Heat loss/m2?

billy_beckham

Hi all,
Just wondering if anyone can give me a rough estimate of heat loss per square metre in a newly built house (ie up too and slightly better than current insulation regs). I have been talking to several different heat pump suppliers and the figure varies from 45w/m to 60w/m. It makes a big difference to size of pump and obviously price!

Thanks in advance.:)

patrido

they might sound miles apart, but do the calculations your self and you'll see how much they can vary depending on what your assumptions are.

you need to pick
- a design temperature difference (the coldest night that you're going to design for).
- u-values for each outward facing surface (and be conservative here, as very few people will achieve the manufacturers claimed u values, due to construction methods, etc).
- how many air changes you think there will be per hour.


- pick the lowest temperature that you're going to design for (people usually pick between zero and minus 3).
- Add 20 (average room temperature) to it. This is your design temperature difference. Say 23.

for each surface in the house (wall, ceiling, floor, windows, doors)
- calculate the area in sq metres
- multiply by the u-value,
- multiply by the design temperature difference.
this is the fabric heat loss. The easiest way to handle walls is to take the gross area and then subtract the area taken up by windows and doors.

to calculate the ventilation heat loss
- multiply the room volume
- by the number of air changes per hour (probably between 0.5 and 2)
- by the design temperature difference
- by 0.33 (specific heat capacity of air at 200 degrees).

add all the numbers up.
add about 10 % for other losses, and maybe another 10 % for contingency. hopefully you'll fall in between 45 and 60, but don't be surprised if you're on the low side, as many suppliers use very conservative assumptions on u values.

there are quick rules of thumb for heat loss, that plumbers use, and simple calculators on the web, but they'll be no more accurate than your two suppliers

billy_beckham

Much appreciated patrido!