Calculating Underfloor Heating Output for 200mm pipe spacing

Hardware Heatpump
1

I thought I would give heatloss.js a go and try putting in all the data for my house.
I quickly discovered that it only supports 150mm pipe spacing for underfloor heating, so thought I would have a look to see what is required to add support for other pipe spacing distances.

(that fact that my house has 200mm pipe spacing is a bit of a sore point - I didn’t realise the importance of the pipe spacing before I started leaning about heat pumps)

The result is this little Python project:

I don’t know if it is of use to anyone else, but I am hoping to use the findings to improve heatloss.js.

3 Likes
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Hi Nicholas, a good idea. I checked it against the CIBSE guide “ The underfloor heating design & installation guide” and information is a good fit. For example the CIBSE guide indicates flow temp of 37.5C with a room temp of 20C and a heat output of 67W/m2 for R=0.05 with a pitch of 100mm (16mm pipe). The MCS derived curve you shared indicates an “eyeball” fit of ~63 W/m2 for a Tf-Tr=17.5C.

The most interesting fact from your curves was the observation that the flow temperature must be ~5C higher than the room temperature to have “effective” heat flow in to the room. The CIBSE guide simply has a MWT cut off at 30C for room temperature ranges from 15C to 24C. Always nice to see the tabulated data as curves.

Thanks for the info.

Frank

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Thanks for doing the cross reference.

If you put it through the algorithm, you get:

>>> watts_m2 = ufh_heat_output(
...     pipe_spacing=100,
...     r_value=0.05,
...     flow_temp=37.5,
...     room_temp=20
... )

>>> print(watts_m2)
62.05763537499999

I have no idea how significant 67W/m² vs 62W/m² is.
Given that there are errors absolutely everywhere, presumably that is close enough!

4

Hi Nicholas, I made a mistake reading the CIBSE table. I will try and correct it later. CIBSE states a heat output of 70.3 W/m2 ( and not the 67 W/m2 I stated previously) for 100mm pitch with MWT of 35C (deltaT=5C). So the difference would now be ~8W/m2. I think it is probably close enough in typical mixed radiator and underfloor heating designs with all the other variables involved. However, it shows a larger difference with low temperature UFH designs running with a MWT of 30C, which seems equivalent of jumping a pitch size or more.
For example, the difference between a 150mm and 200mm pitch is 4.9 W/m2 for a MWT of 30C, 11.6 W/m2 for a MWT of 35C or 19.4 W/m2 for a MWT of 45C.

I have probably confused you with my reply, but I hope it helps.

Frank