Andy Hall wrote:
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.


This is one of those aha! moments - light has dawned and, as usual, the
answer's pretty obvious once you look at it from the right direction.

Many Thanks to Andy and Roger (below)

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