Water Tank and Expansion Vessel Overflow?

paperclap

Thanks all.

With regards to sizes, I'm 99% sure these are 22mm copper pipes, as 3/4 would equate to 19mm... and these are more like 22mm. I measured – thought I saw 21mm weirdly (hence the weird typo), but must've just been because I measured from a distance or my eyes are going!

Have attached a new diagram of the existing set up

Cold water tank
Crap. Hadn't even thought about the tank material itself (fiberglass or asbestos!). I suppose I could take the "easy" route on that one, and simply connect a 22mm speedfit fitting to the existing copper, and pop some 22mm plastic pipe on the end. Still not ideal having the copper pipe hang like that, but I guess it's been that way for years and years... so, should be alright?

Expansion vessel
I can't use a typical overflow pipe or plastic pipe? For what's reason(s)? What is the cap used for?

Bendy_House

3/4" won't be 19mm - it's larger than that, in fact is not far off 22mm, as I (rustily) recall. But could be wrong. I suspect - but don't know - that 3/4" referred to int diameter? Dunno.

Yes, for the large tank, 'all' you need to do is ensure the end sticks out the soffit where any overflow can be seen, and it won't cause any damage. The water coming out this pipe will almost always be just cold water, from a sticking ballcock. Occasionally, it might be luke-warm, since a faulty system could - under rare circumstances (eg a stuck-on immersion heater) - be sending 'boiling' water up to the large CWS, but then it'll be diluted by the large reservoir of cold.

The safety discharge pipe is quite different. If the PRV 'blows', then we are talking of an ejaculation at 3 bar pressure, and a squirt of very hot liquid (must have a lie down...) Because it's at pressure, 15mm pipe is large enough. But, because it's at pressure, plastic ain't.

And it must be aimed correctly. No, not at your neighbour.

grumbler

Bendy_House said:

But, because it's at pressure, plastic ain't.

Mains water pipes are at pressure and can be plastic. So can be even CH pipes. And this particular pipe isn't really at pressure as it's short and open on one end.
Plastic pipe is fine, but it's better to be a proper pipe, not overflow one. 

Section62

grumbler said:

Section62 said:

laurencewhymark said:

It seems a 15mm to 21.5mm fitting doesn't exist!

Now I understand why the previous fitter used 22mm pipe...

grumbler said:

With regard to the pipes, is "21" coming from the water tank in you diagram a typo? Is it 22 actually? If so, just use a 22mm push-fit (or compression) fitting and add a small piece of white plastic 22mm water (not overflow) pipe at the very end. You can replace a bigger part of the copper pipe if you wish.

Wild guess...

Originally there were two tanks, a cold water storage tank and a F&E tank.  Both had 3/4 copper overflow pipes.

The cold water storage tank still has a 3/4 copper overflow.

...

Laurence, you could replace the overflow pipe on the cold water tank, but you'll need to fit a new tank connector.  ...

Why go through this hassle?

JG Speedfit Imperial to Metric Connector 3/4" x 22mm

Because -

a) They aren't cheap

b) Having tried to get one recently I discovered that isn't so easy

c) 22mm isn't the diameter of (cheaper) overflow pipe which should be used for this job.

d) The existing overflow pipe needs extending

e) If Laurence wishes, he could treat the redundant copper overflow pipe as scrap (adding it to the other copper scrap he no doubt has by now) and probably get more cash for that than the correct new materials will cost him.

Bendy_House

grumbler said:

Bendy_House said:

But, because it's at pressure, plastic ain't.

Mains water pipes are at pressure and can be plastic. So can be even CH pipes. And this particular pipe isn't really at pressure as it's short and open on one end.
Plastic pipe is fine, but it's better to be a proper pipe, not overflow one. 

That may be yet another thing I recall incorrectly...

But I'm pretty sure boiler installation instructions do specify copper for the safety discharge pipe. And, when you consider the circumstances where the PRV could be triggered, albeit very rarely, then you'll be glad for copper. 

Yes, PRVs are usually popped when the EV isn't doing its job, so all you get out that pipe is 'hot' water, and not too much of it. But, the potential is always there for the pressure rise to the triggering 3bar to have been caused by a boiler that won't shut off, so boils the system water. And, at 3 bar, I think that's going to be over 100oC?

grumbler

Well, the conditions for this pipe can't be worse than for other CH pipes. They can be only better because it's open on one side. 

paperclap

Thanks again so much all. Always a massive help 

So, I think the best route would be to simply grab some PEX or polybutylene 22mm pipe for both. PEX vs polybutylene – is there a better of the two?

Saves any adaptors, keeps everything "normal". E.g. 22mm instead of the weird 21.5mm.

For the water tank, these:
https://www.screwfix.com/p/mcalpine-straight-overflow-tank-connector-white-22mm/220hr
https://www.screwfix.com/p/jg-speedfit-22bpex-78028-push-fit-pe-x-pipe-22mm-x-3m/78028

For the expansion vessel, these:
https://www.screwfix.com/p/jg-speedfit-22bpex-78028-push-fit-pe-x-pipe-22mm-x-3m/78028
Then, some JG push-fit fittings. Hopefully can grab individuals, rather than 5 or 10 packs!

Probably cost £40 overall me reckons.

Section62

grumbler said:

Well, the conditions for this pipe can't be worse than for other CH pipes. They can be only better because it's open on one side. 

See page 17 of this document -

"Speedfit pipe should not be used to provide the discharge from unvented cylinders, unvented water heaters and sealed systems via the temperature relief and pressure relief valves."

https://www.johnguest.com/sites/jg/files/2022-01/RWC11339_JG-Speedfit-Technical-Specs-Guide_v11.pdf

paperclap

Well sh*t.

So, is the only option to us mere mortals 21.5mm overflow pipe (if possible in other cases), or 22mm copper pipe?

Section62

laurencewhymark said:

Thanks again so much all. Always a massive help 

So, I think the best route would be to simply grab some PEX or polybutylene 22mm pipe for both. PEX vs polybutylene – is there a better of the two?

Saves any adaptors, keeps everything "normal". E.g. 22mm instead of the weird 21.5mm.

For the water tank, these:
https://www.screwfix.com/p/mcalpine-straight-overflow-tank-connector-white-22mm/220hr
https://www.screwfix.com/p/jg-speedfit-22bpex-78028-push-fit-pe-x-pipe-22mm-x-3m/78028

For the expansion vessel, these:
https://www.screwfix.com/p/jg-speedfit-22bpex-78028-push-fit-pe-x-pipe-22mm-x-3m/78028
Then, some JG push-fit fittings. Hopefully can grab individuals, rather than 5 or 10 packs!

Probably cost £40 overall me reckons.

No.  You've been misled by some poor advice.

You should use overflow pipe for the cold water storage tank.  21.5mm is the standard for this application and there is a whole range of (low cost) fittings for it.

Overflow pipe (from Toolstation) costs £2.28 per 3m length.  Speedfit 22mm B-PEX costs £9.48 for a 3m length.

The solvent weld 21.5mm tank connector costs 78p. Elbows and straight couplers likewise cost 78p each.

A Speedfit 22mm 90 degree elbow costs £2.78 (single)

There is zero point spending extra money on 22mm pipe simply to avoid using the standard 21.5mm pipe which is designed for the job.

You should leave the CH pressure relief pipe to be done by a suitably qualified plumber or heating engineer.  This is (I believe) a legal requirement, but happy to be corrected if anyone knows for certain otherwise.