Hello,

I have a really old combi boiler and I think the expansion vessel
needs replacing. What happens to them? Does the membrane eventually
perish?

The vessel is at the back of the boiler in a really awkward place, so
I was thinking of buying an external one. The manual says the fitted
vessel is 8L but says if you add radiators and increase the volume of
the system, a larger vessel will be required, which makes sense.

Now I do plan to add a few radiators and I also plan to download one
of those spreadsheets to calculate what size radiators I need, so I
may upgrade a few of the existing ones. Since a 12L vessel is only one
pound more than the 8L one at tool station, does it make sense to buy
the 12L vessel to give me some spare capacity?

I note that Screw fix sell a EV isolating valve with drain. Are these
useful? Is the idea that you isolate the vessel and depressurise the
CH once a year and measure the pressure in the vessel and top it up if
necessary?

I know I need to fit the vessel as close to the boiler as possible on
the return pipe. Does it have to be plumbed in 22mm or could I use
15mm?

I know there are two spreadsheets recommended here for CH sizing: one
in Myson, what's the other and where can I find them?

Thanks,
Stephen.

On 05/02/2019 11:05, Stephen wrote:
Hello,

I have a really old combi boiler and I think the expansion vessel
needs replacing. What happens to them? Does the membrane eventually
perish?

The vessel is at the back of the boiler in a really awkward place, so
I was thinking of buying an external one. The manual says the fitted
vessel is 8L but says if you add radiators and increase the volume of
the system, a larger vessel will be required, which makes sense.

Now I do plan to add a few radiators and I also plan to download one
of those spreadsheets to calculate what size radiators I need, so I
may upgrade a few of the existing ones. Since a 12L vessel is only one
pound more than the 8L one at tool station, does it make sense to buy
the 12L vessel to give me some spare capacity?

Yes


I note that Screw fix sell a EV isolating valve with drain. Are these
useful? Is the idea that you isolate the vessel and depressurise the
CH once a year and measure the pressure in the vessel and top it up if
necessary?

Not necessarily annually, but a kit does make checking and
repressurising much easier. On my system I just fitted the valve and
drain when fitting the external cylinder.


I know I need to fit the vessel as close to the boiler as possible on
the return pipe. Does it have to be plumbed in 22mm or could I use
15mm?

15 mm is fine, and I don't think it has to be particularly close to the
boiler. Remember, it is not there to control water hammer, it is *just*
for expansion.


I know there are two spreadsheets recommended here for CH sizing: one
in Myson, what's the other and where can I find them?

Thanks,
Stephen.

On Tuesday, 5 February 2019 11:05:20 UTC, Stephen wrote:
Hello,

I have a really old combi boiler and I think the expansion vessel
needs replacing. What happens to them? Does the membrane eventually
perish?

yes

The vessel is at the back of the boiler in a really awkward place, so
I was thinking of buying an external one. The manual says the fitted
vessel is 8L but says if you add radiators and increase the volume of
the system, a larger vessel will be required, which makes sense.

Now I do plan to add a few radiators and I also plan to download one
of those spreadsheets to calculate what size radiators I need, so I
may upgrade a few of the existing ones. Since a 12L vessel is only one
pound more than the 8L one at tool station, does it make sense to buy
the 12L vessel to give me some spare capacity?

I note that Screw fix sell a EV isolating valve with drain. Are these
useful? Is the idea that you isolate the vessel and depressurise the
CH once a year and measure the pressure in the vessel and top it up if
necessary?

I know I need to fit the vessel as close to the boiler as possible on
the return pipe. Does it have to be plumbed in 22mm or could I use
15mm?

I know there are two spreadsheets recommended here for CH sizing: one
in Myson, what's the other and where can I find them?

Thanks,
Stephen.

The expansion vessel is to handle the change in volume as the water heats up. This is a slow process so there's no problem with connecting it via 8mm pipe. I know no reason it must be close to the boiler.


NT

"Stephen" wrote in message
...
Hello,

I have a really old combi boiler and I think the expansion vessel
needs replacing. What happens to them? Does the membrane eventually
perish?

Yes. This allows water into the side of the expansion vessel that normally
contains air. This means that the expansion vessel can no longer accommodate
changes in pressure that are caused by the pump and by thermal expansion
(because water is much less compressible than air), so the over-pressure
valve dumps the excess pressure by venting to the drain. Once the water
cools down, the baseline pressure is then much lower than it should be.

Hence a sudden loss of pressure which needs to be manually topped up and
then disappears again next time the heating turned on is a sure sign that
the expansion vessel membrane has failed.

On Tuesday, February 5, 2019 at 12:42:39 PM UTC, NY wrote:
"Stephen" wrote in message
...
Hello,

I have a really old combi boiler and I think the expansion vessel
needs replacing. What happens to them? Does the membrane eventually
perish?

Yes. This allows water into the side of the expansion vessel that normally
contains air. This means that the expansion vessel can no longer accommodate
changes in pressure that are caused by the pump and by thermal expansion
(because water is much less compressible than air), so the over-pressure
valve dumps the excess pressure by venting to the drain. Once the water
cools down, the baseline pressure is then much lower than it should be.

Hence a sudden loss of pressure which needs to be manually topped up and
then disappears again next time the heating turned on is a sure sign that
the expansion vessel membrane has failed.

You might get away with re-pressurising it.

In article ,
writes:
On Tuesday, 5 February 2019 11:05:20 UTC, Stephen wrote:
Hello,

I have a really old combi boiler and I think the expansion vessel
needs replacing. What happens to them? Does the membrane eventually
perish?

yes

The other thing is that the air leaks out, and may just need
pumping back up with a bicycle pump. That happened to mine
after about 16 years, but the membrane isn't leaking.
You can tell if the membrane is leaking, as water will come
out of the tyre valve used to pump it up - that won;t happen
if the membrane is intact.

The vessel is at the back of the boiler in a really awkward place, so
I was thinking of buying an external one. The manual says the fitted
vessel is 8L but says if you add radiators and increase the volume of
the system, a larger vessel will be required, which makes sense.

Now I do plan to add a few radiators and I also plan to download one
of those spreadsheets to calculate what size radiators I need, so I
may upgrade a few of the existing ones. Since a 12L vessel is only one
pound more than the 8L one at tool station, does it make sense to buy
the 12L vessel to give me some spare capacity?

I note that Screw fix sell a EV isolating valve with drain. Are these
useful? Is the idea that you isolate the vessel and depressurise the
CH once a year and measure the pressure in the vessel and top it up if
necessary?

I know I need to fit the vessel as close to the boiler as possible on
the return pipe. Does it have to be plumbed in 22mm or could I use
15mm?

I know there are two spreadsheets recommended here for CH sizing: one
in Myson, what's the other and where can I find them?

Thanks,
Stephen.

The expansion vessel is to handle the change in volume as the water heats up. This is a slow process so there's no problem with connecting it via 8mm pipe. I know no reason it must be close to the boiler.

Connecting it near to the pump inlet reduces cavitation
in the pump, reducing noise and increasing impeller life.
This probably also requires a low impedance path to the
air reserve, which an 8mm pipe is going to restrict.
If you fit multiple pressure vessels, only one needs to
be connected near the pump inlet.

If that's impossible, it's not a big deal - just the pump
may generate more noise and impeller have a shorter life.

Other very remote possibility of connecting the vessel in
a bad position is that if the boiler has a water pressure
sensor to trip it out on a loss of system pressure, having
the vessel connected a log way from the pump or sensor could
cause a momentary pressure change at the sensor sufficient
to trip the sensor at pump switch-on or switch-off, or if
a motorised zone valve closes.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On Tue, 05 Feb 2019 11:05:24 +0000, Stephen wrote:

Hello,

I have a really old combi boiler and I think the expansion vessel needs
replacing.

How do you know that? The common symptoms are the pressure gauge heading
north towards the red zone and the pressure relief valve spewing out
excess water. Seen either of those signs?




--
Leave first - THEN negotiate!

NY wrote:

Yes. This allows water into the side of the expansion vessel that normally
contains air. This means that the expansion vessel can no longer accommodate
changes in pressure that are caused by the pump and by thermal expansion
(because water is much less compressible than air), so the over-pressure
valve dumps the excess pressure by venting to the drain. Once the water
cools down, the baseline pressure is then much lower than it should be.

Hence a sudden loss of pressure which needs to be manually topped up and
then disappears again next time the heating turned on is a sure sign that
the expansion vessel membrane has failed.

I have a condensing system boiler. Last winter I noticed that it
seemed to need the pressure topping up regularly, but no leaks
were detectable.

During the summer, where it was still in use to heat my
unpressurised hot water tank, no topping up was necessary.

Back in the heating season the issue has returned.

In describing the situation it occurs to me that the difference
is that the hot tank is next to the boiler, so is only raising
the temperature of a small volume of water, compared with the
whole radiator circuit, which latter would thus cause more
expansion.

Chris
--
Chris J Dixon Nottingham UK


Plant amazing Acers.

On 05/02/2019 19:57, Chris J Dixon wrote:


I have a condensing system boiler. Last winter I noticed that it
seemed to need the pressure topping up regularly, but no leaks
were detectable.

During the summer, where it was still in use to heat my
unpressurised hot water tank, no topping up was necessary.

Back in the heating season the issue has returned.

In describing the situation it occurs to me that the difference
is that the hot tank is next to the boiler, so is only raising
the temperature of a small volume of water, compared with the
whole radiator circuit, which latter would thus cause more
expansion.

Chris

Yes, it looks like your system has *some* expansion capacity but not
enough to cope with the whole system getting hot. Re-pressurising the
air in the expansion vessel will likely fix that.

Can you get at the air valve on the expansion vessel? If so, bleed
enough water out of a radiator for the water pressure to drop to zero.
Then check the air pressure with a car tyre pressure gauge. It needs to
be somewhere in the region of 0.7 to 1.0 bar (10-15 psi). If it's a lot
lower than this, top it up with a car tyre pump. then use the filling
loop to re-pressurise the wet part.
--
Cheers,
Roger
____________
Please reply to Newsgroup. Whilst email address is valid, it is seldom
checked.

On Tue, 5 Feb 2019 13:30:45 -0000 (UTC),
(Andrew Gabriel) wrote:


The other thing is that the air leaks out, and may just need
pumping back up with a bicycle pump. That happened to mine
after about 16 years, but the membrane isn't leaking.
You can tell if the membrane is leaking, as water will come
out of the tyre valve used to pump it up - that won;t happen
if the membrane is intact.


The vessel is at the back of the boiler and really difficult to get
to. I had to buy an extension piece to screw into the schrader valve
last time, so that I could reach it. No water came out of the valve
and I did use a bicycle pump but that was over a year ago, so I guess
the pressure has fallen again since then. I thought an external
expansion vessel might just be simpler and easier to access.

Connecting it near to the pump inlet reduces cavitation
in the pump, reducing noise and increasing impeller life.
This probably also requires a low impedance path to the
air reserve, which an 8mm pipe is going to restrict.
If you fit multiple pressure vessels, only one needs to
be connected near the pump inlet.

Would a short length of 15mm be sufficiently low impedance?

Thanks.

On Tue, 5 Feb 2019 18:29:59 -0000 (UTC), Cursitor Doom
wrote:


How do you know that? The common symptoms are the pressure gauge heading
north towards the red zone and the pressure relief valve spewing out
excess water. Seen either of those signs?

Some times there is water out of the PRV and sometimes the system
needs repressurising via the water filling loop.

In article ,
Roger Mills writes:
On 05/02/2019 19:57, Chris J Dixon wrote:


I have a condensing system boiler. Last winter I noticed that it
seemed to need the pressure topping up regularly, but no leaks
were detectable.

During the summer, where it was still in use to heat my
unpressurised hot water tank, no topping up was necessary.

Back in the heating season the issue has returned.

In describing the situation it occurs to me that the difference
is that the hot tank is next to the boiler, so is only raising
the temperature of a small volume of water, compared with the
whole radiator circuit, which latter would thus cause more
expansion.

Chris

Yes, it looks like your system has *some* expansion capacity but not
enough to cope with the whole system getting hot. Re-pressurising the
air in the expansion vessel will likely fix that.

Can you get at the air valve on the expansion vessel? If so, bleed
enough water out of a radiator for the water pressure to drop to zero.
Then check the air pressure with a car tyre pressure gauge. It needs to
be somewhere in the region of 0.7 to 1.0 bar (10-15 psi). If it's a lot
lower than this, top it up with a car tyre pump. then use the filling
loop to re-pressurise the wet part.

I did this. I measured the water extracted by running it into a
2 litre pepsi bottle. It took two cycles of bleeding water out
and pumping up to get 2 litres out (and therefore approx 2 litres
of air in the expansion vessel). I had expected to need to get
more than that in an 8 litre expansion vessel, but 2 litres
returned the system back to normal pressure range during operation.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]