Can you fit carpets with underfloor heating?

Through OpenEcoHomes I recently visited two homes with underfloor heating and was surprised to find that one had a rug covering most of the floor and another had wall to wall carpet. I used to think that carpets were very bad with underfloor heating because they tend to block the heat coming up from the floor. However, many carpet manufacturers cite research commissioned by the Carpet Foundation that shows that carpets and underlay with a combined TOG value of up to 2.5 'allow underfloor heating systems to work efficiently'. For example Jacaranda quotes the research here. Is this really true? As usual, the answer is 'it depends'. In this case, it depends on what is supplying the heat. If you have a gas boiler, fine. If you have a heat pump then carpets are a bad idea.



A carpet acts like a quilt on your floor.
TOG values are also used to rate quilts and blankets for beds. The higher the TOG value the cosier the quilt. With a good quilt I can be warm even when the bedroom is cold because it reduces the heat flow. Carpets do the same thing for floors with underfloor heating. The thicker the carpet the less heat gets through into the room. This chart shows how the heat flow decreases as you increase the carpet TOG value. It is based on data from Jupiter assuming that the average pipe temperature is 50 °C.

Based on data from Jupiter using screed replacement tile [1]. Higher TOG values reduce heat flow and surface temperature.

If you need more heat, you need to run the pipes hotter.
How much heat you need depends on how well your home is insulated and the weather. Your underfloor heating installer should work this out for you, based on worst case weather. If you need more heat you can do this up to a point by increasing the temperature of the pipes. The thicker your carpet, the hotter the pipes have to be to get the same amount of heat through. For example, suppose you need 90 W/m2 heat flow to keep your room warm. With no extra covering, you could run your pipes at 38 °C and this would be fine. With ceramic tiles you could run them at 40 °C. If you put on a carpet and underlay with combined TOG value 2.5, your pipes need to be more than 60 °C to get this flow of heat through [2].

You can supply high temperature heat with a gas boiler but not a heat pump
And 60 °C is absolutely OK if you are heating with a gas boiler. You can feed the pipes with water at 65 °C with a  return temperature of 55 °C and the boiler will function perfectly (though if the return is hotter than 55 °C a condensing boiler will be less efficient as it does not condense the steam properly). On the other hand heat pumps are considerably less efficient at such high temperatures. The reason that underfloor heating is recommended with heat pumps is because it works at low temperature - but that is only true if you don't swaddle it with carpet!

If you have a heat pump - avoid swaddling your heating with carpets and rugs
So, yes carpets are fine with underfloor heating provided you can supply high temperature heat to your pipes. However, if you want to run a heat pump efficiently you need to keep the pipe temperature down and carpets are a bad idea.

With underfloor heating the tiles are nice and warm
So why don't we like bare tiles? They are hard, it is true, and probably not suitable for a kitchen if you drop as many things as I do. However I think the main reason we think we don't like tiles is because they are cold underfoot, and this is irrelevant with underfloor heating because then they are nice and warm.

[1] Chart derived from: 'Output table for Jupiter Underfloor heating System IDEAL' with mean water temperature 50 °C and pipes at 25 cm centres. This table uses R values (W/m2/K). These are multiplied by 10 to get TOG values. Jupiter only gives data up to R=0.15 (TOG=1.5). I have extrapolated up to TOG 2.5 by fitting a curve.

[2] These values are derived from the same data from Jupiter by interpolation and extrapolation.