15mm or 22mm copper pipe for central heating?

I want to buy a new Glow Worm 24cxi condensing combi. To keep costs down, I want to get all the pipework ready before a CORGI plumber commissions the boiler. The manufacturer states that the central heating pipe and return pipe should be 22mm. All the pipework on my existing central heating system is 15mm. I read on several internet articles that to ease the load on the pump, the central heating loop should be fitted with 22mm pipes, and each radiator should be connected to the main loop with 15mm pipes. I will have 9 radiators in total (about 42,000 BTU's output).
Does anyone know the pro's and cons of fitting 22mm pipes for the main heating loop rather than keeping the existing 15mm pipes?
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On 16 Jan 2004 10:32:20 -0800, simon_london snipped-for-privacy@yahoo.co.uk (simon_london) wrote:

Are you happy with its DHW output of 9.8 litres/minute? This is right at the bottom of the scale and you might find the results disappointing. You might want to consider one of the slightly larger models if you want to get a decent shower out of it or fill a bath at any reasonable rate. From a heating perspective it doesn't matter because these all modulate downwards.

This is just over 12kW.
You might just get away with this with a condensing boiler if the runs from the boiler before the radiators start to branch are fairly short.
With a conventional boiler, the flow temperature is generally at about 82 degrees and the return at 71. The amount of heat transferred through pipes is proportional to the temperature drop and flow rate. Generally on a conventional boiler up to about 30kW or so, if the initial boiler pipe runs are short, 22mm is used and then 15mm or even less thereafter.
However, if the radiators are generously sized, a condensing boiler can run at lower temperatures and with a drop of 20 degrees. Thus the heat transferred can be greater for a given flow rate. The limiting factor with pipes is that you want to keep the flow velocity less than a certain amount, otherwise the system becomes very noisy.
If you want to try to use 15mm throughout, you should calculate the flow velocities.
There is a paper on how to do this. If you can measure your pipe lengths and take radiator outputs from manufacturer data, then you can work out whether you can get away with this.
http://www.cda.org.uk/megab2/build/Pub150%20UKCB.pdf

.andy
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On 16 Jan 2004 10:32:20 -0800, simon_london snipped-for-privacy@yahoo.co.uk (simon_london) wrote:

Another thing you might want to look at is whether the syetem is plumbed in such a way that you can split it into 2 sections and just run 22 to the point where they seperate. This might be a good compramise between increasing flow and hassle of re-plumbing.
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in such a way that you can

Mine was already like that. It had 15mm upstairs and 15mm downstairs, joining at a big T into the 22mm run to the boiler. As this occured near the understairs cupboard, I took the liberty of inserting a couple of zone valves at the splitting point and converting the Y-Plan to S-Plan Plus.
Christian.
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