Delivering the heat

How is the heat delivered to the building?

In regions where heating is more important than cooling, heat is usually delivered in the form of hot water which is circulated around radiator circuits. This is a practical and effective method of delivering heat from gas boilers, and has been one of the most common heating system.

Heat pumps usually deliver heat to water the same way. However, to maintain a high energy-efficiency, the heat emitter system should be designed so that the water temperature is as low as possible. This means that radiators will need to be sized larger than normal. The pipework may also need to be slightly different to that of a boiler design system.  In modern very well insulated houses however, the radiators may not need to be so big, so may be of a more acceptable size.

Underfloor heating

Since heat pumps prefer lower operating temperatures, underfloor heating is a good choice if it is practical and possible to install. Since the area of the floor is so large, the temperature of the floor, and water pipes below it, may barely feel lukewarm. This is ideal for a heat pump. The design should however be heat pump compatible.

In general a heat pump compatible design involves more pipe (closer pipe spacing) below the floor, and no mixing valve on the manifold.  Manifolds tend to be large on heat pump instalations, as illustrated in this picture.   The



There are several variants of underfloor design. Pipes embedded in a screed is generally the best method for ground floors. For upstairs, beam and block or similar concrete systems can be good for heat pumps, however, houses seem more commonly built with timber joists. The two main types for use with timber joists are either aluminium spreader plates or a biscuit screed. Usually a timber covering would be laid over this.

The floor covering will have an effect on the performance. For the best energy-efficiency, the heated water should not be too warm. Stone, tile or polished concrete are probably the best surfaces to choose for the best energy-efficiency. However, the heat demand should be considered. With an old building the floor may need to be quite warm. If the floor needs to be as warm as around 28ºC (82F), it may be uncomfortable on the feet. In contrast, in a very well insulated building, the floor surface may need to only be say 22 or 23ºC (72.5F), so feet may feel cold.

Timber floor coverings are an option, but this acts as a part-insulator. Any small air gaps between the screed and timber are also a hindrance to heat flow. The result is the need for warmer water causing a lower energy-efficiency. In a low-energy house, the heat demand will be low, so all these issues become less of a problem.  If the heat demand is low, there may be some advantage to the warmer ‘feel’ of timber is such houses. By contrast, in older houses, it may be difficult to get enough heat from a timber floor. There are different choices of timber, and some new products boast high conductivity. Thinner hardwoods and machined board can be the best.

Carpets and rugs can be used but should be thought-through. For older buildings stone floors give the best performance, but there is nothing wrong with some strategically-place rugs, so long as much of the floor is free to radiate. Fitted carpets are also possible, but they should be not too thick. TOG (insulation) rating values should be available, and special underlays are available for underfloor heating. Again, this is not ideal, but in bedrooms that are not kept so warm, then a carpet may be fine.

The ‘Emitter guide’ was devised to help with heat pump-friendly underfloor and radiator design. You can download it here on the IDHE website

This can help compare the performance of different types of underfloor heating, and help with sizing factors with radiators.

For harder-to-heat older buildings, fan-assisted convectors can be used. The modern designs are a lot better and quieter than they used to be. However, beware of the controls fitted to some that try to stop them blowing uncomfortably cold air. If a heat pump is set to a nice low and efficient water temperature, this could be colder that the convector will accept. This detail should be checked.

Air distribution system

Air-air heat pumps can blow air into a room. It is a matter of personal choice as to their comfort level. Moving air may not be ideal during the heating season, however, if well positioned this may not be a problem. Again, if the building is poorly insulated, then the air flow will need to be high. This may be uncomfortable. In a passive house, the airflow may be tiny, so perfectly acceptable.

Response time

Whilst considering all this, it may be worth considering the heat-up response time. Traditionally we have installed boilers that are big enough to be able to raise the temperature of a room fairly quickly.

It can be costly to install an energy-efficienct fast-response heat pump. Furthermore, if a heat pump is ‘driven’ very low and lean, the efficiency (COP) promises to be much better (higher).  This leads towards the use of smaller heat pumps that are inherently slow-response.  This means a bit of an attitude shift towards automatic and more continuous heating.  Many heat pump controllers have a holiday setting. Its worth using this so that the house has time to warm up for when you arrive home.