Air Source Heat Pumps

An Air Source Heat Pump, also known as an Air to Water Heat Pump, works in precisely the same way as a GSHP, except they take the heat out of the outside air, instead of the ground.  This might sound strange in the winter months, but given that the refrigerant in a heat pump boils at around -20 °C, even the coldest weather feels hot to a heat pump!

Some ASHP’s in particular are designed for radiator based heating systems and can be used together with most electric boilers, oil-fired boilers or equivalent if docking is required. Some systems (usually Swedish) will work to outdoor air temperatures of minus fifteen degrees centigrade meaning that no extra heat source will be required due to the UK’s milder climate. The heat pump includes an advanced control system for optimal control. This is started by a signal from another controller, return line sensor or thermostat. Some high quality pumps can also be controlled by a specially designed controller. This connects and disconnects the additional heat source whilst also controlling switching between room heating and hot water heating.

The main advantage of an Air Source Heat Pump is that you do not have the cost and disruption of laying the ground arrays. A disadvantage is that the ASHP only goes up to a maximum of 14kw capacity; a larger output than this and the fans become so large due to the air volume requirement the machines become impossible to site in most locations. Also ASHPs are not quite as efficient in the coldest weather, and are not capable of passive cooling. Having said that, in the south of England, where recent average winter temperatures have been around 6°C, this is not such a great problem. Indeed, it is arguable that the ASHP is more efficient in spring than a GSHP as it is likely that the temperature underground will have cooled to less than 6°C due to the effect of having freezing water running through the pipes throughout the winter months whilst the outside temperature will be significantly higher.  For pool owners, an ASHP is usually the cheapest option for pool heating. In the winter months you could expect an average COP of 3½/1, and in the summer an amazing average of around 8/1.

 

ASHP outdoor module



Indoor control module for ASHP

Below is a chart showing the typical comparisons of emissions and costs of heating a well-insulated house of 100 square metros by fuel type.

Heating and HWS System	Annual Fuel cost	Annual CO² emissions in tons
ASHP	£305	1.6
Condensing Gas Boiler	£450	3.0
Normal gas Boiler	£500	3.5
LPG	£1055	4.5
Oil	£1200	4.8
Electricity (storage & panel)	£960	6.2

The above figures were taken during a controlled experiment by Powergen and utilise the current fuel cost as of the 1/1/06. Any increases in energy costs since that date has not been included.

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