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Calculating a radiators BTU?

(16 Posts)
Greencheese Sun 02-Nov-14 20:49:47

Hi, I was wondering if anyone has any advice? <hoping piglet john is about>

I'm trying to calculate what radiators to buy. We have a through lounge dining room with 1 radiator in each room. Do I calculate the BTU like they are just 2 individual rooms? I've just the B and Q calculator and it says I need them to be 5249 for my rooms of 4m x 4.5m each.

Am I doing this right?


Greencheese Mon 03-Nov-14 21:59:42

Little bump, anyone? Thank you.

Anticyclone Mon 03-Nov-14 22:15:18

Are you planning to replace the current rads?

If so, it makes sense to treat each room separately. If they are both the same size, same windows etc, that makes it even easier as you only need to do one calculation.

So you've done the calcs on a 4 x 4.5m room and it recommends a 5249 BTU radiator? There is you answer.

I would say that you should buy a radiator that is at least 6000 BTU for each room, if not more, because you can always turn a radiator down (assuming you will have adjustable thermostatic valves on them?) but you can never make them more powerful than their max BTU. Having it more powerful than you need gives wriggle room!

PigletJohn Tue 04-Nov-14 08:00:19


How big are the radiators you currently have, and do they heat the rooms enough on a frosty night?

In very approximate terms, a single rad 600mm high and one metre long has a nominal output of 1kW max, but because of the way it is measured, its output will really be less. A long low radiator keeps the room more comfortable than a short high one.

1kW is 3412 BTU.
BTU are frightfully 1950's.

I'd have a 2kW rad in each end, with TRVs.

Greencheese Tue 04-Nov-14 19:19:31

Hi, we've just bought the house and are totally redecorating the downstairs. There are 2 big radiators in but they are single ones and are really grubby so I wanted to get rid. We've not actually had the heating on really yet so it's hard to tell.

I've seen one in B&Q it's a column one 300mm by 1500mm. It's 4575 BTU but I guess that's not powerful enough?

Whats TRV? I'm really rubbish at this!

burnishedsilver Tue 04-Nov-14 19:31:20

If you take your measurements to a plumbing supply place they'll work out the BTU for you and quote you for the most suitable rads.

I would be reluctant to buy them from B&Q tbh.

A TRV is a thermostatic valve.

Greencheese Tue 04-Nov-14 19:45:52

Ahhhh right thanks. I'll do that then. I've looked at screw fix but they seemed pricier than B&Q. I really like the column radiators bit I think this is where it's causing me problems.

PigletJohn Tue 04-Nov-14 19:50:46

300mm is a foot. They are tiny things. They would look OK in a Wendy house.

Greencheese Tue 04-Nov-14 21:49:39

Ha, I did think that, I wanted a 400 one really but can't find one. It's for under the window. I may have to rethink this. Where is is now would be behind the sofa, it's double glazed so I thought it should be ok under the window. Who'd of thought buying radiators would be so complicated! grin

PigletJohn Tue 04-Nov-14 21:56:42

under windows is not the best place.* It will be in the way of curtains. A long low one behind the sofa will not take up much room if it is flat - or are you struck on columns?

You could put one at each side of the window perhaps.

*people who learned plumbing in 1960 think it is. They have very warm gardens.

Greencheese Tue 04-Nov-14 23:04:26

Ha, I did try to do a bit of google research and read that under windows was ok these days with good double glazing etc, or is that not true?

I did think of on on the side funnerly enough as it's a square bay window.

My thought was have a column radiator because if it's going to be seen it may as well look nice ha.

PigletJohn Tue 04-Nov-14 23:45:44

Temperature gradient means that heat loss is much greater through window. Even a double glazed window is far far worse than a wall.

Also curtains won't look right and may funnel heat to the window and then to the garden.

Greencheese Wed 05-Nov-14 18:25:22

Maybe I am better getting a tall one next to the bay window then, but then again that probably won't be much different than being under it. If we leave it where it is it will be behind a sofa. Grrrrr Bloomin radiators.

PigletJohn Wed 05-Nov-14 18:30:34

you can have radiators behind sofas. Preferably long low flat narrow ones that do not project much.

What width have you got to each side of the window?

If under the window, you will need curtains down to windowsill height, not floor length, and preferably a shelf or windowboard to direct the rising hot air out into the room and not behind the curtain.

Greencheese Wed 05-Nov-14 19:11:39

In the bay there is probably 60cm each side. So the sofas won't stop the heat going into the room? Ok, that's worth considering then. Thanks.

Deux Wed 05-Nov-14 23:30:01

Here is a BTU/KW calculator. I used this when trying to work out our requirements. We are really toasty now.

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