Maximum Wet UFH Output

8 Jan 2014
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United Kingdom
Is there a way to reliably calculate the maximum output possible out of a wet underfloor heating system? Can it take into account a maximum floor temp of 27C?

It would be on a gas boiler, so flow temp can be set as required.

This is for an extension, the net floor area available for UFH piping is likely to be 17m2 after deducting lost areas, kitchen cabinets etc.

Everywhere I read mostly references 100w/m2 max solid floor but for tiles. We are likely to use vinyl tiles, ala Karndean, which I understand needs a floor temp sensor and limit of 27C.

The floor will be new construction, slab, 100mm insulation board with UFH on top, 65mm conventional sand and cement screed.

There is a possibility of extending the UFH coverage into an existing solid floor but it depends on screed depths and slab depth as the what is possible / feasible....

There is a lot of glass in the extension, like 14m2 including the roof light.

Thanks very much for any pointers.
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100w/m is about as much as you'll get out of UFH, but as it's going into a new extension which needs to comply with the current insulation standards laid out in Building Regulations that should be fine. Have you consulted Building Control about your plans? With that much glass, they'll want to see calculations demonstrating that you can achieve the minimum required U-values
Using a private inspector. He has plans hasn't raised anything yet. All the building elements are designed to meet building regs u values.

But the amount of glass pushes up heat loss and I wonder whether 100w/m2 will be enough. It is short of rough heat loss calcs I made..

Edit: it is made a lot worse if I can't do anything for UFH in the existing solid floor area to be opened up into extension area. That is maybe 12m2 and part original 1930s and part 1980 built. Would definitely need a rad or plynth heater to supplement if nothing can go in there. But even putting to one side just the new extension part looks under.

Can UFH cope with being undersized better than radiators or is 1w of rad output, same as 1w Ufh as science would seem to indicate....

The 14m2 glazing does fit within the 25% floor area rule when adding in existing apertures built over. Just about anyway.

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Looking at my rough heat loss calcs there are 4 big drivers of the overrun.

1) Old area opened up that presently has old solid floor and no heating (exclude that and treat in isolation for now, as it may need different heat solution)
2) Glazing area even at 1.3 - 1.5 manufacturer overall U values, are a lot more lossy than walls.
3) Ventilation losses at 0.76 air changes is almost equal to the overall glazing losses

4) U values per building regs for extensions vs practical in use U values in guidance like Energy Saving Trust Domestic Sizing documement. See below comparisson:

E.g. Building Regs U value / Energy saving Trust U value guidance for domestic sizing for most current build
Walls 0.28 / 0.35
Roof 0.18 / 0.35 (2x)
Floor (with area/perimiter adjust) 0.22 / 0.67 (3x)
Windows 2.0 / 1.6
Doors 2.0 / 1.8

So excluding discussion of the first 3 points, there is also a lot of variation in heat loss depending whether the theoretical U values from building regs are used or practical reckoners like Energy Saving Trust U values are used. Walls and floors being 3 times the U value for example.

I have used the more pessimistic U values in the domestic sizing document which included things like perimeter adjustments for heat loss of the floor, but they end up being a lot lot higher. And Like 3 times so which is the better guidance

What is realistic for U values building reg requirements or practical U value guidance like in used in the EST domestic sizing doc?


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