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?
On 16 Jan 2004 10:32:20 -0800, simon_london email@example.com
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
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
If you want to try to use 15mm throughout, you should calculate the
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.
On 16 Jan 2004 10:32:20 -0800, simon_london firstname.lastname@example.org (simon_london)
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.
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.
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