Plastering over 8mm microbore central heating pipes

[DaveyH]

Hello all,

What’s the best way of plastering over 8mm copper microbore central heating (radiator) pipes to avoid cracking due to heat expansion?

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• DaveyH
• 15 Jan 2010
• 1

I have read lots and lots of post on this forum and there appears to be a few of choices.

Some people say just plaster over them.
Some people say wrap the pipes in duct tape and plaster over them.
Some people say wrap the pipes in denso tape and plaster over them.
Some people say run the pipes through flexible conduit and plaster over them.

I quite like the idea of running the pipes through flexible conduit as I can see that option definitely working. However the pipes are only 8mm and the narrowest flexible conduit I can find in the usual DIY shed (ie Wickes, B&Q, Screwfix and Toolstation) is 20mm in diameter which is way to too big for 8mm pipe and if used would result in there only being about a 1 mm coverage of plaster due to the depth of the chases (about 21mm). Any other ideas or suggestions of who sells thinner flexible conduit?

Thanks in advance.

 

[ColJack]

as there are 2 side by side, try the oval conduit instead..

 

[NickB_99]

Some people say just plaster over them.
Some people say wrap the pipes in duct tape and plaster over them.
Some people say wrap the pipes in denso tape and plaster over them.
Some people say run the pipes through flexible conduit and plaster over them.

I quite like the idea of running the pipes through flexible conduit as I can see that option definitely working.

I would suggest definitely not going for the first option - you need something on them.
Duct tape may be little thin also and now the pipes in place, may be difficult to wrap any tape right round.
I don't think it's just the expansion of the pipes - plaster directly in contact with copper pipes is not good. I think it is possible for them to corrode in time. (Tho some posts seemed inconclusive it had actually been seen)

Would it be possible to use flexible conduit as you suggest, but run a stanley knife down the length so you just have a reduced thickness and place over?

Keeping the plaster out of contact as far as possible (with conduit) can only be good from corrosion/expansion effects.

This is only my limited opinion of course

 

[ColJack]

why does it need to be flexible?
does it not go in a straight line, or something close to a straight line?

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[gday2uk]

use this http://www.tlc-direct.co.uk/Main_Index/Cable_Accessories_Index/Capping/index.html then plaster over. Ideally the metal one in case anyone ever tries to drill the area in the future.
Your channeling look too shallow as well.

 

[Richard C]

I’d be tempted to cover them with steel (electrical) capping, the same as I do with cable drops. I use Bonding plaster over the capping, tape over the whole lot & then finish skim; 10mm depth will be plenty enough. If you re-shape the depth of a suitably wide capping, it should allow the pipes some degree movement behind to allow for expansion & contraction & the Bonding plaster + tape will help but the problem you’re likely to have is the heat & I suspect it will still crack.

 

[noseall]

I agree with Richard, it will be very difficult to avoid cracking.

Take all the precautions mentioned and you will reduce the risk.

However, the only sure way would be to have plasterboard covering them entirely, i.e. full boards dabbed across. I appreciate this is probably not an option, but if future cracking is likely to drive you nuts then you may need to consider it.

 

[trowelmonkey1]

tricky one this,

surely the best option would be to use plastic capping? as this would transfer less heat than metal capping and also to use a size of capping that would allow the pipes enough room to move/expand without moving the capping, maybe screwing and plugging the capping will ensure it doesn't move at all while the pipes do their thing.

as Rich C recommends, use Bonding to fill in then as it likes plastic capping better than hardwall but PVA the brick/blockwork before hand to stop any cracking.

 

[ColJack]

overboard the entire wall, problem solved...

 

[DaveyH]

Thanks for the replies so far everyone.

ColJack, your idea of using 20mm oval conduit is interesting but the pipes have already been attached to the wall, radiator and boiler. I Suppose I could run a stanley knife down the oval conduit open it and place it over the pipes. I have a few offcuts of 20mm oval conduit lying around after the re-wire. So I might do little test later on. Another option could be to run each pipe in its own piece of 16mm oval conduit.

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• DaveyH
• 16 Jan 2010
• 1

Can someone confirm that the denso tape option definitely works? Surely the expansion of the pipes will only be a few thousandths of a millimeter per meter of copper pipe and if that’s the case I can see the denso tape absorbing that amount of movement no problem, especially if it’s wrapped loosely (and not tightly) around the pipes (ie loosely allowing for more movement under the plaster).

PS. The chases are about 20mm deep so the 8mm pipes will have about 12mm plaster coverage (the photographs don't show this very well).

 

[Richard C]

Surely the expansion of the pipes will only be a few thousandths of a millimeter per meter of copper pipe and if that’s the case I can see the denso tape absorbing that amount of movement no problem, especially if it’s wrapped loosely (and not tightly) around the pipes (ie loosely allowing for more movement under the have about 12mm plaster coverage (the photographs don't show this very well).

It’s far more than that I’m afraid; longitudinal expansion of copper piping can be calculated by the formula: tube length (m) x temperature rise ( °C ) x 0.0177; an approximation for standard hot water lines is to allow 1mm expansion for each 1m of straight length of pipe run.

That’s why it’s important you allow the pipes to move behind whatever you use to cover them + allow a little depth for radial expansion; plaster doesn’t bend at all & even the smallest amount of movement will crack it. That just leaves the problem of heat transfer & Gypsum plaster doesn’t like to be subjected to much more than 50 degrees before it starts to crack.

 

[Alastairreid]

It’s far more than that I’m afraid; longitudinal expansion of copper piping can be calculated by the formula: tube length (m) x temperature rise ( °C ) x 0.0177; an approximation for standard hot water lines is to allow 1mm expansion for each 1m of straight length of pipe run.

cheers Richard, i'm going to hit the customers with that formula next time they want me to block out pipework with plaster.

 

[seasickstevie]

The oval tubing ColJack suggests is the best idea. Easy to slice down the back with a Stanley knife and clip onto the pipe.

I'd keep the runs in separate tubing though to reduce the heat loss from flow to return.

 

[DaveyH]

Thanks again for all the comments/ideas chaps.

My conclusions regarding conduit.

16mm oval conduit is no good, it grips the 8mm pipe because although its 16mm wide internally its only 8mm deep.

20mm oval conduit will definitely work as its 10mm deep so the pipe will be able to expand inside that no problem at all. Putting 2 runs of 8mm pipe in this is possible but not really practical as your pipes need to be perfectly straight and their isn’t much room left inside for movement/expansion.

25mm oval conduit might work well for 2 runs of 8mm copper pipes, but I haven't got any 25mm oval so I can't test it out.


I’ve been doing a bit of goggling and I’ve found a company selling almost all sizes of flexible conduit going. This stuff might be ideal.

http://www.brianwards.co.uk/online-...essories/conduit/9mm-flexible-conduit-1m.html

Wouldn't you have thought the manufacturers of 8mm copper tubing would sell suitable conduit for it though. Surely they must know that half of all the 8mm pipe they sell gets plaster slapped over the top of it. Why should I have to go to a boat company like Brian Wards to locate a suitable conduit, bonkers totally bonkers. One for Dragons Den I think.

 

[markbentley99]

Hi

I am not a tradesman of any sort, however I was told the following, it is not the plaster that is the main cause of corrosion but the concrete wall it is in contact with?, I was told to cover the pipes though.

With regard to the plastering, I was thinking of using the following http://www.hotline-chimneys.co.uk/products.asp?partno=HRP, I know it is for flues and fireplaces but I cant see why it would not work over pipes and it only needs a 10mm depth, so might be worth a try (unless someone advises against it (I will be trying it though!).