To install a ground-source heat pump, trenches are dug and the collectors (these are from Worcester Bosch, 0844 892 3000) laid within them. However, you need a big garden, as for every 1kW of heat output, the system needs around 50m² of ground.?The alternative is to drill a borehole up to 100m deep, requiring around 4m² of garden.

While they both produce heat, heat pumps are not like boilers. Boilers are a well-understood technology — a good supplier will be able to advise you on the right boiler type and size for your home. But ask what heat pump you need and everyone you speak to will give you a different answer. So there is some learning to do, and some rules to follow.

Calculate the Heat Load

If you are retrofitting a heat pump, you will need to have a heating survey carried out to calculate the maximum amount of heat your house will require. For a new build this will probably come from the SAP calculation. Some heat pump suppliers will do this free of charge or deduct the cost if you order from them. Others will charge up to £500, but you might find this more independent advice valuable.

Undersize the Heat Pump

Heat pumps will generally be sized to meet the space heating only, and it is good practice to undersize the heat pump to around 85% of the peak load. Peak load is the amount of heat needed to meet typical winter conditions, i.e. outside air temperature of -2°C. If that calculation gives a peak load of, say, 12kW, then 85% would be 10.2kW, so we might be shopping for a 10kW machine.

The reason we undersize is to prevent the heat pump from ‘short cycling’. That is, switching on and off within a short period of time, which not only wears the machine out but also significantly reduces efficiency.

Assess Ground Conditions

Ground-source heat pumps collect low-grade heat from the ground and lift the same quantity of energy to a higher, more useful temperature. The quantity of energy stored in the ground is directly affected by the quality of the soil. Dry, sandy soils will only allow around 10W/m² to be extracted, while wet, clay soils will allow up to 35W/m². To install a 10kW heat pump on dry, sandy soil will need an area of at least 1,000m². But only 285m² if we have wet, clay soil.

Choose the Right Heating Distribution System

Heat pumps can only effectively lift water temperature to around 45°C. Beyond that an electric heating element is needed, which significantly reduces efficiency (potentially to the point where it is cheaper and produces less CO? to use a gas boiler).

The heating distribution system will determine the temperature the heat pump needs to operate at, so low flow temperature systems are best. Underfloor heating and some skirting heating systems, such as Climaboard, operate effectively at 35-45°C and are ideally suited to heat pumps.

Do I Need a Supplementary Heat Source?

If the house is well insulated and the right heating distribution system is installed and the heat pump is only dealing with space heating, then the answer is no. If you change those conditions, then a separate heat source is needed.

Air, Ground or Water?

They use the same compression/expansion technology, moving heat from one place to another, but each is best used in certain situation.

The least efficient but cheapest option, air-source heat pumps can heat air and, less efficiently, water. Ideal for heating swimming pools in summer or for warming homes with a low heating requirement.
The second most efficient option, but you need a large garden to install the pipes for a ground-source heat pump.?Alternatively you can drill a borehole. Ideally suited to underfloor or skirting heating.
The most efficient option, but you need access to a stream or spring to install a water-source heat pump.?Can be used much of the year to provide heating, domestic hot water and warmth to swimming pools.

What’s COP?

COP is the Coefficient of Performance. The COP is partly a measure of the efficiency of a heat pump, but also a measure of the conditions under which it operates. So the quoted COP will be for the machine in a given set of circumstances. An air-source heat pump, for instance, will often be quoted as ‘A-2 / W35 COP = 3.6’. This translates as an Ambient (or air) temperature of -2°C and a Water (or output) temperature of 35°C means the machine will have a COP of 3.6. So under these conditions, for each kilowatt (kW) of electricity the machine consumes it will produce 3.6kW of heat.

Change the circumstances the machine operates in and the COP will change: increase the ambient temperature and the machine has to do less work and the efficiency rises; increase the water temperature and the machine has to do more work and the efficiency falls.

What to Pay

Expect to pay around £5-10,000 for a ground-source heat pump, although if you have to drill a borehole, then the cost could be anything up to £15,000. Air-source heat pumps cost around £5-8,000.

A Guide to UFH

James Dale, Technical Manager at Underfloor Warehouse (0116 258 1410) shares his tips for combining a heat pump with underfloor heating.

Due to the low water temperatures required by underfloor heating to generate satisfactory ambient air temperatures, a heat pump is the ideal partner compared to other heat sources.

However, airtightness and the level of insulation within a building influence the efficiency, economy and effectiveness of heat pumps and underfloor heating. These two factors must be dealt with so the building has low heat losses and therefore low energy use —especially in renovation projects.

The output of the underfloor heating system is determined by the supply water temperature necessary to meet the heat requirement of the building, so higher heat losses mean a less efficient heat pump. Another factor that reduces the efficiency of the system is the floor finish. Ideally a tile or stone would be used to maximise efficiency, but this is not always practical.

Tim’s Analysis: A Good Idea?

Heat pumps are a good, reliable technology when used in the right way. A properly designed and sized machine, dealing only with space heating, needs no back-up system (gas or oil boiler) and, with proper servicing, will last 15 to 20 years. The best system will be a combination of a heat pump for space heating and solar thermal panels for domestic hot water. The Energy Saving Trust does not recommend air-source heat pumps for houses that are poorly insulated or that have mains gas available, and there are good reasons for that. It is not a simple replacement to a gas boiler. To work effectively a heat pump needs the right conditions. Get those conditions in place and buying the heat pump is easy.

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