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

Thermostats, Volumisers, Buffer Tanks, TRVs, Pumps, Piping, Valves, Radiators & Controls

Last Post by JamesPa 11 months ago

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Bash

(@bash)

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Joined: 12 months ago

Posts: 228

Topic starter 12/11/2025 6:36 pm [#2590]

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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Joined: 4 years ago

Posts: 5476

12/11/2025 6:51 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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Bash

(@bash)

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Topic starter 12/11/2025 8:45 pm

Posted by: @jamespa

↑

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.

Yes, an elbow is what I should have said.

The 22mm copper pipe from the boiler curves slightly into the ceiling and then the push fit elbow connects to the horizontal run of plastic pipe under the 1st floor floorboards.

The copper pipe is about 1.5m into the ceiling which is directly below the first radiator, so I'd expect the first tee from the plastic pipe to be no more than 2m.

The HP will be fitted on the same wall as the boiler and the run of 28mm primaries will be around 2m, so about 4m or so to the ceiling where the 22mm copper and plastic pipes are joined.

If I need to run at dt6, would that only be required in the colder months?

This post was modified 11 months ago by Bash

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JamesPa

(@jamespa)

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Joined: 4 years ago

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12/11/2025 9:27 pm

Posted by: @bash

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If I need to run at dt6, would that only be required in the colder months?

In principle yes. Some heat pumps (I think including Daikin) appear to modulate the water pump to achieve a quasi fixed DT. So if you set it at 6 for the cold months it may stay at 6 even in the warmer months. There are probably some installer settings to control the water pump behaviour which you may well be able to tweak if it comes to that.

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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