Hello everybody,

I recently found u-wert.net and it took a little while to get used to it, but now I get some results out of the heat-calculation.

On other (not so detailed) sites on the internet, I read the rule-of-thumb to multiply the cubic meters of the room with 85 (for living-room) to get the needed power to heat the room.

Why do the answers from u-net differ that much from the rule-of-thumb answer?

Hi
Such rules of thumb can lead to extremely false results due to wide variations of the u-value of walls,floor, roof etc.
The mentioned rule of thumb might apply to buildings with a comparable building/insulation standard.
A variation of the temperature also leads to different results (here 20 degrees usually)

Depending on the purpose of your calculations better do it correctly and precisely using the u-value software- it might take some time to get used to it though.

Andreas Teich
Building energy advisor

Hello Andreas,

Thanks for your answer.

I agree with you that such rules of thumb are based on assumptions (applied isoliation, weather conditions) which are not necessarily true.

Nevertheless I would expect the results be reasonably close to each other. When I do the calculations for our living area (5m x 9m x 2.5m), I find:
- rule of thumb (85 W/m3) 9562 W
- installed heating equipment (high temp radiators) 4656 W
- u-wert calculations 2180 W

I will re-check to see if I entered the correct data.
Thanks for your response,
Albert

I just realized that you probably mixed up the units:
A rule of thumb for the heating energy demand says 85 W per m2 (not m3).

This only applies for older buildings which are not insulated.
These assumptions were a gross simplification used ages ago for a very rough calculation of radiators etc.

Nowadays, according to building regulations, you MUST calculate precisely, according to the relevant building standard etc, which nearly always leads to far lesser figures.
Better use a software like U-Wert.net or others.
You can phone me in case of difficulties (WhatsApp 00436506021975)

Andreas Teich
Building energy advisor, building consultant

The 85 W/m3 is indeed questionable. I found this on three sites (all Dutch):

- https://www.mijnkluswijzer.nl/radiator-capaciteit-berekenen/
- https://www.cvtotaal.nl/bereken-radiator/
- http://www.radiatorstunter.nl/advies.php

The first and last state W/m3 (the last one uses 75 W/m3). The second one is addresses the unit; "Wattage per vierkante meter (m3)", translated "Watt per square meter (m3)". Note that the 'square' does not correspond to the 'm3'. There could be a mixup of units. If the rule-of-thumb would be 85 W/m2, it would make for my living room 3.8 kW, which would bring the three results closer together.

I am glad that you confirm that a precise calculation leads to "far lesser figures". It gives me partial confirmation that the data I entered to u-net is not completely wrong.

My thanks for your support and to this site. In my opinion it is very doable for a technically skilled person to get results. My difficulty at this moment is my lack of knowledge of building materials and their German equivalent.

Thanks for your offer to further assist me. I'll be back on this forum with a question or two...

Kind regards,
Albert