Mixture of copper and plastic CH pipework. Will it be ok, or need changing?

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Last Post by JamesPa 1 hour ago

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Bash

(@bash)

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Joined: 1 month ago

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Topic starter 12/11/2025 5:36 pm

We are getting closer to our heat pump install and I have been reading (and reading!) in the hope I can catch any potential pitfalls before the installation begins.

One slightly concerning area is the existing central heating pipework.

We had an extension fitted to our 1950's semi 20 yrs ago. The boiler was fitted in our new utility room connecting to the old radiator circuit somewhere in the middle of the house where the old back boiler was housed.

I can see that our primary 22mm copper CH pipework tees off into what looks like 22mm plastic pipe using a right angled push fit connector in the ceiling above the boiler (there is a small void left by the gas fitter in the new boiler cupboard). It also appears we only have one central heating circuit for the whole house, certainly from the boiler and I'm assuming connected to the old CH circuit.

From the plentiful reading I have done, this copper to plastic connection and pipe run will have a limiting impact on the flow rate.

I'm hoping it will probably be ok, although not perfect and that I can still get decent efficiency from the HP and possibly still run at DT5.

We are due to be installed a Daikin EDLA08EV3. Heat loss calculation (which we feel is pretty accurate) was 6.2KW. Radiators are all 15mm copper connections and are all being replaced. Design 42.5c @ -2

What are your thoughts?

Thanks.

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JamesPa

(@jamespa)

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Posts: 3363

12/11/2025 5:51 pm

According to the Heat Geek cheat pipe sizing sheet (which seems to have disappeared form the heat geek site but is reproduced here 22mm copper is good for 6kW @DT5. That means that 22mm plastic, and its a single pipe (you describe it as teeing off via a right angle connector which I dont exactly understand), then is a bit marginal, but the HC calcs are based on 0.9m/s and many would say you can go a bit faster. Of course the run before it splits to the first rads is fairly short, its also less likely to be a problem, and if you have to run at DT6 the performance penalty is only 1.5% or thereabouts.

If it were my house I would very definitely 'suck it and see'. Very likely it will work at DT5, if not a tweak will almost certainly get it to work. It might be worth replacing that plastic elbow (if that is what it is) with a swept bend or copper elbow though whilst its drained down.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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