"Andy Hall" wrote in message
...
On Wed, 29 Dec 2004 20:58:19 -0000, "Mike" wrote:
Basically the flow rate is too low, as is the return temperature. You
have about 25 degrees temperature drop on a design that should be
about 12 degrees.
You don't say what pipe size your main 40m runs are,
Partially 28mm, but about 15m of flexible 22mm plastic was needed to poke
under a floor we couldn't get under properly. Did wonder at the time if
I
might need to put in parallel runs of this pipe so there is a rope left
to
allow more to be dragged through. However couldn't see a way of
parallelling these up without causing more resistance. Need a sort of Y
joint whereas all I could find were Ts or swept Ts.
You'd have to compare the resistance of adding the fittings vs. having
the extra pipe. Also, for the plastic section, you may need to ask
the manufacturer because characteristics may well be different to
copper.
One relatively simple thing that you could try is to go and check all
the radiator lockshield valves and open them. If they are closed
down too low then this will restrict flow. Of course it may be that
some have to be quite a bit for balancing, but at least if you could
improve the flow it would help.
Two are flat out, rest are balanced to maintain all radiators at roughly
the
same temperature drop.
If the others are closed almost down in an effort to get a reasonable
temperature drop across these two, it sould indicate inadequate
pipework for the size/distance of them.
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.
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.
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 ?)
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'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.
Many thanks
Mike