BTU calculator

SMcGill

I’m trying to estimate what BTU radiator I need for a bedroom but the free online calculators are giving me different answers ranging from 1700 to 4100! Can anyone recommend a good calculator please? Or someone with the knowledge tell me what BTU they think I need?

The bedroom is over a heated room and directly under a thatch roof (around 1m thick) with two outside walls. It’s 12ft x 13.5ft and 9.5ft high ceiling, there’s one window about 3ft x 4ft which is single glazed with secondary double glazing.

Astria

Desired internal temperature? Based on what outside temperature?

I would say 500W of heating would be sufficient giving the parameters you have given.

Less if very well insulated, more if poorly insulated.

FreeBear

Astria said:

Desired internal temperature? Based on what outside temperature?

I would say 500W of heating would be sufficient giving the parameters you have given.

Less if very well insulated, more if poorly insulated.

https://starsapp.co.uk/basic-heat-loss-calculator/ suggests something in the 7-7500BTU range

https://www.bestheating.com/btu-calculator is saying 4-4500BTU

(I assumed a 220mm thick solid wall with both).

I'd go larger rather than smaller (and certainly not 500W/1700BTU). If the radiator is oversized, the room will heat up quicker before the TRV kicks in. It also future-proofs you for the day heat pumps are the only option.

A quick test - Get an electric fan heater (say 1KW), run it for an hour and see what temperature you get up to.

SMcGill

A quick test - Get an electric fan heater (say 1KW), run it for an hour and see what temperature you get up to.

That sounds a practical approach, thanks. I just don’t understand why two calculators with the same info put in come out with such different answers.

So if it takes say 1.5 hours to get to a comfortable temperature then I need a 1500W radiator? The cottage I live in is timber frame and rendered with lath & plaster internal walls, the thatch ought to help insulate the upstairs bedrooms.

FreeBear

SMcGill said:

A quick test - Get an electric fan heater (say 1KW), run it for an hour and see what temperature you get up to.

That sounds a practical approach, thanks. I just don’t understand why two calculators with the same info put in come out with such different answers.

So if it takes say 1.5 hours to get to a comfortable temperature then I need a 1500W radiator? The cottage I live in is timber frame and rendered with lath & plaster internal walls, the thatch ought to help insulate the upstairs bedrooms.

Timber frame - Any idea what the infill is between the timbers ?

With an old property like this, I'd suggest going bigger as the insulation is likely to be poor to non-existent  - I'm thinking a 7-7500BTU radiator may be more suitable.

To convert KW to BTU, multiply by 3412

markin

Now would be a good time to move the pipes and do 50mm internal insulation then?

SMcGill

Timber frame - Any idea what the infill is between the timbers ?

Brick in some areas, wattle & daub in others. Lime render and some cement render which I know isn’t great but it’s in good order so for now it will stay.

Now would be a good time to move the pipes and do 50mm internal insulation then?

Thanks, I have paid for some pre-app advice from my local council as the cottage is listed so when I meet the conservation officer I’ll ask about that. My instinct is they’ll say no, but who knows.

FreeBear

SMcGill said:

Timber frame - Any idea what the infill is between the timbers ?

Brick in some areas, wattle & daub in others. Lime render and some cement render which I know isn’t great but it’s in good order so for now it will stay.

Now would be a good time to move the pipes and do 50mm internal insulation then?

Thanks, I have paid for some pre-app advice from my local council as the cottage is listed so when I meet the conservation officer I’ll ask about that. My instinct is they’ll say no, but who knows.

I had a feeling the building might be listed - That does restrict what you can do to some extent. You could try asking if woodfibre or cork insulation on the internal walls would be acceptable. With a coat of lime plaster over the top, it might be acceptable to the Conservation Officer.