In the ASHP the heat is recovered from the outside air, and the heat pump then uses this heat and compresses it for use to heat the water for your central heating system. The amount of heat extracted from the air depends on the outside air temperature. In very cold climates where average temperatures in winter is -5° Celsius or less, it is usually not efficient to rely on the ASHP so a more typical GSHP is required. But in Ireland, it is clear very few days in winter are actually as cold as this, so this could be quite a suitable alternative for moderate climate conditions.

Air Source Heat Pump Benefits And Drawbacks

Accepting that the heat pump principle can give more units of heat in return for the unit of energy expended, the ASHP has some advantages and disadvantages when compared to the GSHP versions mentioned on the previous page. The biggest advantage for the ASHP is that it requires no ground works in order to create heat! So, no digging up of the garden, no filling, relaying grass, and all that messing about. And, given the costs for the ASHP and GSHP are broadly similar, savings of 20-40% can be achieved given the minimal installation required.

The downside is that by their nature there are more moving parts to an ASHP. Coupled to that is the fact that the evaporator must be located outside, further adding to wear and tear from the elements. Doubtless most manufacturers will maintain that their units are completely weather proof, but I would take that with a pinch of salt. Regardless of their claims, I would imagine that the units with a separate evaporator will survive longer, as the heat pump itself remains inside and should be subject to only minimal wear and tear.

Another disadvantage is that in many designs of ASHP rely on a defrosting cycle which is required when cold weather has frozen the coils used to trap the air. During this cycle to bring the coils back to operating temperature, many heat pumps actually turn off and cannot output any heat until the defrost cycle is complete. Certainly, as a householder, I think it is a reasonable request that a heating solution gives me heat when I want it, not when it decides it is ready! So this is an important point to consider when choosing an ASHP.

ASHP Space Requirements:

If you opt for a single box unit, i.e. combining the evaporator and pump in one single housing, then the entire unit will be located outside the property and the brickwork will be drilled so that the pipes can be connected inside. This would be a good option if you have no space at all inside the house.

In two box systems only the evaporator is mounted outside, and will typically be smaller than the combined unit. The pipes are connected through the wall to the pump which is located inside. Most of these systems take up no more than 800mm², so the would easily fit into the space vacated by an oil boiler, for example.

ASHP Heating Options:

Although I have seen some claims that an ASHP can deliver hot water at up to 80° Celsius, I very much doubt if this could be achieved regularly and with enough volume to satisfy a radiator based system. Some manufacturers are up front about it, stipulating that the pump only be used with under floor pipes. Others are willing to say there system will work with any heat distribution system, but if using radiators then these would need to be over sized to achieve a reasonable heat output.

There is also evidence to suggest that the ASHP and radiator solution has been used for many years on the continent. Perhaps there are particular heat pumps on the market which can deliver high temperature water at the required volumes to support radiators? I suspect that the while this may be possible, a larger heat pump may be employed to overcome the heating restrictions that smaller pumps would have, therefore a larger capital cost and higher ongoing running costs are to be expected. Not only that but is has been well documented that once heat pumps are asked to heat the water to a higher degree, their overall co-efficient of performance drops. So an ASHP with a quoted CoP of 4.5 will only deliver a CoP of 2.5 if it needs to heat the water to higher temperatures.   

There is another radiator option which could be used with a heat pump, called a hydronic radiator. These radiators are mains powered but only require a small volume of water at lower temperatures to create a large heating output, of up to 6Kw per unit. The heat created is actually circulated in the room by internal fans located in the unit. They appear to be a good choice if the homeowner wanted to get the most out of the ASHP. However, I have it on good authority that with costs of EUR400 to replace a standard radiator with one on these units, retrofitting my 15+ radiators with these would also be extremely costly. Although the potential for using a smaller sized ASHP with the hydronic radiators reduces the capital cost and ongoing running costs, the overall initial investment would be very high for a retrofit. Another issue to bear in mind is the fan noise - as with all fan based system a certain amount of noise will be heard from the fans, so it is not considered a silent heating solution.

Which Air Source Heat Pump?

In preparing this list, I have noted that in most cases the ASHP costs the same as the GSHP, according to most manufacturers. So the guide price is for the heat pump, less grant of EUR4,300 and typical cost to install either inside or outside a suitable boiler room. 

MANUFACTURER / MODEL	PICTURE	COMMENTS	PRICE
Dimplex ASHP		5-25Kw power CoP of 4.0 quietness of pump: < 50dB life expectancy: ~ 25years single and three phase	EUR14,500
Sweco ASHP	<not available>	15Kw power CoP of 4.3 Direct Expansion system 80deg hot water from D/X less ground area required	EUR20,500
Heliotherm ASHP		CoP of 4.5 internet controllable only for use in under floor systems various sizes available	EUR11,500
Waterfurnace ASHP		up to 15Kw power up to cop of 4.5 established brand	EUR12,950

IF you have a groundwater or well water source, you may have another option to heat your home. Lets look at the options for a water source heat pump.......