Screfix radiator BTU figures

Are the BTU figures quoted in the Screwfix brochure accurate? For a given size of radiator they seem a fair bit higher than other sources.
For example, for a bedroom of 2.5w 4l 3.3h, 2x1 dbl glazed window, solid wall, 2.5 outside wall, radcalc gives a figure of 5258 BTU's.
Screfix quote 5480 BTU's for thier 600x600mm dbl radiator. Several other radiator sources indicate I would need a 700x700 dlb panel dbl convector radiator.
The radiator currently in the room is 760x800, bit it is pretty old.
Thanks, Ian
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On 31 Jul 2003 15:18:47 -0700, snipped-for-privacy@yahoo.co.uk (I.Wilson) wrote:

We've had this thread before, Ian, not that long ago.
They have published the gross figures for a temperature difference of 70 degrees - i.e. mean water to air temperature of 90 degrees with 100 degrees in and 80 out. This is obviously not a real world case.
If you look at brochures of radiator manufacturers they publish figures adjusted for a MWTA of 60 degrees so of course the outputs appear lower for a given size. Even this is high so they have tables of correction factors. For example if the heating is conventional with 82 degree flow and 70 degree return, the mean water temperature is 76 degrees and MWTA for a 20 degree room is 56 degrees, so really the radiator should be derated to 0.89 of the table values.
I don't know whether Screwfix are being deliberately misleading - probably not - just incompetence - but straight away it puts them at 22% apparently higher output figures than, say, Myson, and 37% more than reality.
Have you checked the heat losses using U values e.g. from the Building Regulations Part L1 Approved Document. ? It's simply a matter of taking the area of each element in sqm., multiplying by the U value and the temperature difference gives the loss or gain in Watts. From experience, the calculator programs are not always accurate so I always do a manual sanity check.
Did you take losses due to heating the air into account?
.andy
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(I.Wilson)

Just looking at the Myson web page. Thier 600x600 dbl panel dbl convector with grill quotes a BTU of only 2365. This is for "Std. output @ Dt50 degrees C 908W\m". What is this 50 degrees figure? Is it the MWTA?
Why does a radiator with a single convector have a better output than one with a dbl convector? I'd have thought it would be the other way around. The Myson dbl panel single convector 600x600 rad has an output of 3349 BTU's.
http://www.myson.co.uk /

Ok, you've now lost me ;-) I didnt quite understand the above two bits. Whats U value? Whats heat loss into the air got to do with it? Surely this is what radiators are meant to do?
What about taking into account the construction of the building when calculating the BTU requirement? I live in an old Edinburgh victorian tenement.
Ian
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Ok, I've downloaded the Myson program. I assumed an internal temp of 20C since it will be a bedroom/study, and an air change of 2 per hour (there is an old blocked-up fireplace with a vent/grill).
The internal walls are 150mm brick plastered on both sides with approx 1 inch of plaster (full of old horse hair!). For this I guestimated U-value of 1.8.
The external wall is 600mm thick for approx 1.2 x 3.4M, and 400mm thick for 1.5 x 1M (the rest is aliminium DG window). For this I assumed a worst case and just used the solid 18" stone option which has a U-value of 2.5.
Its a first floor flat so there are flats above and below. The floors and ceilings are timber joists with 1 x 6.25 inch floorboards above and plaster-and-lath ceilings below. There is an air gap above the ceiling. The area between the joists is filled with a mix of old cinders and plaster, approx 150 mm thick. This provides excellent sound and heat insulation so I used the lowest U-value from the list - 1.36.
This gave a heat loss of 1314W and a required emitter output of 1698W (5807 BTU). (Radcalc gave a BTU figure of 5258) The smallest radiator to suit is a Premier HE 26DC29 690h x 743w. This is a dbl panel dbl convector, and provides an output of 1822W (6216 BTU). Downside is that it costs 110 ;-)
Ian
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On 2 Aug 2003 03:18:23 -0700, snipped-for-privacy@yahoo.co.uk (I.Wilson) wrote:

These seem reasonable, Ian.
The other thing that you can do now you have the figures tabulated is to look and see which surfaces are the major heat losers. Then you can pay more attention or be more conservative about the U values for those and less concerned with the lower losing surfaces.

You probably need to derate this. The figures in the table assume an MWTA of 60 degrees which is a bit high for a conventional UK system.
Assuming a flow of 82 and return of 70, which is normal, the mean water temp. is 76 so the MWTA to your room is 56 (call it 55). If you look at the Myson data sheet there is a small table and if MWTA is 55 you need to derate to 0.89 of the 60 degree values in the main table. Thus your output is really going to be 1622W, which is a bit low. Did you assume -3 for outside? Also if the position is exposed, then upping the requirement a bit (say 10%) is often done.
I would go for the next width up or possibly even the one above that. The other alternative might be to use two smaller ones?
Another point is that 600mm radiators tend to be much more available than 700mm since they are more widely used AIUI. They also tend to be less expensive per watt since presumably they are made in higher volumes.
If you want to buy on-line www.discountedheating.co.uk has Myson for about 20% less at least than your 110 figure.
Another option is Plumb Center's own brand, Centerrad. I bought some of these for my workshop and they are fine. They are actually made by Myson but in slightly different sizes. The prices are lower anyway, they have some branches in Edinburgh and you may be able to get a bit of discount if you ask, especially if you are buying some other stuff with it.

.andy
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