I am going to have to fit a PRV in the cold supply for the benefit of a new thermostatic shower mixer (max allowed pressure 5 Bar) current cold pressure 7.5 bar and hot pressure regulated to 3 bar by a pressurised hot water system.
If I do this, do I also have to fit a mini expansion vessel (as in 0.5 or 1lt - like the one linked below) into the cold supply?
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As I see it the water that has just entered the house at 3 or 5 C will go through the PRV and be reduced to 3 bar. It may sit in the cold feed pipe for some hours and warm up to say 20C. How is this expansion accommodated?
About a year ago boiler and complete heating system replaced and pressurised tank installed.
A month ago the same plumbing firm refurbished the family bathroom and installed a thermostatic mixer shower.
Two weeks ago I noted water from the tank tundish/vent. Reported this to the plumber concerned and he said it would be a warranty claim on the tank and I would have to call the tank manufacturer.
This I did and when the rep arrived he looked at the system installation and asked what the cold water pressure was. (ask me one on sport) so he tested it and found it to be 7.5bar.
He diagnosed the cold was forcing its way through the shower mixer (which I have since found is cleared to 5 bar) into the hot circuit and had over-pressured the system resulting in the safety valve doing its job and relieving excess pressure, at 7 Bar
The tank manufactures rep says the main expansion vessel needs replaced (overstressed), the safety valve needs replaced (overstressed) and a PRV required on the cold feed (he mused on why they might have blanked off the regulated cold supply on the main tank PRV).
Original plumber has agreed to come back and do the work described as required by the tank manufacturer, but having looked much more into how these systems work I thought that a small expansion vessel would also be needed in addition to the PRV.
No one has mentioned this yet and I just wanted some more opinions before I went into battle with the plumber.
(on a separate subject the manufacturers rep noticed another part of the installation which does not meet the regs and is also going to be corrected - cold feed to an en-suite coming from a T at the cold entry to the tank. Not allowed apparently as there is a possibility of warm tank water entering cold feed and risk of legionella. Another item that would have been solved by using the port provided on the PRV for the tank........
And this plumbing firm has a good reputation in the area..........
That does indeed seem odd. Balanced hot and cold mains supplies are a key benefit of unvented systems. You wouldn't really get that with separate PRVs (although you could get arguably get close enough).
No expansion vessel is required on the cold as there's no expansion to accommodate. The only reason is it required on the hot side is due to the expansion that arises from heating the water.
You should take your cold water supply for the shower from downstream of the same PRV for the hot water system thus ensuring the pressures are exactly equal.
I understand why a large expansion tank is required to accommodate the expansion of 200lt of water as it is heated from say 5C to 60C.
However on the cold side the entire volume of the cold supply pipe inside the house (2 or perhaps 3 litres) will be warmed from 5C to house ambient - say 20C. This must cause some expansion (volume increase may only be an egg-cup full) and I am curios as to how this is dealt with.
I could invent a hand-wavy explanation about cold taps never being off long enough, or leaking a bit at high pressures, but the fact is that people (including me) put double check valves in cold supplies with no ill effects. And no expansion vessels.
As you touch on, the expansion to be accommodate is a function of initial volume and temperature rise which, compared to the hot side, is relatively small - particularly given that the expansion is volumetric and hence is exponential.
There are a few graphs, with the formulae behind them, at
There will be some expansion, but an egg cup full would seem rather excessive. Less than a teaspoon full seems more likely, and there will be enough elasticity in the pipes to accommodate that tiny extra volume in those circumstances.
Thanks, to you, John Rumm and Mathew Newton for the info. With that explanation, along with the link provided by Mathew and a bit more Google searching around those principles I can see why a small expansion tank is not needed on the cold supply. Much happier now.
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