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?

https://www.advancedwater.co.uk/prod-775-147-0006.html

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?


--
Chris B (News)

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.


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?

https://www.advancedwater.co.uk/prod-775-147-0006.html

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.Â* It may sit in the cold feed
pipe for some hours and warm up to say 20C.Â* How is this expansion
accommodated?


By the expansion vessel you are going to fit..
Isn't there a way to take a cold feed also off the other side of the
mains pressure hot water?


--
€œThere are two ways to be fooled. One is to believe what isnt true; the
other is to refuse to believe what is true.€�

€”Soren Kierkegaard

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.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

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..........


--
Chris B (News)

On Tuesday, 3 March 2020 21:50:42 UTC, Chris B wrote:

(he mused on why they might have blanked off
the regulated cold supply on the main tank PRV).

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).

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 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.

On Tuesday, 3 March 2020 18:34:48 UTC, 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.


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?

https://www.advancedwater.co.uk/prod-775-147-0006.html

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?


--


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.

On 03/03/2020 23:33, Mathew Newton wrote:
On Tuesday, 3 March 2020 21:50:42 UTC, Chris B wrote:




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 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.


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.


--
Chris B (News)

Chris B wrote:

On 03/03/2020 23:33, Mathew Newton wrote:
On Tuesday, 3 March 2020 21:50:42 UTC, Chris B wrote:




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 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.


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.

--

Roger Hayter

On Wednesday, March 4, 2020 at 9:10:11 AM UTC, Chris B wrote:

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.

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 https://www.engineeringtoolbox.com/e...nks-d_885.html that illustrate it quite well.

On Wed, 4 Mar 2020 09:10:07 +0000, Chris B wrote:

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.

The coefficient expansion of Water is highly nonlinear but from 10 to
20 C it's about 0.0001 / deg C. So 1 litre of water warmed by 10 C
expands by about 1 ml.

I'd be surprised if your incoming underground fed mains water is as
cold as 5 C even in a severe winter. 10 to 15 is much more likely.

--
Cheers
Dave.

On 04/03/2020 09:10, Chris B wrote:
On 03/03/2020 23:33, Mathew Newton wrote:
On Tuesday, 3 March 2020 21:50:42 UTC, Chris BÂ* wrote:




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 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.


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.Â* 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.

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.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 04/03/2020 10:02, Dave Liquorice wrote:
On Wed, 4 Mar 2020 09:10:07 +0000, Chris B wrote:

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.

The coefficient expansion of Water is highly nonlinear but from 10 to
20 C it's about 0.0001 / deg C. So 1 litre of water warmed by 10 C
expands by about 1 ml.

I'd be surprised if your incoming underground fed mains water is as
cold as 5 C even in a severe winter. 10 to 15 is much more likely.

No.

underground is seldom more than 9C and surprisingly stable year round

But it depends on how deep the pipes are and where the water in fact
comes from ... Borehole water is obviously colder than recycled London
urine...

--
Future generations will wonder in bemused amazement that the early
twenty-first centurys developed world went into hysterical panic over a
globally average temperature increase of a few tenths of a degree, and,
on the basis of gross exaggerations of highly uncertain computer
projections combined into implausible chains of inference, proceeded to
contemplate a rollback of the industrial age.

Richard Lindzen

On 04/03/2020 10:02, Dave Liquorice wrote:
On Wed, 4 Mar 2020 09:10:07 +0000, Chris B wrote:

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.

The coefficient expansion of Water is highly nonlinear but from 10 to
20 C it's about 0.0001 / deg C. So 1 litre of water warmed by 10 C
expands by about 1 ml.

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.


I'd be surprised if your incoming underground fed mains water is as
cold as 5 C even in a severe winter. 10 to 15 is much more likely.



--
Chris B (News)