I've tried a few but they seem to give wildly differing numbers (and one
gave the same answer regardless of the inputs!), can anyone suggest one
that works?

--
Dave S
(The return email address is a dummy)

On Thu, 30 Dec 2004 22:01:09 +0000, Dave
wrote:

I've tried a few but they seem to give wildly differing numbers (and one
gave the same answer regardless of the inputs!), can anyone suggest one
that works?



The Myson and Barlo ones are OK.

The Myson one is not on their site at present, but I have a copy and
will happily send it to you if you drop me a mail.

One of the problems with some of these programs is that they have
incorrect U values for given object types - so it's always as well to
check the numbers against the list in the Building Regulations
Approved Documents.

The other factors that can introduce error are that some programs do
not account for floor heat loss correctly. The reasonably correct
approach is based on the combination of outside wall lengths and floor
area and there are correction tables of modified U values to cope with
it. Computer programs should do all of this as well, but if not,
there will be an error. Some programs have correction factors for
whether the heating is on all day, exposure of the house and so on.
The defaults may not be sensible.

Finally, you do need to make sure that heat loss by air exchange is
reasonably accounted for. This is a big chunk of heat requirement in
most properties.



--

..andy

To email, substitute .nospam with .gl

Andy Hall wrote:
On Thu, 30 Dec 2004 22:01:09 +0000, Dave
wrote:


I've tried a few but they seem to give wildly differing numbers (and one
gave the same answer regardless of the inputs!), can anyone suggest one
that works?




The Myson and Barlo ones are OK.

The Myson one is not on their site at present, but I have a copy and
will happily send it to you if you drop me a mail.

One of the problems with some of these programs is that they have
incorrect U values for given object types - so it's always as well to
check the numbers against the list in the Building Regulations
Approved Documents.

The other factors that can introduce error are that some programs do
not account for floor heat loss correctly. The reasonably correct
approach is based on the combination of outside wall lengths and floor
area and there are correction tables of modified U values to cope with
it. Computer programs should do all of this as well, but if not,
there will be an error. Some programs have correction factors for
whether the heating is on all day, exposure of the house and so on.
The defaults may not be sensible.

Finally, you do need to make sure that heat loss by air exchange is
reasonably accounted for. This is a big chunk of heat requirement in
most properties.



Thanks Andy, email sent.

Do you happen to know whether the U values in the myson software are
correct?
Estimating air exchange is tough, any suggestions on how to approach it?

--
Dave S
(The return email address is a dummy)

On Thu, 30 Dec 2004 23:20:20 +0000, Dave
wrote:



Thanks Andy, email sent.

On its way.


Do you happen to know whether the U values in the myson software are
correct?

The ones that I used were, but I didn't check all of them and had to
add a couple.

Estimating air exchange is tough, any suggestions on how to approach it?

I used the standard numbers given in the program, which are an
industry standard. If the house is exposed or the windows are older
sash types or you have gaps in the floorboards then it would be an
idea to up the numbers a bit.

You can also look at the various correction factors and adjust to your
pattern of use and location.

You don't need to include the typical 3kW or so for the hot water.
This is used for older cylinders with basic coil.

Are you going for a condensing boiler? If so, you can design the
radiator provision around a 70 degree flow and 50 return. This will
keep the boiler at the efficient end of the scale and also give you
plenty of head room for very cold weather. It does imply larger
radiators of course. Don't forget the correction factors from the
manufacturer data tables. These are normally 0.89 for 82/70 degree
operation and 0.6 for 70/50.




--

..andy

To email, substitute .nospam with .gl

Andy Hall wrote:
On Thu, 30 Dec 2004 23:20:20 +0000, Dave
wrote:

.... snipped

Are you going for a condensing boiler? If so, you can design the
radiator provision around a 70 degree flow and 50 return. This will
keep the boiler at the efficient end of the scale and also give you
plenty of head room for very cold weather. It does imply larger
radiators of course. Don't forget the correction factors from the
manufacturer data tables. These are normally 0.89 for 82/70 degree
operation and 0.6 for 70/50.


I can size the rads for a 70 flow but can't see how to achieve a 50
return. If the house is heating from cold (and all TRVs are open) the
return will presumably be much cooler than when the house is up to
temperature and all/most TRVs shut; is it really possible to design for
a specific return temperature?

--
Dave S
(The return email address is a dummy)

On Tue, 04 Jan 2005 08:55:27 +0000, Dave
wrote:

Andy Hall wrote:
On Thu, 30 Dec 2004 23:20:20 +0000, Dave
wrote:

... snipped

Are you going for a condensing boiler? If so, you can design the
radiator provision around a 70 degree flow and 50 return. This will
keep the boiler at the efficient end of the scale and also give you
plenty of head room for very cold weather. It does imply larger
radiators of course. Don't forget the correction factors from the
manufacturer data tables. These are normally 0.89 for 82/70 degree
operation and 0.6 for 70/50.


I can size the rads for a 70 flow but can't see how to achieve a 50
return. If the house is heating from cold (and all TRVs are open) the
return will presumably be much cooler than when the house is up to
temperature and all/most TRVs shut; is it really possible to design for
a specific return temperature?

OK.

The radiator calculation is to first of all determine heat loss using
-3 degrees as the outside temperature (considered worst case)
inlcuding air change losses etc. and correction factors for exposed
position, intermittent use (needs more during start up) etc.

You will have figures in watts for each room.

If you work on 70 degrees flow and 50 return, then the mean water
temperature is going to be 60 degrees. Let's say you want 20
degrees in the room. The Mean Water To Air temperature will be 40
degrees. Most radiator manufacturers give a correction factor of 0.6
for that. You could be a purist and use slightly different numbers
for temperatures in different rooms but it probably isn't worth it.

From this, you pick your radiators. So for example, if you needed
1200W in a room, then you would need a 2kW nominal radiator.

A condensing boiler, by virtue mainly of its heat exchanger design,
can manage a dT of 20, sometimes as much as 25 degrees.

If everything were precisely matched (i.e. radiators and boiler
exactly matching heat loss), then the boiler would reach a steady
state of 70/50 degrees and would be operating in the condensing range
as well.

In practice, as you say, you won't have these levels of heat loss
because the system will have a design margin and weather will
typically be warmer.

Under those circumstances, when the system is heating from cold, it
will go flat out to begin with. Some boilers will allow you to set
the maximum flow temperature and you could set that to 70 degrees and
keep the return under 50. This is good from the efficiency point of
view (lower return temperatures, especially below the condensing dew
point are more efficient) or you could set the control at 82 degrees
and the system will heat more quickly.

With a modulating boiler, the burn rate will reduce accordingly.

If the heat demand reduces (e.g. because TRVs start to close, then the
boiler detects the load reduction by monitoring of the temperatures
and again reduces burn rate.

The point of designing around 70/50 is that you can operate the boiler
in a more efficient range. Efficiencies of condensing boilers tend
to be better than many non condensing types anyway since the heat
exchanger is usually larger. The efficiency increases as return
temperature reduces, but in the condensing range the *rate* of
improvement of efficiency increases with falling temperature.





--

..andy

To email, substitute .nospam with .gl

The message
from Dave contains these words:

I can size the rads for a 70 flow but can't see how to achieve a 50
return. If the house is heating from cold (and all TRVs are open) the
return will presumably be much cooler than when the house is up to
temperature and all/most TRVs shut; is it really possible to design for
a specific return temperature?

You are looking at this from the wrong perspective. The 70 flow, 50
return is an element you use in calculating the required radiator sizes.
In operation you won't initially get the 70 flow as the boiler has to
heat the return from cold but you should get (ignoring any losses in the
pipework) a 20 degree rise across the boiler.

If the rise is too low decrease the pump speed, too large increase the
pump speed. Unless you have a continuously variable pump (do such
animals exist?) you are not going to get this exact and as the TRVs
start closing off the circuit resistance will increase mimicking the
effect of decreasing the pump speed so I think you should aim low* if
you have a condensing boiler and want to benefit by recovering any of
the latent heat out of the steam in the flue.

Not having a condensing boiler myself I haven't had to deal with the
practicalities of this but ISTR from long ago that to get any condensing
effect the return must be below 53C and that it is not an on/off effect
but develops over a temperature range.

*This may not necessarily be correct but keeping the return temperature
low is critical in getting the best out of a condensing boiler. No doubt
someone more knowledgeable will be along in a minute to pull my
contribution apart but don't take any notice of dIMM unless he is backed
up by a good majority.

--
Roger

"Dave" wrote in message
...
Do you happen to know whether the U values in the myson software are
correct?
Estimating air exchange is tough, any suggestions on how to approach it?

I use Myson V1.05, not sure if that is the latest version or not. Noticed
recently that there is a design program available at
http://www.centralheating.co.uk for £15.

Find the Myson useful for the maths because it takes into account your mean
water temperature and the room temperature but then transfer the results to
a spreadsheet. There is a "funny" with this version. The required outputs
are presented on a Dt60 basis but with a confusing product reference comment
to Dt50 and so have to be factored down to Dt50 before they are useable with
current rad data.

Jim A

Jim Alexander wrote:
.... snipped
Find the Myson useful for the maths because it takes into account your mean
water temperature and the room temperature but then transfer the results to
a spreadsheet. There is a "funny" with this version. The required outputs
are presented on a Dt60 basis but with a confusing product reference comment
to Dt50 and so have to be factored down to Dt50 before they are useable with
current rad data.

Jim A


Thanks for the warning, where does it say that it's on a Dt60 basis?

--
Dave S
(The return email address is a dummy)

On Tue, 04 Jan 2005 09:19:53 +0000, Dave
wrote:

Jim Alexander wrote:
... snipped
Find the Myson useful for the maths because it takes into account your mean
water temperature and the room temperature but then transfer the results to
a spreadsheet. There is a "funny" with this version. The required outputs
are presented on a Dt60 basis but with a confusing product reference comment
to Dt50 and so have to be factored down to Dt50 before they are useable with
current rad data.

Jim A


Thanks for the warning, where does it say that it's on a Dt60 basis?

That's part of the EN 442 test standard. Normally it is mentioned in
the radiator data sheet as well. You can deduce it anyway, because
there is an accompanying table of correction factors for different dT
values. For 60 degrees it is 1.0





--

..andy

To email, substitute .nospam with .gl

"Dave" wrote in message
...
Jim Alexander wrote:
... snipped
Find the Myson useful for the maths because it takes into account your
mean water temperature and the room temperature but then transfer the
results to a spreadsheet. There is a "funny" with this version. The
required outputs are presented on a Dt60 basis but with a confusing
product reference comment to Dt50 and so have to be factored down to Dt50
before they are useable with current rad data.

Jim A


Thanks for the warning, where does it say that it's on a Dt60 basis?

It doesn't, you need to check the data sheets to find out.

Jim A