Converting CH to flow-return (using plastic pipe)?

[Jon]

Hi,

Looking to update the CH system in my Dad's 1960's bungalow and hopefully make it more efficient. It's three bedroom, with two of the bedrooms and the kitchen radiators having been updated to flow-return pipework a few years back. The rest of the bungalows Ch pipework was installed as a single pipe loop.

I was thinking of re-plumbing the radiators which were on the the single loop to flow-return, using as much plastic piping as possible to avoid soldering and save on cost due to copper prices.

The easiest way to re-do the pipework I thought, is to take it down the wall from the loft to each radiator, then box it in down the walls. I've just seen these on screwfix and thought they'd save me a lot of messing about.

(link to screwfix)

I could connect one of these each to the 22mm pipe from the flow and return of the boiler. Then feed 10mm plastic pipe from the manifold to the radiators which need re-plumbing, and back again for the return flow.

The questions I have are:

1) Is this a valid way to redo the plumbing?

2) Will this be ok seeing as the rads which are already flow/return are plumbed in with 15mm copper?

3) Are the manifolds any good/reliable?

4) Can I take 10mm plastic pipe straight to the radiator valves, or should I change to copper for the run against the skirting board once I've run it down the wall?

5) Is there a better way of doing this?

Thanks,

Jon

[Andrew Gabriel]

Why?

[Andy Hall]

In principle, yes.

It's not possible to say without doing some calculations and looking up of data. Assuming that the radiators are adequately heating the room then from the sizes and looking at manufacturer data sheets you can identify their approximate output in watts.

From this can be calculated the volume of water that needs to flow through per second to supply this heat. There is then a design principle that the velocity of the water through the pipes should not exceed 1.5 metres/sec. Above this speed, there will be more noise and the flow begins to become screwed up anyway.

You can then refer to tables for the different sizes of tube and the lengths involved to determine the tube diameter required for a given run and volume of water/sec. to do the job.

Approximately speaking, 15mm copper tube can supply the need for 6kW over a few metres. 10mm will do about 2.5kW. 8mm is only 1.5kW etc. The internal diameter of plastic tube may be a bit smaller than copper, I am not certain on that.

Download leaflet P150 from this site and it explains how the calculations are done.

should be able to get equivalent figures for plastic from suppliers such as John Guest. I've found them pretty helpful.

Note that you don't have to do the calculation exactly to the size, the point is to make sure that the chosen pipe size is about or more than enough and is not something like 50% of the size needed. If you undersize, then either the radiator won't warm sufficiently because the flow rate is too low or you will have to reduce flow through all the other radiators and increase pump speed, making the system noisy. If it's way undersize you will get both problems.

If you have just one radiator that is too much more 10mm tube, you could run that one in 15mm and the rest in 10mm.

I've found Speedfit stuff to be completely reliable, although have only used 15 and 22mm sizes. However, the 10mm uses the same principle so no reason to suggest it would not be good.

You can get 10mm radiator valves but may have to search for them a bit. It should be possible to use those directly with plastic, but check with the valve manufacturer.

There are reducers of different types for copper to allow 15mm valves to be used, but I don't think they will work with plastic

Assuming that the pipe sizing is adequate, I would use 10mm copper instead. It is very easy to work and you can use a hand bender to make the bends. Screwfix 17898 for example. You can get brass manifolds as well.

[robgraham]

Thanks for that link on copper, Andy - useful bookmark for later study !

Rob

[Onetap]

Yes, but...

The plastic expands a lot more than copper when heated; look up the coefficients of thermal expansion (mm expansion per m length per degC temperature rise). The result of this is that, although it may be installed straight & level, the plastic becomes wavey when hot. Generally, don't use it anywhere it will be visible. It's virtually impossible to get it straight if you buy the coiled stuff anyway.

It's very useful under floors, much easier to install, but then is liable to be gnawed by rodents.