replacing old radiators with new and btu calcs


7 Jun 2011
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United Kingdom

Getting the correct btu calculation drives me mad. For the current job mentioned below I've used b&q. hlab, all with different results, so which of these calculators would you advise.

I've slowly replaced the radiators in the house but never 100% sure whether I've got the correct size. In some forums they have recommended the B&Q calc but I'd like to double check.

The Job

I'm replacing an old double rad (H750mmxW900mmx55mm) in the kitchen (4.4m x3mx2.4m, north facing, 2 outside walls, wood floor,double glazed,no french doors). The boiler is in the kitchen as well.


I'm replacing an old double rad (H450mmxW2100mmx65mm) in the dining room (4.4mx3.7mx2.4m,south facing, 2 outside walls, wood floor, double glazed, no french doors).

New replacements:-

I'm interested in the stelrad compact style vertical P1 to replace the kitchen radiator (H 1600mm x 500mm (3642 btu)) this is due to change in wall space and Compact style horizontal K2 (H300 x 2000 (5930 btu))
in the dining room.

many thanks
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Why bother with inaccurate calculations when you can do it more accurately, quicker and easier?

How often has the room been too cold in the past two years and nothing you could do about would make it nice and warm?

A. Never. -> replace with same size
B. Once or twice -> one size up
C Occasionally -> two sizes up


If you don't have a condensing boiler yet, you should increase the size of every rad 10% or so in preparation for the steamer that will one day arrive.
I'm quite interested in any response to this as i'm replacing rads in my house too.
Problem is i've never lived there as we've just got it and are updating it.
Rads are old 70's 80's type and i'm replacing them with Stelrad compacts.

Problem is they dont make the same sizes now. I can get near but then think newer rads will be more efficient so do i need to?

I'm trying to use BTU calcs but get varied results. e.g from 4000 from one calc to 6500 from another. (3.75m * 3.75m * 2.4m high, 1 outside wall, concrete floor, frenchdoors, double glazed and cavity wall ins). This makes a big difference in needing a K2 or P+ rad and the size.

Also the previous owners just had a Viessmann air source heat system installed which drives the rads in combination with existing pumps. I'm not sure how that will affect the BTU needed. The viessmann is meant to run constantly afaik.
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Why oh why do you folks want to use outdated BTU units when everything is now rated in kW ?

BTUs are now only used by thick plumbers who are too set in their ways to use the currently used kW.

Radiators are always 100% efficient!

Modern types use fins which increase output about 25% over non finned types.

Anyone without a condensing boiler should increase the rad outputs by about 25% to compensate for the reduced flow temperature from current boilers which are designed for 70 C flow rather than 82 C from old boilers.

As Ben says the best sizing is by subjective assessment of the heat output although an engineer would measure achieved temperatures.

Thanks Agile. I'm using BTU as that's what the radiators seem to be referenced in and as a DIYer i'm not up to date with current trade methods :(
I'm more than happy to use KW if there's anything you can point me at reading to explain it.

I can't test the current temps as am not yet living at the house so dont know if a room gets warm or not.

I expected the newer fin rads would be better than the ones i have in so thats good to know cheers.

Indeed i expect the AWHP system runs cooler than a condensing boiler as its running at a constant.

I didnt want to overspec the rads a) due to size of them and b) due to wasted energy, but if b) isn't much of an issue (especially if using TRV's) then perhaps i'll go that way.
Why oh why do you folks want to use outdated BTU units when everything is now rated in kW ?
People like to wait til the dust has settle before taking decisions about something new.
This modern kW stuff has only been around for about 30 years. Be patient, give them some time; I'm sure it will all change after WWIII.
Heat pumps usually give a heat output of around 50 C and work best with underfloor heating.

If used with rads then they have to be carefully sized to ensure they give adequate output with just 50 C flow temperature but you can adjust the flow rate for 50/40 C differentials.

That can mean quite big rads and perhaps professional assistance might be better to avoid expensive mistakes with larger and more expensive rads.

All rads are quoted in kW but they still put the BTU outputs on the data because they know that there are still many older plumbers who left school at 16 with only three GCSEs and dont really understand simple physics and have not kept updated. ( Even though gas calculations are all in kW which they have to do every five years if Gas Registered! )

This modern kW stuff has only been around for about 30 years. Be patient, give them some time; I'm sure it will all change after WWIII.

The 1964 Wilson government's policy that British Industry would adopt metric units "within ten years" was announced in Parliament on 24th May 1965 by the President of the Board of Trade at that time, Rt Hon Douglas Jay MP through a Parliamentary Written Answer to a question by John Horner, Labour MP for Oldbury and Halesowen.

Its actually over 40 years Ben!
You know me: always erring on the side of caution and very careful with my words.
30 years, 40 years, it's still less than a lifetime; we don't want to do anything impulsive, do we?
Act in haste, repent at leisure old chap.

Look at the bright side: when Armageddon strikes the planet, we will have at least another decade before Britain gives in.
It's actually over 40 years Ben!
A bit older than that!

The Watt was recognized by the Second Congress of the British Association for the Advancement of Science in 1882. The 11th General Conference on Weights and Measures in 1960 adopted it for the measurement of power into the International System of Units (SI). (Wikipedia)

So it's a British unit not a newfangled continental idea.

The only country which still sticks stubbornly to BTUs is the USA, and they also measure flow rates in gallons. But at least that is logical.

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