In an earlier contribution to this discussion,
John Phillips wrote:

On 2007-05-15, DukeD wrote:
I now want to give the Ground floor flat its own combi boiler, and
isolate that circuit from the rest of the house. In view of the fact
that the entire system has worked fine for ten years, is there any
point in adding any 22mm pipe to the ground floor circuit when I add
the new boiler? Can I simply use 15mm for the feed and return from
the new boiler? The furthest radiator will be just under twelve
meters' pipe-length away from the new boiler.

I'm not an expert but what I think matters is that you have an
acceptable flow rate in the pipes. For any particular pipe diameter
the upper limit (there is a lower limit too) is determined by pipe
erosion (and flow noise).

For 11 C drop across the radiators the CDA (Copper Development
Association) seems to say in table 2 of:

http://www.cda.org.uk/megab2/build/p...ng-systems.pdf

that a 15 mm pipe will deliver up to 9.2 kW total output to radiators
(that's 0.2 kg/s of water flow - beware of some fairly obvious
incorrect tabulation in table 2).

I suspect that may possibly be a little high from the noise POV. In
my recent upgrade to a small CH system I used 15 mm pipes where the
total power carried was 6.5 kW or less, and 22 mm elsewhere. It's
adequately quiet at that level.


The rule of thumb often quoted here is a max of 6kW for 15mm pipe.
--
Cheers,
Roger
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