I've noticed that my direct unvented cylinder discharges a small amount
when heating overnight.

It's a redring one, with air bubble expansion - I've already replensihed
the bubble, and while it has helped, hasn't stopped the discharge.

Basically, a slow trickle (a drop every few seconds) is discharged from
the pressure-relief valve (not the temp/pressure valve). I've also got
no record that it has been serviced since installed 3 years ago.

I'm guessing that this is a sign that the pressure-relief valve has worn
out and needs replacing.

The other thing is that it's supposed to have a protective anode, but
I've no idea where it is. There's relatively little corrosion visible,
but I did drain the system down and the stuff that came out of the
bottom of the cylinder was absolutely filthy. Additionally, when I
refilled the cylinder again, there was some leaking from around the
immersion heaters (though this stopped pretty quickly once the cylinder
had pressurised).

As you might expect, this is starting to sound expensive - any idea how
much a full service/inspection +/- valve replacement +/- new seals,
anode and other bits is going to cost, but more importantly are there
any hidden gotchas that are going to have plumbers rubbing their hands
with glee?

On Tue, 24 Jan 2006 20:54:15 +0000, Mark wrote:

I've noticed that my direct unvented cylinder discharges a small amount
when heating overnight.

It's a redring one, with air bubble expansion - I've already replensihed
the bubble, and while it has helped, hasn't stopped the discharge.

Basically, a slow trickle (a drop every few seconds) is discharged from
the pressure-relief valve (not the temp/pressure valve). I've also got
no record that it has been serviced since installed 3 years ago.

I'm guessing that this is a sign that the pressure-relief valve has worn
out and needs replacing.

The other thing is that it's supposed to have a protective anode, but
I've no idea where it is. There's relatively little corrosion visible,
but I did drain the system down and the stuff that came out of the
bottom of the cylinder was absolutely filthy. Additionally, when I
refilled the cylinder again, there was some leaking from around the
immersion heaters (though this stopped pretty quickly once the cylinder
had pressurised).

As you might expect, this is starting to sound expensive - any idea how
much a full service/inspection +/- valve replacement +/- new seals,
anode and other bits is going to cost, but more importantly are there
any hidden gotchas that are going to have plumbers rubbing their hands
with glee?

Does the installation come with a pressure gauge?
Does the dribble only happen at night?
Did things improve when the air-bubble was renewed?


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

Ed Sirett wrote:
On Tue, 24 Jan 2006 20:54:15 +0000, Mark wrote:

Does the installation come with a pressure gauge?
No.

Does the dribble only happen at night?
Yes.

Did things improve when the air-bubble was renewed?
Yes, but it didn't fix the problem.

M

Mark wrote:
Does the dribble only happen at night?
Yes.

Then more likely the pressure reducing valve is letting by, rather than
a duff pressure relief valve. Mains water pressure generally increases
at night & there's no draw off, which would remove any excess pressure
that's crept through.

Does the installation come with a pressure gauge?
No.
You'll probably find there's a blanked-off tapping on the pressure
reducing valve/ multi-function valve where you could fit one. Maybe the
gauge came with the kit and it was left in the back of a plumber's van.
A gauge helps diagnosing problems like this.


Did things improve when the air-bubble was renewed?
Yes, but it didn't fix the problem.

Unlikely to be an air bubble problem. You should refill the cylinder
SLOWLY when you replenish the air bubble. Try recharging it again (air
is free, at present) and then maybe a new pressure reducing valve.

Aidan wrote:

Unlikely to be an air bubble problem. You should refill the cylinder
SLOWLY when you replenish the air bubble. Try recharging it again (air
is free, at present) and then maybe a new pressure reducing valve.

If it's the type you can turn to test it, turn it quickly so that it
opens then snaps shut quickly. Sometimes this will get them to seal
properly again.

If you do replace it make sure it's of the correct type.
www.hrpc.co.uk is where I got one from before.

Is the cold water downstream of the pressure reducing valve? If so,
open the cold water first thing in the mornign to see if the intial
pressure is greater than normal. That'll give an indication as to
whether the mains pressure is leaking through over night.

On Wed, 25 Jan 2006 08:13:49 +0000, Mark wrote:

Ed Sirett wrote:
On Tue, 24 Jan 2006 20:54:15 +0000, Mark wrote:

Does the installation come with a pressure gauge?
No.

That is all too common. It does not help any.
£10 to add one - probably a 1/4" BSP hole on the _inlet_ gizmo for
this.


Does the dribble only happen at night?
Yes.

Looks like a weakish PRV - the heating is happening at night so even with
a new bubble it letting by.


Did things improve when the air-bubble was renewed?
Yes, but it didn't fix the problem.

Most unventeds have two pressure relief valves. You _must_ fit the correct
replacement it will be anything from 6 to 10 bar. If you are unlucky its
the combined T&P releif valve (c. £60).

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

wrote:

If it's the type you can turn to test it, turn it quickly so that it
opens then snaps shut quickly. Sometimes this will get them to seal
properly again.

That's the pressure relief valve, aka thesafety valve.

The problem is most likely with the pressure reducing valve.

If the pressure relief valve wasn't reseating, it would be dripping all
the time. However the easing lever/knob should be operated at least
once a year to make sure it still moves as intended. It might start
dripping after doing that, but it's too bad if it does.

Aidan wrote:
That's the pressure relief valve, aka thesafety valve.

The problem is most likely with the pressure reducing valve.

If the pressure relief valve wasn't reseating, it would be dripping all
the time. However the easing lever/knob should be operated at least
once a year to make sure it still moves as intended. It might start
dripping after doing that, but it's too bad if it does.

When the water is heated and there's no demand for hot water (e.g. at
night or early morning) the pressure in the cylinder will rise so it
is possible for the relief valve to only leak under those conditions,
but that usually is also an indicator that the bubble isn't doing it's
job.

For what it's worth (see Ed's post) the pressure reducer on my unvented
system was 1bar and the safety valves, both the PRV and TPRV, were
correspondingly low rated. I'm not sure placing a 6 - 10 bar PRV when
an e.g. 3 bar one is required is a good idea. Also my TPRV was more
like 30 quid but I'm sure I could have paid double/triple had I gone
elsewhere.

wrote:
For what it's worth (see Ed's post) the pressure reducer on my unvented
system was 1bar and the safety valves, both the PRV and TPRV, were
correspondingly low rated. I'm not sure placing a 6 - 10 bar PRV when
an e.g. 3 bar one is required is a good idea. Also my TPRV was more
like 30 quid but I'm sure I could have paid double/triple had I gone
elsewhere.

All our flats originally came with a combined pressurised heating/hot
water system and one was leakign for months over an entranceway. One
day it was not leaking and a few weeks later the hot water tank blew up
and gutted the flat and damaged neighbouring properties. So don't
whatever you do just block up the valve!

wrote:

When the water is heated and there's no demand for hot water (e.g. at
night or early morning) the pressure in the cylinder will rise

Get away.

so it is possible for the relief valve to only leak under those conditions,

No. That is the pressure relief valve working normally, relieving the
excess pressure caused by the water expanding and the air bubble being
inadequate to absorb the expansion. It is not 'leaking'. It is not
failing to reseat, which would cause it to drip continuously. Operating
the easing knob, as you have suggested would not effect this problem.


For what it's worth (see Ed's post) the pressure reducer on my unvented
system was 1bar and the safety valves, both the PRV and TPRV, were
correspondingly low rated. I'm not sure placing a 6 - 10 bar PRV when
an e.g. 3 bar one is required is a good idea.

It isn't a good idea, but then that is not what Ed said. He wrote;
"Most unventeds have two pressure relief valves. You _must_ fit the
correct
replacement it will be anything from 6 to 10 bar. "

He was referring to the Pressure Relief Valves which are factory
pre-set at something "from 3 to 10 bar", according to the cylinder
design pressure rating. You have to get the right one, set to the right
pressure, for your cylinder. A PRV set at 6 bar would be little use on
a cylinder designed for 3 bar and which fails at 5 bar.


Also my TPRV was more
like 30 quid but I'm sure I could have paid double/triple had I gone
elsewhere.

Sure you got the right one?

Ed Sirett wrote:
On Wed, 25 Jan 2006 08:13:49 +0000, Mark wrote:

Ed Sirett wrote:
Does the installation come with a pressure gauge?

No.

That is all too common. It does not help any.
£10 to add one - probably a 1/4" BSP hole on the _inlet_ gizmo for
this.

Now added. The regulating valve is 1.5 bar nominal - and a gauge
attached to the output reads 1.6 bar. So this valve seems to be working
satisfactorily.

Unfortunately, this won't read the cylinder pressure because there's a
non-return valve in the way, but it's better than nothing.

Does the dribble only happen at night?

Yes.


Looks like a weakish PRV - the heating is happening at night so even with
a new bubble it letting by.

That was my original thought.

I've also checked the immersion stat - It was originally set to 60 C.
Checking the installation manual for the cylinder, however, it suggests
the maximum allowable temperature is 55 C - so I've made the appropriate
adjustment.

I'll see what happens with this adjustment.

Most unventeds have two pressure relief valves. You _must_ fit the correct
replacement it will be anything from 6 to 10 bar. If you are unlucky its
the combined T&P releif valve (c. £60).

I think it's the pressure (not temp) relief valve as the discharging
water is cold (admittedly, I can't be absolutely sure as both valves
discharge through the same pipe and tundish).

As an aside, I tested the PRV by twisting the top - and it discharged an
immense torrent - this hosed out of sides the tundish, and all over the
walls, while at the same time overcoming the downpipe, which promptly
overflowed with a rather impressive fountain. (The tundish and downpipe
are quite capable of dealing with a wide open TPRV - but the PRV looks
like it can manage a rather higher flow rate). Is a discharge pipe that
can't actually handle the maximum discharge rate acceptable?

I'm beginning to think that this whole installation is a bit of a
liability...

M

Mark wrote:

Now added. The regulating valve is 1.5 bar nominal - and a gauge
attached to the output reads 1.6 bar. So this valve seems to be working
satisfactorily.

Good move. You need to see what the pressure is when the PRV is
dripping; the pressure might creep up overnight if the pressure
reducing valve is iffy.

Unfortunately, this won't read the cylinder pressure because there's a
non-return valve in the way, but it's better than nothing.

It will indicate high pressure if the pressure reducing valve is
letting by overnight. It won't indicate high pressure if the excess
pressure is caused inadequate expansion capacity.


Looks like a weakish PRV - the heating is happening at night so even with
a new bubble it letting by.

That was my original thought.

I've also checked the immersion stat - It was originally set to 60 C.
Checking the installation manual for the cylinder, however, it suggests
the maximum allowable temperature is 55 C - so I've made the appropriate
adjustment.

I'll see what happens with this adjustment.

Most unventeds have two pressure relief valves. You _must_ fit the correct
replacement it will be anything from 6 to 10 bar. If you are unlucky its
the combined T&P releif valve (c. £60).

I think it's the pressure (not temp) relief valve as the discharging
water is cold (admittedly, I can't be absolutely sure as both valves
discharge through the same pipe and tundish).

You sometimes have to remove the pipework connecting the 2 PRVs to the
tundish to see which is dripping. You have to remove the pipework to
replace the valve anyway.

As an aside, I tested the PRV by twisting the top - and it discharged an
immense torrent - this hosed out of sides the tundish, and all over the
walls, while at the same time overcoming the downpipe, which promptly
overflowed with a rather impressive fountain. (The tundish and downpipe
are quite capable of dealing with a wide open TPRV - but the PRV looks
like it can manage a rather higher flow rate). Is a discharge pipe that
can't actually handle the maximum discharge rate acceptable?

No. Part of the commissioning test is running either PRV (but not both
simultaneously) at full-bore; the discharge pipe shouldn't back-up.
There are design criteria for the size, length & number of elbows in
the discharge pipe. You'll find it in the installation manual. You can
wind up needing big (28mm) expensive copper pipes, which can wipe out
the profit if Mr. Plumber didn't anticipate that one. It is copper,
isn't it?


I'm beginning to think that this whole installation is a bit of a
liability...

On Thu, 26 Jan 2006 20:17:11 +0000, Mark wrote:

Ed Sirett wrote:
On Wed, 25 Jan 2006 08:13:49 +0000, Mark wrote:

Ed Sirett wrote:
Does the installation come with a pressure gauge?

No.

That is all too common. It does not help any.
£10 to add one - probably a 1/4" BSP hole on the _inlet_ gizmo for
this.

Now added. The regulating valve is 1.5 bar nominal - and a gauge
attached to the output reads 1.6 bar. So this valve seems to be working
satisfactorily.

Unfortunately, this won't read the cylinder pressure because there's a
non-return valve in the way, but it's better than nothing.

The usual order is NRV then PReducingV.


Does the dribble only happen at night?

Yes.


Looks like a weakish PRV - the heating is happening at night so even with
a new bubble it letting by.

That was my original thought.

I've also checked the immersion stat - It was originally set to 60 C.
Checking the installation manual for the cylinder, however, it suggests
the maximum allowable temperature is 55 C - so I've made the appropriate
adjustment.

I'll see what happens with this adjustment.

Most unventeds have two pressure relief valves. You _must_ fit the correct
replacement it will be anything from 6 to 10 bar. If you are unlucky its
the combined T&P releif valve (c. £60).

I think it's the pressure (not temp) relief valve as the discharging
water is cold (admittedly, I can't be absolutely sure as both valves
discharge through the same pipe and tundish).

As an aside, I tested the PRV by twisting the top - and it discharged an
immense torrent - this hosed out of sides the tundish, and all over the
walls, while at the same time overcoming the downpipe, which promptly
overflowed with a rather impressive fountain. (The tundish and downpipe
are quite capable of dealing with a wide open TPRV - but the PRV looks
like it can manage a rather higher flow rate). Is a discharge pipe that
can't actually handle the maximum discharge rate acceptable?

I'm beginning to think that this whole installation is a bit of a
liability...

The pressure relief pipes only ever dribble but if the temperature relief
operates this must be handled OK. However as you say the tundish is
capable.

The problem is that the failure modes of unvented cylinders are quite a
problem - set up right they work well. Vented have their problems too
mostly with the storage cistern.



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

On Thu, 26 Jan 2006 12:12:16 -0800, Aidan wrote:

wrote:

When the water is heated and there's no demand for hot water (e.g. at
night or early morning) the pressure in the cylinder will rise

Get away.

so it is possible for the relief valve to only leak under those conditions,

No. That is the pressure relief valve working normally, relieving the
excess pressure caused by the water expanding and the air bubble being
inadequate to absorb the expansion. It is not 'leaking'. It is not
failing to reseat, which would cause it to drip continuously. Operating
the easing knob, as you have suggested would not effect this problem.


For what it's worth (see Ed's post) the pressure reducer on my unvented
system was 1bar and the safety valves, both the PRV and TPRV, were
correspondingly low rated. I'm not sure placing a 6 - 10 bar PRV when
an e.g. 3 bar one is required is a good idea.

It isn't a good idea, but then that is not what Ed said. He wrote;
"Most unventeds have two pressure relief valves. You _must_ fit the
correct
replacement it will be anything from 6 to 10 bar. "

He was referring to the Pressure Relief Valves which are factory
pre-set at something "from 3 to 10 bar", according to the cylinder
design pressure rating. You have to get the right one, set to the right
pressure, for your cylinder. A PRV set at 6 bar would be little use on
a cylinder designed for 3 bar and which fails at 5 bar.


Also my TPRV was more
like 30 quid but I'm sure I could have paid double/triple had I gone
elsewhere.

Sure you got the right one?

Aidan I didn't mean to imply that that the OP should replace the 3 bar PRV
with a 6 bar unit if a 3 bar unit is the correct one. A 1.5 bar unvented
is quite a bit less pressure that the typical unit.

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html

Aidan wrote:

No. Part of the commissioning test is running either PRV (but not both
simultaneously) at full-bore; the discharge pipe shouldn't back-up.
There are design criteria for the size, length & number of elbows in
the discharge pipe. You'll find it in the installation manual. You can
wind up needing big (28mm) expensive copper pipes, which can wipe out
the profit if Mr. Plumber didn't anticipate that one. It is copper,
isn't it?

Erm. No, it's plastic.

There's a short run of 15 mm copper from the relief valves to the tundish.

From there it's 40 mm plastic. About 30 cm straight down, then a rather
impressive network of 6 elbows, then a long horizontal run (actually the
2nd half of it isn't quite horizontal, but it's definitely less than a
1:50 drop), until it joins the main waste pipe.

Mark wrote:

Erm. No, it's plastic.

There's a short run of 15 mm copper from the relief valves to the tundish.

From there it's 40 mm plastic. About 30 cm straight down, then a rather
impressive network of 6 elbows, then a long horizontal run (actually the
2nd half of it isn't quite horizontal, but it's definitely less than a
1:50 drop), until it joins the main waste pipe.

It should be metal, plastic won't tolerate the heat if the T&PRV ever
operates due to over temperature. I'd suggest getting it looked at, in
case there are other errors.

Ed Sirett wrote:

The usual order is NRV then PReducingV.

My book (CITB) shows IV, strainer, PReducingV, tee off for balanced
cold supply, NRV, expansion vessel (not applicable here), PReliefV,
cylinder.

The strainer, PReducingV & NRV usually come as one multi-function
valve, so you can't alter the sequence.

Ed Sirett wrote:

Aidan I didn't mean to imply that that the OP should replace the 3 bar PRV
with a 6 bar unit if a 3 bar unit is the correct one.

I know you didn't.
Adder1 misunderstood what you had written.