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An air source heat pump (ASHP), also known as an Air to Water Heat Pump, works in a similar way to a Ground Source Heat Pump except that it extracts heat out of the surrounding air instead of the ground.


Air source heat pumps are compatible with both radiator based and under floor heating systems and can be used together with most electric boilers, oil-fired boilers or equivalent when docking to an existing heating/hot water system.

Air source heat pumpAll our air source heat pump systems are designed with an advanced controller that balances which systems are running when, distributes heating and/or hot water to the property and compensates for the weather throughout the year by sensing the outside temperature and adjusting output accordingly.

If you do not have suitable land area for a ground source system, or have a very highly insulated new build property, or if you want heating for an outdoor pool then an air source heat pump may well be an ideal solution.

To be certain, contact isoenergy on 01293 821345 or navigate to our contact us page

How it works

An air source heat pump extracts energy in the form of heat from the outside air in the same way that a fridge extracts heat from its inside. This is the same gas compression refrigeration cycle as a ground source heat pump but this time the energy source is outside air drawn across the evaporator heat exchanger by a fan.

This might sound impossible during the winter months, but given that the refrigerant in a heat pump boils at between -15ºC and -30ºC there is always energy to be extracted from the air regardless of the weather! The key is to understand how to do this efficiently and to understand the limitations of air-source devices in these circumstances and to provide for them.

There are two main types of air source heat pump system:

  • An air-to-water system distributes heat via your wet central heating system.
  • An air-to-air system produces warm air which is circulated through ducting by fans to heat your home. Air-to-air systems are generally restricted to spatial heating and cooling only.

Running costs

The main advantage of an Air Source Heat Pump is that you do not have the cost and disruption of laying the ground arrays and you do not need the ground or water source

It is often said that an ASHP is not quite as efficient as a GSHP. However, in the spring and autumn months when air temperatures may be higher than ground temperatures the air source heat pump can outperform a ground source heat pump. That said, as air-temperatures in the UK are generally between 0-5ºC during the bulk of the heating season, ground-source systems are better suited to our climate where there are appropriate source conditions for them to be deployed.

For outdoor swimming pool owners, an air source heat pump is almost always the most economical option for pool water heating as the air temperature is generally higher than the ground temperature for much of the UK outdoor swimming season.

In the winter months you could expect an average CoP (the ratio of energy produced vs. the electricity required) of 3:1, and in the summer up to 5:1.

It should be stressed that not every air source heat pump is optimised for the UK climate. isoenergy recognises that some air-source heat pumps are designed for much dryer winters and are therefore equipped with much smaller heat exchangers. These tend to freeze up more frequently in the UK's damp winter climate. When this happens, the air source heat pump uses electricity to defrost the evaporator heat exchanger (a defrost cycle). The more time an air-source unit spends in defrost mode the less efficient it is overall which results in higher running costs.

isoenergy has rigorously tested all the air source heat pump that we specify to ensure that excessive defrosting is avoided. Furthermore, isoenergy is not tethered to one manufacturer to ensure that we can continue to offer bespoke solutions rather than shoehorning one make or model into any given situation.

isoenergy has years of experience consulting on, designing and installing bespoke air source heat pump systems and can dock to existing fossil fuel systems or combine with other renewables such as Solar Thermal, Solar PV and Ground Source Heat Pumps.


There are several key considerations when looking to install an air source heat pump:

Ideally, the unit should be installed close to the plant room where possible. The unit will require a good flow of air on both the input and exhaust sides and should not be placed underneath trees or where a risk of falling debris may exist. Minimum clearances must be observed to avoid low air flow or the risk of recycling the cold exhaust air.

Power supply
Do you have single phase or three phase electricity and does this run to your proposed location for the air source heat pump.

Associated builders work
The air source heat pump will require a solid base with a condensate soak-away. isoenergy can provide details for this.

Plant room sundries if docking with other systems
Dependant on the configuration of the renewable technologies, you may need buffer tanks or higher capacity hot water storage tanks. It is important to consider how large these are and whether you have enough space in your proposed plant room – again, isoenergy can advise on the appropriate plant room layout

Heating emitters or distribution system
Air source heat pumps can perform better with under-floor heating systems or warm air heating than with radiator-based systems because of the lower water temperatures required. This does not exclude existing radiator systems provided they are taken into consideration in the design phase.




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