Heat pumps and underfloor heating: perfect partners?
Posted by John Barker-Brown on 1 November 2010 at 11:01 am
Underfloor heating is often portrayed as the perfect partner for heat pumps. With its large heat emitting area and low input temperature requirements it seems to provide the ideal choice to offer the maximum efficiency from the heat pump. However, is that always the case?
With any underfloor system to be used with a heat pump, it is important that the system is designed correctly, for example due to the lower flow temperatures of a heat pump system the underfloor pipes are generally installed at smaller centres increasing the pipe density and the type of floor covering used will also have a major effect on performance.
There are a number of different underfloor heating systems which can be installed in a property:
1) Underfloor heating mounted in screed: This is most commonly used on the ground floor, but can be used in first floor applications in buildings constructed using a beam and block design. As the underfloor pipes are mounted in a concrete screed with a high conductivity and lack of air gaps, a low flow temperature can be used and this is generally around 35C. This temperature should be adequate to meet the Building Regulations requirement of 21C in the living area, with an average of 18C throughout, at an external ambient temperature of -3C. As said above the lower the flow temperature, the greater the heat pump efficiency, and COPs (coefficients of performance) of around 4 can be achieved. The use of the screed can also act as a thermal store and so lower cost off-peak electricity can be used, lowering running costs further.
2) Joisted systems: There are a number of different joisted systems for underfloor and these are generally used for first floor and above underfloor systems. I have listed the 4 most common ones below:
i) Over-joist – This is where the pipes are laid on top of the existing floorboards and then covered with a wooden deck. You generally have to run the heat pump at a higher flow temperature to drive the heat through the ‘insulating’ floorboard and into the room. This will increase the required flow temperature and we have found this to be around 45C, meaning the COP is now around 3. With over joist systems there are a couple of further issues which also need to be taken into account. If the pipes are bedded in a dry sand/cement mix to provide a close contact and thermal store to enable the use of off-peak electricity, this has an associated weight which needs to be considered in the joist design. There is also an increase in floor level so staircases, skirting boards etc need to be designed with this in mind.
ii) Between Joist – The underfloor is now installed between the joists, keeping the floor level the same. The same structural issues apply as with over joist if a dry sand cement mix is used. With regards to flow temperatures again these need to be around 45C to drive the heat through the ‘insulative’ floorboard, lowering the heat pump efficiency. Although between joist seems a better solution it is more difficult and hence costly to install.
iii) Plated system – This consists of aluminium plates placed between joists to which the underfloor pipes are clipped to and is probably the most common of joisted underfloor systems. It has the advantage of being quick and easy to install and hence lower costs. However there is no screed to act as a thermal store to maintain the heat into the room when the heat pump is turned off, so the heat pump generally has to run 24 hours a day. The system is still covered with floorboards so there is an effective insulation layer and the flow temperature of the heat pump has to be increased to around 45C with the corresponding drop in efficiency.
iv) Air Void systems – This is where the underfloor is simply mounted between the joists, with no aluminium plates or screed. Due to the fact that as well as having to drive the heat through the floorboard, the heat now has to cross an air void, which is a very good insulator, the flow temperature has to be even high than the 45C. In fact the flow temperature is now so high that the heat pump generally cannot provide this and still maintain Building Regulations temperature requirements. Air Void systems are generally not advised to be used with heat pumps.
So although underfloor heating can be the ideal partner for heat pumps it does depend on how the underfloor heating is installed. For any joisted underfloor heating you have to run the heat pump at 45C, which is the same temperature that radiators need from a heat pump. So, why not install radiators on joisted floors with underfloor on screeded floors instead? It's a much cheaper solution. Don’t get me wrong a radiator/underfloor mix is not perfect, there is a delay in the radiators getting warm as the screed downstairs takes all the heat and the radiators are generally oversized due to the low flow temperatures, however these are minor issues compared to the cost savings involved.
Photo by Bryn Pinzgauer
About the author: John Barker-Brown is special projects manager at British heat pump manufacturer Kensa Engineering.
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