What it costs: Heat pumps

Heat pumps are an increasingly popular way to provide heating in domestic and commercial building. The Code for Sustainable Homes advocates their use, as they significantly reduce carbon emissions. On the other hand, they have higher capital costs and lower output temperatures than gas boilers. Heat pumps work best in well insulated buildings in which demand is low. The noise they make may be an issue for some users.

The components of a ground-source heat pump system are a pipe in the ground (the ground loop), a heat pump and a distribution system. In heating mode the pump takes air of a relatively low temperature from the ground loop, concentrates the heat and passes it to the distribution system.

Understanding the heat capacity of the ground and the operation of the heat pump is important when designing and specifying a system. For example, if the pump takes out more energy from the ground than the sun replaces, the ground could freeze and the system would become ineffective.

A geotechnical survey is recommended to confirm the ground conditions. BS EN 15450 is the standard for heat pump system design.

Pipework is usually high-density or medium-density polyethylene (HDPE or MDPE) and may be specified to BS EN 13244, 1555, 12201, BS ISO 4427 or 4437. Their design life is typically 50–100 years. The circulating fluid is generally a water and anti-freeze mix, which is pumped around by a circulating pump. The pump should be made to BS EN 1151, and should be capable of carrying low temperature fluids. Pumps last about 10 years.

The pipework is usually installed in a closed loop, ideally without any joins to avoid leaks. Installation costs depend on the quantity of pipework and its depth.

An option increasingly considered in commercial buildings is to run ground loop pipework within concrete piles or incorporate it in the foundations. This saves the cost of excavating separately and utilises the thermal mass of the concrete piles.

Heat pump

The heat pump itself comprises a compressor, a condenser, an expansion valve and an evaporator. These components circulate a refrigerant that changes from a liquid to a gas and back again into a liquid.

Heat pumps should be to BS EN 14511 and BS EN 378, the standard for heat pumps with electrically driven compressors for space heating and cooling. The design life of a heat pump, based on the compressor is 15-25 years, scroll compressors being at the higher end. Maintenance involves inspections, servicing and responsive repairs.

Heat pump costs can be reduced by using cheap rate electricity such a seven or 10 hour tariff. Using a buffer tank or hot water thermal store can enhance the efficiency of the system. It’s also worth bearing in mind that 24-hour running reduces start-up loads.

A more sustainable option is to power the heat pump using photovoltaics or wind turbines, although generating enough power may be difficult without a backup.

The output heat of ground-source heat pumps is not as high as gas boilers, with temperatures typically about 35°C at a coefficient of performance (CoP) of 4. Higher output temperatures mean decreased efficiency. Underfloor heating is the preferred system for ground-source heat pumps. If radiators are installed then they should be sized for lower flow and return temperatures. An alternative is to use high-efficiency radiators.

Running a ground-source heat pump for domestic hot water at a temperature of 50°C has a number of cost consequences. First, the CoP would drop to nearer three, resulting in higher running costs. Second, higher demands require a larger area of ground loop at higher capital cost. Third, the design complexity of the system increases, which may mean reduced efficiency.