In an earlier contribution to this discussion,
Stephen wrote:

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?

Yes, or even less. Very few systems with conventional boilers run at a dT of
60 - 55 is more usual.


To convert the toolstation outputs to dT=50, do I just divide by 60
and multiply by 50?

Not exactly - it's not quite linear. The factor given by Stelrad is 0.798 -
whereas 50/60 gives 0.833
--
Cheers,
Roger
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In an earlier contribution to this discussion,
Roger Mills wrote:

In an earlier contribution to this discussion,
Stephen wrote:

Am I right to think
that for condensing boilers you want to aim for a dT=50C?

Yes, or even less. Very few systems with conventional boilers run at
a dT of 60 - 55 is more usual.

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.
--
Cheers,
Roger
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Roger Mills wrote:
Not exactly - it's not quite linear. The factor given by Stelrad is 0.798 -
whereas 50/60 gives 0.833

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.

http://www.stelrad.com/uk/docs/lst_2.pdf

last page (they are *very* big pages!) it's the table near the top at
the left of the rightmost quarter of the page (IYSWIM) and note that the
temperatures are differences to room temperature.

Now I've just found a link suggesting that the return temp. for a
condensing boiler needs to be below 55.

http://www.condensingboiler.org.uk/

so if you have flow at 65, and return at 55, average rad. temp is 60 and
your delta-T is only FORTY...

Andy

In an earlier contribution to this discussion,
Andy Champ wrote:

Roger Mills wrote:
Not exactly - it's not quite linear. The factor given by Stelrad is
0.798 - whereas 50/60 gives 0.833

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.

http://www.stelrad.com/uk/docs/lst_2.pdf

last page (they are *very* big pages!) it's the table near the top at
the left of the rightmost quarter of the page (IYSWIM) and note that
the temperatures are differences to room temperature.


Yes indeed! The 50 and 60 referred to above are Delta-T values - not
radiator temperatures.
--
Cheers,
Roger
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Roger Mills wrote:
In an earlier contribution to this discussion,
Stephen wrote:

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?

Yes, or even less. Very few systems with conventional boilers run at a dT of
60 - 55 is more usual.

To convert the toolstation outputs to dT=50, do I just divide by 60
and multiply by 50?

Not exactly - it's not quite linear. The factor given by Stelrad is 0.798 -
whereas 50/60 gives 0.833

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.

Roger Mills wrote:
In an earlier contribution to this discussion,
Andy Champ wrote:

Roger Mills wrote:
Not exactly - it's not quite linear. The factor given by Stelrad is
0.798 - whereas 50/60 gives 0.833
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.

http://www.stelrad.com/uk/docs/lst_2.pdf

last page (they are *very* big pages!) it's the table near the top at
the left of the rightmost quarter of the page (IYSWIM) and note that
the temperatures are differences to room temperature.


Yes indeed! The 50 and 60 referred to above are Delta-T values - not
radiator temperatures.

Ah Muy bad..

In an earlier contribution to this discussion,
The Natural Philosopher wrote:


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..

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]
--
Cheers,
Roger
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On Fri, 5 Sep 2008 22:53:45 +0100, "Roger Mills"
wrote:

A radiator's output at dT=40 is only 60% of the output at
dT=60.

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!

In an earlier contribution to this discussion,
Stephen wrote:

On Fri, 5 Sep 2008 22:53:45 +0100, "Roger Mills"
wrote:

A radiator's output at dT=40 is only 60% of the output at
dT=60.

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]
--
Cheers,
Roger
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On Sat, 6 Sep 2008 23:12:00 +0100, "Roger Mills"
wrote:

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.

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?

However, many manufacturers are now quoting the output at dT=50. The output
at dT=40 is about 76% of that at dT=50.

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?

Hopefully you won't *actually* need an output of 4.7kW, unless you've got a
very large room

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.

On Fri, 5 Sep 2008 22:06:42 +0100, "Roger Mills"
wrote:

Yes, or even less. Very few systems with conventional boilers run at a dT of
60 - 55 is more usual.

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.

In an earlier contribution to this discussion,
Stephen wrote:

On Fri, 5 Sep 2008 22:06:42 +0100, "Roger Mills"
wrote:

Yes, or even less. Very few systems with conventional boilers run at
a dT of 60 - 55 is more usual.

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?

Yes, that's right.

I have tried the Myson program and it gives slighly lower heat losses.
Any idea which program is nearest the mark?

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.

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?

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.

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?

Yes. Are the cavities insulated? If so, there should be an option for that,
too.

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?

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!
--
Cheers,
Roger
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