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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Could it be that the online calculator's R-Value data is really that out-of-date or am I missing something?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
woter324
  • 176
  • 3
  • When I see heating elements, I think toasters, which are one of the most expensive heaters to use. For that size of a room I would check into what we North Americans call mini splits(small heat pumps). They are cheaper to use than room toasters and also cool the air in summer. – crip659 Jul 20 '23 at 22:20
  • Yes, very strong reco for heat pumps especially in mild Britain. You're getting 3-6 watts of heat for every watt you pay for. The better ones work far colder than Britain will ever see. Widely seen in Ukraine attached to their tower block apartments. Really if this is new to you,check it out. – Harper - Reinstate Monica Jul 20 '23 at 22:34

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