Hello,
I see Screwfix list radiators with outputs calculated at dT=50C, whereas Toolstation list their rads with outputs calculated at dT=60C; this makes comparing them difficult. Am I right to think that for condensing boilers you want to aim for a dT=50C?
To convert the toolstation outputs to dT=50, do I just divide by 60 and multiply by 50?
Thanks, Stephen.
Yes, or even less. Very few systems with conventional boilers run at a dT of
60 - 55 is more usual.
Not exactly - it's not quite linear. The factor given by Stelrad is 0.798 - whereas 50/60 gives 0.833
To amplify a bit more . . . I've just found a note I made a while back, suggesting a flow temp of 70 degC and a return of 50 degC for a condensing boiler. This would give a mean temp of 60, and a dT of 40 - assuming a room temperature of 20. A radiator's output at dT=40 is only 60% of the output at dT=60.
I'm surprised it's anything like linear. I would have expected it to be related to the difference between the rad temperature and the room temperature - which would mean the conversion is more like (50-20)/(60-20) or 0.75.
aHA! Found the table! But it's in a PDF.
Now I've just found a link suggesting that the return temp. for a condensing boiler needs to be below 55.
Andy
No, prolly more like convert to degrees absolute and take the difference, then use some non-linear factor with a power of 33 1/3 or summat. Best get over to the diy wiki and look at central heating design / radiators or suchlike where you should find a formula to calculate outputs from sizes regardless of what the manufacturers say.
Yes indeed! The 50 and 60 referred to above are Delta-T values - not radiator temperatures.
strictly you should take your target room temp - say 20C - subtract it from both rad temps, and then use that..so 60-20=40, and 50-20=30, so output will be 35/40 or .75..
one guesses that stelrad are rating for an output room temp somewhat lower - say 17C or so.
Ah Muy bad..
No! A dT of 60 imples a mean radiator temperature (never achieved!) of around 80, and 50 implies around 70 (more easily achieved).
[I think your subsequent post indicates that you have seen the error of your ways]
Thanks, so I actually need a larger radiator that it first appears from their catalogues. For example if I needed a 4.7kW rad, I ought to be looking for a 4.7/0.6=7.8kW rad according to their stats. How confusing!
If the output is quoted at dT=60 and you're going to run it at a dT of 40, then yes - that's right.
However, many manufacturers are now quoting the output at dT=50. The output at dT=40 is about 76% of that at dT=50. So, if you're looking at a catalogue based on dT=50, you'll need 4.7/0.76 = 6.2kW for the example which you quote.
Hopefully you won't *actually* need an output of 4.7kW, unless you've got a very large room with very poor insulation and draft proofing! [I've got a large L-shaped lounge with quite a lot of glass, and my calculated heat loss for that is less than 3kW]
I was reading a brochure by Grant boilers which says: "Size radiators with a mean water temperature of 60° C". So if the room temperature is
20C that's a dT=60-20=40C. Isn't it?
Thanks. I will save that for future reference. Where did you get these factors? Are they all listed on the stelrad web site you mentioned earlier?
I got my units mixed up. I know you'll tell me off for using BTUs but the catalogues use them. I used an online calculator (I can't remember which) and it recommended 3800 BTU. I downloaded the Barlo program and that said 4700BTU, however I am not sure I put the right numbers into the right boxes and besides, I didn't know what all my walls were made of. I think I saw 4700 in my spreadsheet and thought it was Watts but it was BTUs.
I need to check to see what dT the Barlow program was using, as it might require an even larger rad if I have a smaller dT. It's for a
3mx3mx2.4m room but with a 2mx2m patio door.Thanks, Stephen.
Sorry to start this up again but I have been using the Barlo heat loss program and see it has an option to set water temperature. Does it matter what this is set to? I presume that it makes no difference to the room heat loss calculation but is only used if you ask it to select appropriate radiators from the Barlo range?
I have tried the Myson program and it gives slighly lower heat losses. Any idea which program is nearest the mark?
I notice neither program asks whether the room is north facing. Isn't this significant?
I'm not quite sure what materials to list my walls as. Am I right to think that the downstairs ceiling/upstairs floor should be selected as floorboard/joist/plaster where I have a joist with a floorboard on one side and plasterboard the other?
I know three walls of the house are cavity with a brick outside and breeze block inside so I guess that's the brick/breeze cavity option?
The problem is that the front of the house is tile clad on the outside, with who knows what behind it, I guess a single breeze block? What would this be described as?
Thanks, Stephen.
Yes, that's right.
Not really, but if you want to be safe, take the higher heatloss figure. Have a close look at what U values each program uses for various types of construction. Differences in U value will give differences in heat loss. Also look at air changes per hour; that makes a difference, too.
That sounds about right. Unless there is a large temperature difference between upstairs and downstairs (like heating one but not the other at certain times) the heat loss/gain through the ceiling and upstairs floor won't be much anyway - particularly if there are carpets upstairs.
Yes. Are the cavities insulated? If so, there should be an option for that, too.
The programs may not have an exact match for this - but selecting a brick/block or block/block cavity wall probably won't be far out. I think at least one of the programs allows you specify your own wall type - complete with U value - in which case you can insert an appropriate value. I have a feeling that there is information about typical U values on one of the government sites - where building regs etc. can be found - but don't ask me exactly where - the appropriate government department (the one which used to be Prescott's) seems to re-invent itself too often for me to be to be able to keep up with it!
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