Discuss UFH with high Output and high dT in the Water Underfloor Heating Advice area at Plumbers Forums


Two loop UFH system heating 24M2 with lots of glass.
Loop lenghts 90M each X 16MM OD piping

UFH output 9.52kw.
flow/return 55C/34C, dT 21C
fowrates, 3.0LPM & 3.5LPM = 6.5LPM total

Boiler Vaillant Ecotec plus 438.
flow/return 65C/59C due to high recirc rate required to get the boiler away at ignition conditions with UFH only on.
The UfH output was "validated" by taking gas meter readings which gave a boiler consumption of 11.44kwh which gives a boiler efficiency of 83.2% so reckon reading reasonably accurate.

Query: Is the UFH output of 9.52kw from a two loop system and a dT of 21C plausible? and if so does it indicate that the heat is being dissipated in a badly installed under floor system.

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You would get inconsistent floor temp eg hot spots and cold spots if you run it at a dt 20 ideally should be dt5-7 so floor heat around 2.2kw-3.1kw but as you need to increase the flow this will alter try doubling the flow and see what you get
If the flowtemp was reduced to a more "normal" say 40C what dT would you expect with the present flowrates, 3.0LPM&3.5LPM and loop lengths (90M) and OD (16mm).
The average pipe spacing is 24 m^2 / 90 m = 0.27 m so I'm guessing it's a 200 mm system with 18 m^2 of heated area. IIRC, the maximum output for the heated area should be around 100 W/m^2 (16 mm pipe, concrete screed floor, 200mm spacing). Downwards heat loss should be no more than about 10 W/m^2.

So, if I've understood/guessed correctly, with Tf–Tr = 21°C you are supplying an average of 395 W/m^2 to the room and 533 W/m^2 to the heated area. So, my guess is that the pipework layout was to be designed to operate at Tf – Tr = 4°C – 5°C and emit a total of 1.8 - 2.4 kW, i.e. not far from @Ben-gee's heat loss estimate.

What does your heat loss calculation for the room come up with?

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