Getting the best from underfloor heating

Many years ago, when heat pumps were not so common, I found it a real struggle to get any of the main underfloor heating suppliers to embrace the need for low water temperatures for use with heat pumps.

15 years on, and I’m sure that things have changed dramatically, but I’m still stumbling across things that make me doubt that.
When I visited Germany over 10 years ago, I got the feeling they commonly use much closer spacing between floor pipes than we do in the UK, and I have read of very close spacing (50mm) in Austria.  I realise these are potentially cold countries, but even recently visiting a Spanish heat pump company, they were surprised that we don’t use closer pipe spacing (more pipes).

Over the last 6 months I have heard several references in the trade to; ‘never use closer than 150mm pipe spacing’.   However, CIBSE clearly give ratings for 100mm spacing, as does the MCS Emitter Guide, so why are some in the UK so resistant to putting more pipe into floors.

To basics     COP v heat pump water temperature
Heat pumps are more energy-efficient if the water temperature is low.
Coefficient of performance (COP) = heat output / electrical power input
This graph of an air source heat pump shows how beneficial a low flow temperature is.   Units like this with electronic expansion valves are particularly efficient at very low water temperatures.
How to get a low flow temperature with underfloor heating
The values for the example graph above have been extracted from CIBSE, and show a general trend for a fixed heat output.  It demonstrates what we all should know  – the closer the spacing, the lower the required water temperature for the same heat output.
Putting the two together we could plot pipe spacing v COP.  It would clearly indicate that closer spacing improves COP, so why do some seem reticent to embrace close spacing?  Is that added cost for more pipe really that much??
I have gleaned that some people have the notion that close spacing could lead to warm floors and an overly hot house.  The whole point here is that with closer spacing, we can turn the water temperature setting (heating curve) down, and get the same heat output, thus improving the COP.
I think some of the fears about hot  floors stem from boiler systems, many of which had plenty of extra output capacity for quick warm-up, and hot patches could result. However, heat pumps are generally slower response, with lower water temperatures, so are much more forgiving in this respect.  I recently visited a system where I had used 50mm spacing in an always-open loop in a bathroom.  I asked the owner if they had ever thought the bathroom floor was too warm.  The answer was no, never. 
The reason why I had used 50mm spacing was that we didn’t want a buffer cylinder, so I was trying to ensure there was adequate water quantity, and flow-rate in the floor – effectively using the floor as a buffer.  
Another hot (excuse the pun) topic here is floor coverings. People are now seeing that a carpet will drop the star rating for RHI, so there is an added financial incentive for having solid (tiled etc.) floor, as seems far more common in Germany, instead of carpet.
However, by designing a house where some rooms are tiled, and some are carpet, you are somewhat limiting any changes that future occupants might wish.  (e.g.  if 100mm spacing in a carpeted room, and 200mm in a tiled room).   Underfloor pipes are literally set in stone, so can never be changed.  The only solution that I could think of here would be to interlace the loops (given that there is often more than 1 loop per room).  There is always the option of turning off one of the interlaced loops.  I like multiple interlaced loops.
I have come across an installation where a new owner had fitted thick pile carpet in one room without thinking the affect it would have.  The result was a cold room, remedied only by turning up the water temperature, thus  increasing the running cost, and the reliance on room thermostats to limit heat to the tiled rooms.  
The general design approach for underfloor is to consider the heat required (e.g. watts/sq m) , but unless a buffer cylinder is fitted, we should also consider what heat is being delivered to the floor, given that some zones will be closed for some of the time, and the outside temperature is seldom at design temperature (-2C etc.).   For most of the time, we have far more available heat than the floor needs.  Even with modulating heat pumps, there can still be a tendency for the flow temperature to stray above the theoretical flow/return temperatures. This is another reason for favouring more pipe in the floor.  Furthermore,  MCS requires the heat pump to provide at least 100% of the design at -2C etc.  Due to models only being available in certain size jumps, the heat pump installed is often oversized, so this is an added reason for ensuring that there is adequate pipe in the floor.
For any thinking that buffer cylinders are the perfect answer – they may be an answer, but they are seldom perfect, as shown by this piece of monitoring.     (openenergymonitor.org)
In this example of a system with a simple buffer tank, the heat pump flow/return needs to be approx. 4 degrees hotter than UH flow/return.    This could reduce energy efficiency by 10%, plus the added energy to run a second circulation pump.
A buffer can usually be avoided IF enough zones are always on AND if there is plenty of pipe in the screed.
Another potential worry is the thermal mass of the screed.   I was recently involved in the design of a passive house where a buffer cylinder was not wanted, so I proposed  to use the screed as the buffer, and fitted 9 x 100m loops with average 100mm spacing in a  floor that was 200mm thick.  One might expect some temperature overshoot, and I may not have been so bold as to propose it if it were not for the fact that due to the interlaced nature of pipework,  we could shut off ½ the zones if we needed to.
The result was surprisingly good with all room zones open and control on one master thermostat. The whole house has very even temperatures.  This demonstrates the self-regulating nature of very low temperature underfloor heating.
Another concern I have heard of is the pumping power required  for such a lot of pipe.  In the above passive house example.  If the heat pump needs 10 lit/min to give a 5 degree flow-return dt , then the flow rate is about 1.11 litres/min for each pipe loop.  If we used a more standard 5 loops, the flow rate per loop would be 2 litres/ min.  For the same heat (kW) and same dt, more pipe actually means less pumping power since the heat output per m of pipe is lower, and the flow-rate for each loop  is lower.
Finally, I have also come across the notion that mixer valves and pumps are desirable, even for a heat pump.  Here is an example to illustrate the potential penalty of having a mixer even if the mixer never actually mixes (i.e. its fully open).
The arrangement here with recorded temperatures shows that the flow to the floor is always equal in temperature to the return to the heat pump. In this example, the flow from the heat pump is 7 degrees higher  than the flow to the floor.  The mean floor temperature  was only 23°C (average of 28 & 18).
If the mixing valve and pump were removed, the heat pump’s flow could go directly to the bottom manifold.  
To get the same floor heat output, the heat pump setting could be adjusted down so as to give a working flow of 26 and return of 20.  This is a reduction in flow temperature of 9 degrees, potentially (according to our first graph at top of page ) saving 13%.  
(see top graph – The COP at 35°C is 3.1, at 26C its 3.1, saving 13% in COP). 
All these little details can eat into potential savings. They should be dealt with at design stage.

4 thoughts on “Getting the best from underfloor heating”

  1. This is a great post. Getting buffer vessels to work properly seems key, especially for commercial applications. It seems from your graphs that there is mixing going on which is effectively reducing the buffer temp. Hence the heat pump needs to provide a higher flow temperature to get what you need to the UFH. I wonder if this can be helped by having large pipe entries into the buffer (lower velocities=less mixing), or even a section buffer to prevent natural convection currents? I believe there are cylinder designs out there which serve this purpose (eg in solar hot water applications). I will look into this. Does your system have weather compensation? My experience has been that control engineers have varying knowledge of this and often have slopes which are too shallow ie the flow temp doesn't drop quickly enough with increase external temp.

  2. Yes, the example of a buffer that I monitored was a poor one, and as you say, could be better with intermal distributor/baffles etc.

    Yes, weather compensation usually good, although normal curves not ideal for very high thermal mass building, and also debatable for air-source. I guess it will take a few more years before control systems are developed to a better level.

  3. Have you looked at specifying a superplasticiser (water reducer) in the screed or slab?

    I used a polycarboxylate super plasticiser for my screed, and if I remember correctly it had the effect of increasing the thermal conductivity of the dried screed by something like 40% (in addition to reducing cracking).

  4. Interesting post especially the graph on the pipe spacing. I would always use 150mm.

    Also I would like to add that I find a lot of people are still sceptical of heat pumps for their home as a result of the initial cost of installation.

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