What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's leak

[interesting100]

Hi
Can anyone tell me what sort of pressure an expansion vessel normally runs at, on a normal GCH system, when it is heating

I've just put an 18 litre expansion vessel as a temporary replacement for a first floor open header tank, to see what happens if the system is run at pressure. I've attached it to the cold feed (which is effectively a long pipe to the return side of the pump) and capped off the vent pipe

The amount of water expanding into the open header was less than 3 litres
If this is now expanding into the 18 litre pressure vessel, presumably it will only cause a few psi pressure rise - is this correct ?
If so, the standard advice to change all the TRV's seems over the top,
if the increase in pressure is, say 4 psi.
Is an extra 4 psi likely to make old TRV's leak - eg from the 80's

thanks


W-Bosch 24Ri powered GCH system - s plan valves

 

[interesting100]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

My plumber has had a look at what I've done and told me that the expansion vessel is fitted correctly, but needs to be pressurised to 1 bar when cold, (the same as a combi system), which i have now done without problem.
The hot pressure now goes up to 1.6 bar

Can anyone tell me why the unvented systems need to be pressurised when cold ?
Presumably there is some physics behind it

 

[dancinplumba]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

cos water expands when its heated 🙂

 

[vern]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

Hi
Can anyone tell me what sort of pressure an expansion vessel normally runs at, on a normal GCH system, when it is heating

I've just put an 18 litre expansion vessel as a temporary replacement for a first floor open header tank, to see what happens if the system is run at pressure. I've attached it to the cold feed (which is effectively a long pipe to the return side of the pump) and capped off the vent pipe

The amount of water expanding into the open header was less than 3 litres
If this is now expanding into the 18 litre pressure vessel, presumably it will only cause a few psi pressure rise - is this correct ?
If so, the standard advice to change all the TRV's seems over the top,
if the increase in pressure is, say 4 psi.
Is an extra 4 psi likely to make old TRV's leak - eg from the 80's

thanks


W-Bosch 24Ri powered GCH system - s plan valves

Well you had better uncap the open vent or install a PRV, at least the RI has an overheat stat!

 

[TBServices]

Brilliant.

"I'm just going into the loft darling!"
"Why?"
"I'm just going to turn the boiler into a bomb, won't be long"

You're dangerous. And so's your plumber, if he exists. Why would a plumber come round to pass judgement on your DIY plumbing? Lol. I wouldn't do it and I don't see a reason why anyone else would.

 

[vern]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

Brilliant.

"I'm just going into the loft darling!"
"Why?"
"I'm just going to turn the boiler into a bomb, won't be long"

You're dangerous. And so's your plumber, if he exists. Why would a plumber come round to pass judgement on your DIY plumbing? Lol. I wouldn't do it and I don't see a reason why anyone else would.

Lol tell him how it is Tom......

 

[Whn1]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

If you've just teed it to the cold feed Wont the expansion still be taken p bythe header tank?

 

[snowhead]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

If you've just teed it to the cold feed Wont the expansion still be taken p bythe header tank?

He's taken out the tank, and omitted to fit a very important safety feature, which his "plumber" has also omitted to notice.

 

[Whn1]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

Apologies.. While you're at it change the gas supply to your boiler to a fuel line carrying petrol....

 

[chris watkins]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

My plumber has had a look at what I've done and told me that the expansion vessel is fitted correctly, but needs to be pressurised to 1 bar when cold, (the same as a combi system), which i have now done without problem.
The hot pressure now goes up to 1.6 bar

Can anyone tell me why the unvented systems need to be pressurised when cold ?
Presumably there is some physics behind it

Hi interesting100
I think you should have got a idea of what proper plumbers think of your actions & question, don't hold your breath for any further answers or information, with all due respect, you typify all that is meant by the old saying "A little knowledge is a dangerous thing" & should serve as a warning to other d.i.y'ers not to play around with un-vented systems unless you fully understand them & are competent to do so, this is the Law (Part G Building Regs) & for good reason these systems can be bombs if things go wrong.

If you really do want to know more about these un-vented systems can I suggest a Level 3 plumbing course (4 years) followed by a one day assessment on them cos that's what we have had to do!!

 

[snowhead]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

Apologies.. While you're at it change the gas supply to your boiler to a fuel line carrying petrol....

Is this a smart move to beat the current possible gas shortage, or just a Darwinism?

 

[interesting100]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

I was a little surprised by the above comments - they seem a little premature and unhelpful-
Obviously I fitted the expansion vessel with the normal PRV (pressure relief valve), pressure guage and relief overflow pipe. That's normal procedure for these things and there is usually an expansion vessel control kit to that effect. (which has been fitted)
It might have been more appropriate to enquire if that had been done, rather than to assume that it hadn't ?

So the plumber is not doing anything wrong
We are just seeing if the rest of the system can cope with the pressure

What I was asking, as a general query, is: Why does an unvented system need to be set at 1 or 1,5 bar when cold, rather than the very low pressure of a vented system ?
The pressure at the bottom of a vented header tank system is a few psi, equating to the height of the water column - so why the need for the extra pressure for unvented ?

regards

 

[chris watkins]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

Surely it is you that should provide the relevant information & not for us to have to ask, we can only go on what you provide. Re-garding the information you requested - if your plumber can not provide this then you need to get a new one.
Some boilers require a minimum cold fill pressure, what does your system require? what is the static head? what pre-chacharge pressure are you going to use? have you set this before filling the system? what is the setting on the PRV? what is the water content of your system? & there is lots more.... DO NOT PLAY if your do not fully understand.
I am gone !

 

[Barry98]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

What I was asking, as a general query, is: Why does an unvented system need to be set at 1 or 1,5 bar when cold, rather than the very low pressure of a vented system ?
The pressure at the bottom of a vented header tank system is a few psi, equating to the height of the water column - so why the need for the extra pressure for unvented ?

regards

Ask the plumber on site

 

[Howsie]

What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's

Get a new plumber.

I'm out of here. Still haven't finished that bloody bridge at the bottom of my garden...

 

[zzzjim]

Re: What is the normal pressure in an expansion vessel? - Will 4psi make old TRV's le

Old system used boiling as a safety feature !

( Lots of Dangerous Design-Layout issues..so need proper training)

Stay safe