A standard boiler system works on a short, sharp shock premise with an instant response to the call for heat. A heat pump - whether air source or ground source - delivers a constant flow of heat. The efficiency comparison is rather like driving a car stop/start through town as opposed to a smooth, even cruise at one speed.

Conventional boilers produce hot water at a temperature in excess of 65°C compared with 35 - 45°C from a heat pump. These lower temperatures will adversely affect the performance of underfloor heating (UFH) if they are not factored in prior to installation.

Significant Factors

The first consideration is the amount of pipe in the floor and the spacing required. A greater amount of pipe may be required for use with a heat pump due to the lower water temperature. Tube spacing for a conventional boiler might typically be at 200mm centres and a heat pump at 150mm centres. However, there are so many variables that this can only be determined when the design has taken into account such factors as room size, glazing and floor covering.

It is important to establish what floor covering will be used when the pump is installed. Many floor coverings including most tiles, slate, laminates and solid wood work well with UFH. Carpet and underlay are acceptable as long as a tog of more than 2.5 is not used.

For any underfloor heating installation, levels of insulation are essential to the efficiency of the system, but when a heat pump is to be used this is even more critical. Insulation levels should ideally ensure that less than 45 watts of heating are required per square metre of floor space.

If the future heat pump system is to include radiators, these must be sized correctly so that they continue to work efficiently after the changeover. As a rule of thumb, radiators will need to be around one third bigger than when used with a conventional boiler. The alternative is high efficiency radiators that should be sized according to manufacturer specification.

Along with the future proofing required at design stage, it is also worth looking at practical issues to see if heat pump technology will be suitable, as there are particular installation requirements. For example, an area approximately two and a half times the size of the property is needed to lay the ground loops for a GSHP. An ASHP requires space outside, pipework and wiring from the external unit back into the property and both types require a plant or utility room to house the equipment.

Installer Perspective

Dominic Taylor-Lane of Purus Navitas, an experienced installer of underfloor heating and heat pumps recently worked on a refurbishment project where the householders decided to install a conventional boiler with a view to changing to a ground source heat pump in the future.

Dominic explains how the system was future-proofed in this instance. 'The owners have the space for boreholes so, although a conventional boiler has been installed, the ground source heat pump is an option for later. For this reason the plant room was purpose-built to be large enough to house a ground source heat pump and buffer cylinder. Pre-insulated pipe was run into the plant room terminating in a manhole cover close to the proposed borehole site to minimise any future disturbance to the work already done.'

Of insulation requirements Dominic comments 'The internal walls were lagged during the refurbishment so in this instance the underfloor heating pipe spacing specifications made by Nu-Heat were similar to a normal specification. The first floor radiators run off a different circuit, but we installed oversized aluminium radiators so they will work with a heat pump in future.'

As increasing numbers of customers think about their long-term energy needs, future-proofing is a serious matter. Ultimately, as long as the initial design and installation of UFH has taken into consideration future integration with a heat pump, the final system will run efficiently and is likely to last for a long time to come.