Working in partnership

Only underfloor heating can compliment the energy-saving performance of heat pumps. Robert Ewels examines the best route to successful installation and commissioning

The 2010 report by the Energy Saving Trust into the performance of heat pumps in the UK judged that system design, installation and commissioning were critical to their success both in reducing carbon emissions and in reducing costs for users.

Critical to that successful design and installation is the selection of underfloor heating as the heating medium, since it is only underfloor heating which can operate effectively with the low flow and return temperatures generated by heat pumps.

The water circulating within an underfloor heating system is typically between 35º and 45ºC, compared with a traditional temperature of 70-80ºC for radiators meaning that the heat pump works more efficiently. This is due to the heat pump needing less electricity to reach the lower water temperatures required for an underfloor heating compared with a radiator system. Less energy input results in a higher co-efficient of performance (CoP).

The Underfloor Heating Manufacturers’ Association (UHMA) has produced a heat emitter guide for domestic heat pumps which shows that, with a heating circuit flow temperature of 35ºC, a ground source heat pump operates at a likely seasonal performance factor (SPF) of 4.2, whereas with a heating circuit flow temperature of 60ºC, that SPF is reduced to just 2.8.

In addition, because it generates radiant heat, underfloor heating is a more efficient and comfortable solution than either radiators or HVAC units which rely on the principle of convection and heating air.

In a commercial project in particular where underfloor heating is sited within a screeded floor or other surface, the large thermal mass of the floor works to retain that heat and release it smoothly and consistently, concentrating it in the 2m occupied zone.

In contrast, radiators and HVAC units demand a degree of over specification to take into account the fact that to achieve a comfortable temperature of 20-21ºC in the occupied zone, the temperature in the unoccupied zone near to the ceiling is likely to be closer to 28-30ºC. This further reduces the efficiency of a heat pump.

It also inevitably means that those working in the building have warmer heads than feet, contradicting the best practice advice for optimum employee awareness and performance that the feet should be warmer than the head.

Underfloor heating also eliminates the peaks and troughs associated with convected heat when windows or doors are opened and hugely reduces the incidence of dust and allergens being circulated in moving air patterns.

The benefits have been perfectly, if rather extremely, demonstrated by REHAU in Germany where underfloor heating has been installed to replace HVAC units in a 100m long x 20m high aircraft hangar at Munich airport. Previously, the workforce had to work in insulated layers of clothing because the occupied area of the building was always cooler than the roof space and each time the hangar doors were opened there was a sudden and intense dip in temperature.

In contrast, the workforce now only requires insulated clothing when they work at height on a plane. The dip in temperature when the doors are opened is shorter and much less severe.

Compared with specifying heat emitters to partner with a gas-fired boilers, specifying with a heat pump is much more critical since the implications for efficiency are so much greater. As both the Energy Saving Trust and UHMA reports demonstrated, a heat pump which is not matched effectively to the heat emitters will simply use more energy.

For underfloor heating, the key factors to consider are heat losses, floor coverings and whether there is a requirement for heating as well as cooling.

Underfloor heating systems can operate very effectively with a heat pump to provide cooling in the summer or to counter high levels solar gain simply by reversing the flow. With REHAU’s latest set of controls, it is even possible to have auto switching between the two to maintain ideal comfort conditions.

However, there are implications in terms of how the pipework is spaced to optimise the heating and cooling performance and it is advisable to seek advice from the system supplier.

In terms of design considerations, a key recommendation is that a storage buffer vessel is installed between the heat pump and the underfloor system. This not only ensures a minimum flow of water through the heat pump but also ensures that the heat pump is steadily charging the water in the buffer vessel.

A buffer vessel also helps to smooth out the peaks in demand and eliminates the risk of inefficient temperature spikes or overshoots contributing to increased efficiency.

It is always advisable for any installation of underfloor heating with a heat pump to be specified as one complete system. This doesn’t mean choosing a package from one supplier but rather adopting an integrated approach and involving specialist underfloor heating designers at an early stage to consider the overall system performance and not just the performance of individual components in isolation. 

• Robert Ewels is business team manager, REHAU Ltd

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