In Sefaira's User Interface we apply a COP value that we call “nominal” which means that the COP applies only when certain (OAT and SWT) conditions are met. Any time when any of one of two (OAT or SWT) is different we have to calculate the COP value and we call this value the adjusted COP (COP adjusted). Depending on the OAT and SWT values COP adjusted can be either lower or higher than “nominal” COP. The process of how we are calculating the adjusted COP will be explained later in the document


How do we calculate the electric energy consumption of the “Air To Water Heat Pump” boiler?

In the ERL, for each hour of the annual simulation we are tracking the following:

  • Is DHWS delivering heating energy to the HVAC system? 
  1. If “YES” we do the following:
    1. We read the current Heating Energy Capacity (HECAP) of the DHWS
    2. We read the current OAT
    3. We read the current SWT
    4. We calculate the adjusted COP (COPadj) as a function of OAT and SWT
    5. We calculate the “Air To Water Heat Pump” boiler electric energy consumption (ELCON) as: ELCON = HECAP/COP adj
    6. We add ELCON to the  “AirToWaterHeatPump” “Exterior:Lights” object meter (ATWHPmeter)
    7. After the simulation is done when we parse the energy breakdown we add the total of ATWHPmeter to the electricity end use.
  2. If “NO” we skip the whole calculation and we add zero to the ATWHPmeter for that hour.

Adjusting the instant COP based on hourly factors

Nominal set of curves as a function of OAT and SWT is based on the set of curves that is provided in the following article

Nominal set of curves means that if user selects COP value equal to 4 (when SWT=35C and OAT=7C) a family of curves showed below will be used to calculate the COPadjusted for different OAT and SWT


When user selects a different COP value than nominal (COPdiff) set of curves used used for that COPdiff value is adjusted as



For COP = 5 all curves are shifted up based on the factor that is equal to 5/4 meaning an “Air to Water Heat Pump” with higher COPnomial than 4 will have all other curves more efficient.


Formulas used for creating curves SWT = f(OAT)

SET COP_NOMINAL = CURVE_ADJUSTMENT * 3.44278776825230 * (2.718281828^(0.021772039290835 * OAT))

SET COP_30C_SWT = CURVE_ADJUSTMENT * 3.80319445274466 * (2.718281828^(0.022756197434888 * OAT))

SET COP_40C_SWT = CURVE_ADJUSTMENT * 3.08171162234967 * (2.718281828^(0.020548125533303 * OAT))

SET COP_50C_SWT = CURVE_ADJUSTMENT * 2.41487315422090 * (2.718281828^(0.019071242300744 * OAT))

SET COP_60C_SWT = CURVE_ADJUSTMENT * 1.77487315422090 * (2.718281828^(0.016071242300744 * OAT))

SET COP_70C_SWT = CURVE_ADJUSTMENT * 1.20487315422090 * (2.718281828^(0.010071242300744 * OAT))



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