I am calculating the required specification of the heating components in a single-story extension @ 31m² in the South-East of England (in-land).
I've used several online "BTU" radiator calculators, but the results vary wildly from one site to the next and with no formulas published, I do not know how they are calculating the value. The construction choices lead me to believe that none of them factor in the latest June 2022 Part 'L' building regulations.
So, I am attempting to work it out for myself. I've followed this guide and after inputting the energy details of each layer of each surface, I've arrived at a heat loss value for the extension of:
1908 + 20% = 2290 Watts
or
6510 + 20% = 7812 BTU/hr
Considering online calculator results average out at 4371W or 14915BTU, I fear I have made a mistake.
Could it be that the online calculator's R-Value data is really that out-of-date or am I missing something?
Thank you in advance for any advice.
Key inputs:
- External design temperature: -1°C
- Internal design temperature: 21°C
- Air change: 2 times / hr
- 3 external walls
- Glazed French door (4.3m²)
- Windows, including roof lantern (4mx2m) (15.1m²)
- 150mm of 0.032W/mK (or better) insulation in all faces
- Warm deck flat roof
