BTU Calculator

16 Nov 2004
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United Kingdom
Hi chaps,

I am attempting to spec the rads for my house and have been told the existing rads are too powerful for the rooms.

I have tried a few BTU calculators off the web, but they all seem to have slightly different input parameters ie some ask for window sizes others room aspect or floor type or outside wall size etc etc.

Because of this they then give a wide range of answers - anything between 4850 BTU to 6890 BTU for one particular room. :confused:

Have any of you guys in the know found a reliable calc to use to size rads ?


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Mears is over generous if you put all the factoe in - but hardly anyone does. It's plastic thing you wouldn't want to buy anyway.
Dont forget toheat the air - need to know air changes/hr.
There was one on the Myson site. Try Google.

As a very average start, try 50 watts per cubic metre.
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I would rather have rads that have too many BTU's than be short of a few. I have a new rad in my living room, its just not big enough, & Im now stuck with it.
I thought most plumbers/heating engineers used

Bedrooms BTU = width/ft x length/ft x height/ft x 4
Living rooms/Bath BTU = width/ft x length/ft x height/ft x 5
go to find heat loss calculator.its easy to use.and free....i always add on 5% and fit quality trvs.and an auto by-pass valve

btus are a little bit old hat nowdays.but us dinosaurs love them,the more the merryier. good luck[/url]
Just do what the customer wants, after all they are allways right, so fit enormous radiators in every room to give them the mental feelgood factor, it's all old hat this working out heat losses, everyone knows you just go down to Bill&Que or Prickes and pick a rad which "looks about right" you can get a bargain heatpack with a cracking Halstead Cold condersending boiler whoopee bleedin dooooooo
Best to oversize, if it looks good. You can always turn it down but not up. Also if there are kids about a big radiator runs cooler. :idea:
I can confirm that M.H.Mear's will be launching a new domestic heating calculator in February 2006. The new Calculator will replace the existing model 15 (Metric Domestic Central Heating Calculator).

The design of the new calculator is different and the maths is based on "First Principles". The accuracy of the new calculator is 100% accurate and exact when calcultating heat requirements. We will be advising that heating engineers etc add 10% to the calculated sums as to give customers a bit extra heat in very harsh conditions.

The New Calculator will be called, "1st Principle Heating calculator" and will be available in metric only.
well I`m just going to start a "corgi" argument :LOL: Using my Mears model 6A :rolleyes: and my M8 who used to teach maths, Using heat loss tables.....we worked out the calcs for a house, and the wasn`t much in it ;) Over to the Pendants
Mears said:
The accuracy of the new calculator is 100%
No calculation of something like heat losses from a room can be 100% accurate. The whole process is based on approximations, particularly in relation to air changes and u values of materials, and so it is foolish to talk about 100% accuracy.
Exactly so. Different aspect, wind chill factor, slight dampness due to rain or building defect, non 'standard' construction, so the U values aren't right. It all makes a difference, then we can start on what makes you feel comfortable - how warm were you to start with, what are you wearing, and so on - for ever!! Not really a science, more of a best guess!!

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