On 03/03/2020 19:08, John Rumm wrote:
On 03/03/2020 18:34, Chris B wrote:
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.

What make / model of unvented cylinder is it? Most also have a low
pressure take off on the cold inlet for just this purpose.


OK full story. House under restoration.

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..........


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Chris B (News)