A report in the latter part of 2010 from BSRIA questioned the viability of heat pumps working with a combination of underfloor heating (UFH) and radiators. Chris Weightman, Nu-Heat’s Senior Development Engineer looks at the issues that surround such a system and what practical solutions are available.

The main concern about the integration of UFH and radiators is the basic thermodynamic law that heat output is related to a difference in temperature (in this instance between the radiator and the room) and the heat transfer area. Put simply, radiators are designed to run at high temperatures because they comprise a comparatively small area. UFH, on the other hand, has a relatively large area for heat transfer and therefore can afford to run at lower temperatures, making the system a more suitable partner for the low temperatures associated with heat pumps.

Why radiators?

It is often assumed that a request for a heat pump to work with radiators on the first floor and UFH on the ground floor is to do with cost. However, it is frequently down to practical issues. For example, in a retrofit situation it might be impractical to disrupt the flooring in the upstairs accommodation. In new build, UFH might be considered at a stage of the build that is too late for installation. And of course people can be set in their ways, feeling for unsubstantiated reasons that radiators are better in a bedroom than underfloor.

Design

If radiators are felt to be the required solution, the key is to make sure that the company designing and supplying the heat pump and UFH is involved in the radiator sizing and specification, as this has to be correct to ensure that the system performs well and achieves energy efficiency. Otherwise, the system could mistakenly be sized to high flow temperatures and fail to reach its full potential.

Radiator solution

Jaga RadiatorConventional thinking is that for radiators to work with heat pumps, they need to be grossly oversized (compared to when used to a boiler) in order to cope with the heating requirement. This is of course impractical in many situations, either from the perspective of available space or the need to maximize usable wall space for other purposes.

The only realistic solution at the moment is to use ‘high output ‘radiators that are able to achieve the required output using the lower flow temperatures. Nu-Heat, for example, often designs systems integrating UFH on the ground floor with Jaga Strada radiators on the first floor. These radiators feature an intelligent temperature monitoring function ensuring that the low energy fans integral to the radiators boost output by precisely the right amount. From the perspective of energy efficiency they do use a small amount of electricity, but this is only 20 watts and is not continuous.

The ultimate aim with this configuration is to keep the UFH and radiator temperature exactly the same in order to maintain the efficiency of the heat pump system – that is, low temperatures and a good Coefficient of Performance (CoP).

Controls

With a standard heating system, the timing of the UFH is very different to the timing of the radiators. UFH has a slower response when coming on and going off, whereas radiators have a more instantaneous response. With such a difference in response needs, the controls are also very different.

Where a heat pump is integrated, there is no timing involved with the heat pump element of the system due to the integral weather compensation facility, meaning, in essence, that the heating is on all the time. As the heat fed into the system is matched to the heat load of the property, the system should be balanced, so there should be no conflict between the timing of the radiators and the UFH.

However, whilst in an ideal world a heat pump system should be perfectly balanced, experience shows that in reality the thermostat for the UFH and the thermostatic radiator valve (TRV) are often used to handle any fine tuning.

Buffer tank

With the possibility that part of the heating system will turn off, a buffer tank will be needed, so that the heat pump always ‘sees’ something to work against. Where radiators might be deemed unsuitable because the total volume of the system might be low, a buffer tank will offer a practical solution.