On Wed, 29 Dec 2004 21:54:11 -0000, "Mike" wrote:
I have been assuming the main reason for the closing down is most radiators
are fed from quite short runs from the pump/zone valves under the stairs
(shortest is 2m, longest about 5m, all in 15mm pipe) but these two
radiators are further away at the end of about 20m (10m each way) of 15mm
(plastic) pipe. For this reason they have their own pipe all the way back
to the zone valve.
OK. I think I'd contact the pipe manufacturer or see if it's on the
web site and see if you can get the equivalent figures to the CDA ones
for copper. I believe that plastic pipe has slightly narrower
internal bore and may also have different resistance behaviour. As
a first approximation, you could treat it as copper and plug the
numbers into the CDA formulae and see what you get.
It's good that they have their own home runs though.
You could look up the radiator manufacturer data for the radiators (or
ones close to them). There is not an enormous difference between
vendors for typical finned radiators. THe main effect is the number
of panels and which have fins. If you find something close in size,
you can scale it. This should give you a gross heat output assuming
the EN testing method. You then need to look at the derating table
for the mean water to air temperature correction factor. For 82
degree flow and 70 return, (MWTA about 55 degrees) the normal derating
is to multiply by 0.9. So if you determine by size that a radiator
has a nominal output of 1kW, its actual output under what should be
the design conditions for your system will be 900W. You can then
take that figure and plug it into the CDA method and just check that
the pipework to those two radiators is adequate for them. A rough
rule of thumb is that 15mm pipe should be good to carry 6kW over
distances of a few metres (but not 40 :-) ).
I think my worst case is the pair of radiators (about 3kW total) at around
20 m there and back of 15mm.
My gut feel is that that ought to be OK, but I'd run the numbers to
check.
Beyond that, you could look through the design guide published by the
Copper Development Association and do the calculations for the pipe
runs.
http://www.cda.org.uk/Megab2/build/Pub150%20UKCB.pdf
If you plug in the numbers for the main pipe runs, that should give
some clue if there is something inadequate.
Thanks - will do. Sounds to be what I need.
It's slightly laborious but you don't have to be hugely accurate.
In effect, the system is adrift by a factor of 2:1, so it should be
possible to deduce what is going wrong. You can work back to
determine the pump requirement given the resistance and from that see
if it is within the range of the pump.
From a first pass I need a head of over 8m and the pump obviously isn't up
to that.
Can I put another pump in series on the return to the boiler to see if the
problem improves ? (i.e. do pump heads add ?)
I don't believe they do, unfortunately. If you put in a more
powerful one then the first is going to end up making that worse I
think.
Also I appear to be right on the velocity limit of the section of 22mm pipe
from the boiler. The article you quoted mentions long term erosion of the
pipe but does anything silly happen to the resistance effects of the pipe
for a quick test.
I was afraid it might be. Unfortunately the plastic is a bit worse
than the copper as well. THe design point is to stay between 0.5 and
1.5m/sec, although sludging shouldn't be an issue if you are taking
care of the system with inhibitor. Beyond 1.5m/sec the noise level
increases but that may not be an issue if it's outside or under the
floor. However, the head required to increase the flow starts to
increase dramatically.
I'm thinking the proper solution is a separate pump at the boiler for
upstairs and downstairs systems with separate pipework as well,
unfortunately still with the 22mm length unless somebody knows where I can
get flexible 28mm pipe in 20 m lengths, but I'd like to get as much
understood with the current system before changing it.
It may be that the 28mm is adequate, especially if copper for this
amount of heat. Another option would be to run in two pairs of
22mm from the end of the 28mm and split those upstairs to downstairs.
it would be reasonably easy to calculate for that, but you could still
be towards the pump limit.
A second pump and pipe run would be the other option, and you could be
pretty confident of each working and adjustability for each as well.
I have never seen barrier pipe or even non-barrier in 28mm coils. You
could contact Hepworth or John Guest and see if they can do it to
order.
--
..andy
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