Why air source heat pumps

Air Source Heat Pumps (ASHP) are a less expensive alternative to the Ground Source Heat Pump.  They are much easier to install and only require a small space at the side of a building.

The performance from an ASHP will vary according to the actual conditions on any given day.  That is to say the amount of energy (output) is not constant.  Because of this the ASHP is fitted with auxiliary boost heaters to supplement their output in very cold weather.

How they work
An ASHP uses the variable energy found in the ambient air temperature.  Typical average UK winter air temperatures tend to be around 4 or 5oC with only limited periods of very cold weather. The heat pump boosts this low energy via its refrigeration circuit to a usable level for the provision of heating and hot water for the home.

This works by taking advantage of the energy created when a refrigerant gas is compressed to a liquid. The refrigerant is first passed through a heat exchanger where it is warmed by the air.  This heat exchanger is called the evaporator.  The now slightly warmed refrigerant gas is compressed to a liquid which dramatically increases the temperature of the refrigerant.  The hot refrigerant liquid is then passed through another heat exchanger which is then used to warm the system water (Underfloor Heating or Radiators).  This second heat exchanger is called the condenser. 

In order to start the cycle again, the refrigerant liquid must be converted back to a gas and this is done by the expansion valve.

 

Air Source Heat Pumps
How varying air and water temperatures affect the performance
The output from an ASHP is not fixed and it will vary depending on certain conditions.  The certified output of an ASHP is measured for an air temperature of 2oC and a heated water temperature of 35oC.  However, if the air temperature falls or the water temperature rises then the output of the ASHP will fall.
The variation in output is approximately 250 watts for every degree Celsius change in air temperature.  However the rate of variation is not constant and will vary depending on the actual air temperature and the actual water temperature.  This is illustrated on the air source heat pump power graph.

A Radiant ASHP will work down to an outside temperature of -7oC and we even have models that work down to -20oC. 

All Radiant Heat Pumps can be ordered in Heat and Cool models as required.  The cooling process simply reverses the refrigerant gas circuit.  This is ideal when summertime cooling is required.

We also have special models that capture the waste heat in cooling mode to heat the hot water tank.

Caution
Because the output can vary it is important that you have an air source heat pump with an auxiliary heater.  All Radiant ASHP have built in auxiliary heaters

The best way to use an ASHP is as part of a system integration with a boiler or other heat source.  This will give you all the benefits of a heat pump with none of the drawbacks.

 

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The power graph will show the amount of power in kilowatts that is available at a given source and heated water temperature.  It is therefore easy to plot the output for any source temperature against any system flow temperature.  It is part of the design tool used to specify the correct machine for any given application.  Most manufacturers will provide these graphs to system designers.

These are typically electric elements rather like an immersion heater that will be switched on to boost the output.  These are very common in ASHP installations and also in some GSHP installations.  They will either be fitted in the heat pump cabinet or alternatively in the buffer tank.