Andrew Grimsley, Nu-Heat's Technical Director explains how this can be achieved:

"There are many factors to consider when designing a combined underfloor heating and heat pump system, but how successful it is will basically depend on the insulation levels of the property and, therefore the flow temperature the heat pump has to generate."

Flow temperatures

All heat pump types operate using similar principles - by harvesting energy from the environment and 'compressing' it to a temperature that can be used for a home's hot water and heating needs.

Probably the largest single factor affecting the efficiency of a heat pump is the flow temperature that it is asked to produce. The higher this temperature the more work the compressor has to do and the less efficient it becomes. As a result, a heating system that can operate with lower flow temperatures, such as underfloor heating which typically operates at around 55oC, allows the pump to maximize its effectiveness and minimize both its carbon production and the fuel costs for the homeowner.

When underfloor heating systems are specifically designed to be fed by a heat pump, additional tubing and more efficient floor constructions can be used to allow even lower flow temperatures, typically 35oC - 45oC, whilst still achieving the required air temperature inside the property (averaging 21 oC in living areas). Due to the smaller surface area of the heat emitter, a conventional radiator system requires a significantly higher flow temperature to achieve the same internal air temperature. Consequently underfloor heating and heat pumps are perfect partners as they are both well suited to the low temperatures involved in maximizing efficiency.

When operating UFH with a GSHP, an open flow weather compensated system is preferred, with an external sensor checking any deviation in outdoor temperature, comparing flow and return temperatures on the UFH, then adjusting accordingly.

Insulation, insulation, insulation!

With underfloor heating, warmth passes into the room from the floor and it is therefore important to reduce building heat loss, including downward heat losses into the ground or the floor below. Recent changes to Part L of the Building Regulations have focused attention on the importance of insulation levels within domestic dwellings and in a new building that meets the regulations, there will always be an adequate level of floor insulation, and in these circumstances pumps can provide 4 to 5 kilowatts of free energy for every 1 kilowatt of electricity used to power them.

Usually, the aim should be to insulate the building so that less than 50 watts of heating are required per square meter of floor space. This will then ensure that the UFH water temperatures can be kept to a minimum and the heat pump can operate at a higher Coefficient of Performance (COP) -typically 4 - 5 for a ground source unit. In general it is more cost effective to increase insulation levels than it is to install a larger pump and buildings that exceed the requirements of Part L of the Building Regulations are most suitable.

In theory, there is nothing to prevent a heat pump from working in a building with a higher heat loss, such as a property that requires up to 80 watts per square meter. However, higher heat loss requires higher heating water temperatures from the heat pump - typically 55°C rather than 35 - 45°C, meaning the heat pump's COP may suffer although the heat pump may still be sufficient to heat the property.

Heat pumps and underfloor heating - project compatibility

Installers can quickly determine whether heat pump technology is suitable for a project by completing a series of short questions, such as detailing the approximate internal size of a property compared to the available outside space, and confirming the level of insulation, as set out in Nu-Heat's online 'heat pump guide'. The answers selected will give a general guide to the likelihood of a heat pump being able to deliver both the heating and domestic hot water needs of the property. Visit www.nu-heat.co.uk/heatpumptool for more information.

Conclusion

Heat pumps will deliver the maximum efficiency in a building insulated to high standards with very low heat losses and where high-resistance floor coverings are not used.