Expansion vessel failure

[jeff the gasman]

It’s much the same as tyres deflating when a car isn’t being used.
Think it’s called Graham’s law of absorption or something similar.

 

[Johntheo5]

You can get condensation building up in the air side of the expansion vessel over time that gets in there when its pumped up. If the air in the air side has leaked out so that the air gap has gone, its possible that its only the condensation that's left in the air side that you're seeing come out of the valve.

Personally, i think its unlikely in my opinion, but possible. If its a significant volume of water coming out then it's probably just a failed expansion vessel.

I've looked after four close relations oil fired systems for years (I have a OV system), one of which has a upside down EV with the air side on the bottom and I might get a eggcup full of water or less every year from the schrader valve when I drain it. EVs when supplied should have been precharged with nitrogen and not air.

Here are a few calcs that I've used for years, and my metod of calculation.might find interesting.

 

[danlightbulb]

look up your boiler spec and see what size EV is installed

The one fitted to the combi is the flat pan type 6 litre one, pre-charged to 0.75 bar as standard. Worcester Bosch part number 87161055450.

I am about to try to pump it up, will try for 0.75 bar to match the factory setting. Will report back shortly.

 

[Johntheo5]

Good Luck!. The air&water sides are all one, now.

 

[danlightbulb]

Good Luck!. The air&water sides are all one, now.

Well yeah, I think this will only prove that the vessel is gone, which we're 99% sure of already.

 

[Johntheo5]

The one fitted to the combi is the flat pan type 6 litre one, pre-charged to 0.75 bar as standard. Worcester Bosch part number 87161055450.

Pre charge of 0.75bar but do they give a filling pressure?, some but not all boiler manufacturers do give this but others are more coy, reason being that if the filling pressure is the same as the pre charge pressure then the EV can satisfy a much greater system volume, big downside of this is that there is no water reserve to make up for microscopic air bubbles collapsing etc leading to frequent top ups.....people get fed up with this so install a auto fill system which is just a drop tight PRV, if even a very small leak develops then no clue as the pressure remains spot on but it wlll very rapidly lead to corrosion/sludge and pin holed rads etc.
A 6L EV with pre&fill pressures of 0.75/0.75bar will rise to 2.3bar (hot) with a system volume of 154L.
A 6L EV with pre&fill pressures of 0.75/1.25bar will rise to 2.3bar (hot) with a system volume of 81L but with a 1.33L water reserve.
A 6L EV with pre&fill pressures of 0.75/1.0bar will rise to 2.3bar (hot) with a system volume of 113L but with a 0.75L water reserve.

A 8L EV with pre&fill pressures of 0.75/0.75bar will rise to 2.3bar (hot) with a system volume of 205L.
A 8L EV with pre&fill pressures of 0.75/1.25bar will rise to 2.3bar (hot) with a system volume of 108L but with a 1.78L water reserve.
A 8L EV with pre&fill pressures of 0.75/1.0bar will rise to 2.3bar (hot) with a system volume of 151L but with a 1.0L water reserve.

 

[danlightbulb]

Closed all upstairs rads to not lose too much system water.

I am a keen cyclist and have a range of pumps, none of which would seal very well against the EV schrader valve. I resorted to a mountain bike shock pump which has a screw on fitting, but is very low volume so it took ages. Pumped it for a while and checked pressure with a digital gauge: 1.67 bar. So step 1, it is allowing inflation. I left it for 20 minutes, and checked the pressure again. Obviously lose a little bit of air every time the pressure gauge is pushed on but it is still reading 1.66 bar. So its holding pressure, at least short term.

Then I deflated it back down to approx 0.8 bar and closed up the valve cap. Closed my drain connection, and began slowly refilling. Opened upstairs rads, filled some more.

Shall I refill the kitchen rad that I set up as a temporary EV last night?

And now fire up the system see if it works ok?

 

[nitro23456]

No keep it like that forever

 

[Johntheo5]

What you now have IMO is a EV with effectively no diaphragm, it may be full of air or the air may have dissipated into the system, much the same result as if you had attached a schrader valve to your temporary rad, are you now only getting air out of the schrader valve, if so and you depressurize by keeping the schrader valve open then once all the air has been expelled and assuming good diaphragm there will be no pressure whatsoever at the air end, it has to work that way, no ifs or butts.

I would strongly recommend doing this ASAP otherwise your whole system will eventually get air in it (if diaphragm punctured).

 

[danlightbulb]

What you now have IMO is a EV with effectively no diaphragm, it may be full of air or the air may have dissipated into the system, much the same result as if you had attached a schrader valve to your temporary rad, are you now only getting air out of the schrader valve, if so and you depressurize by keeping the schrader valve open then once all the air has been expelled and assuming good diaphragm there will be no pressure whatsoever at the air end, it has to work that way, no ifs or butts.

I would strongly recommend doing this ASAP otherwise your whole system will eventually get air in it (if diaphragm punctured).

Sorry John, please clarify what you're saying I should do? You want me to test for air pressure on the shrader valve again and fully deflate the vessel?

I'd have to then re-do all the draining and pumping up procedure?

Also - should I now reinstate the temporary EV rad as a rad, or keep it as an EV for now as backup?

 

[Johntheo5]

No, fill up your system as before, you can leave the temporary rad as is. keep around 0.75bar at the water end, it doesnt really matter.

You have a 6L EV, and assuming a intact diaphragm, then the max volume of water that can be in the air end, even if magically introduced into it is 6L??, get a bucket with line scribed in it at say a 7L level, hold it under the schrader valve and release all the pressure, if the pressure doesn't fall to zero with/without 6L in the bucket then diaphragm is holed?.

The pressure HAS TO fall to zero at the air end otherwise the diaphragm is punctured??.

 

[terryplumb]

I

Closed all upstairs rads to not lose too much system water.

I am a keen cyclist and have a range of pumps, none of which would seal very well against the EV schrader valve. I resorted to a mountain bike shock pump which has a screw on fitting, but is very low volume so it took ages. Pumped it for a while and checked pressure with a digital gauge: 1.67 bar. So step 1, it is allowing inflation. I left it for 20 minutes, and checked the pressure again. Obviously lose a little bit of air every time the pressure gauge is pushed on but it is still reading 1.66 bar. So its holding pressure, at least short term.

Then I deflated it back down to approx 0.8 bar and closed up the valve cap. Closed my drain connection, and began slowly refilling. Opened upstairs rads, filled some more.

Shall I refill the kitchen rad that I set up as a temporary EV last night?

And now fire up the system see if it works ok?

Did you achieved 1.66 bar at the EV ,With an open drain cock on the central heating system ?
If you did the membrane can't be perforated .

 

[danlightbulb]

No, fill up your system as before, you can leave the temporary rad as is. keep around 0.75bar at the water end, it doesnt really matter.

You have a 6L EV, and assuming a intact diaphragm, then the max volume of water that can be in the air end, even if magically introduced into it is 6L??, get a bucket with line scribed in it at say a 7L level, hold it under the schrader valve and release all the pressure, if the pressure doesn't fall to zero with/without 6L in the bucket then diaphragm is holed?.

The pressure HAS TO fall to zero at the air end otherwise the diaphragm is punctured??.

I think I get you but not 100%.

If I press the shrader valve now, I would expect some air to come out first (the air Ive just put in, which will be at the top of the vessel). Then the water that was in there before, if it hasn't drained away, might also come out.

If that water keeps coming, the diaphragm is holed I agree.

But then I will have let all the pressure out so I will need to repeat my draining and pumping up procedure again to refill it with air?

And also the vessel would have drained down with my system, so the wet side should be full of air too now (the vessel is upright so air will rise to the top and stay there?)

I

Did you achieved 1.66 bar at the EV ,With an open drain cock on the central heating system ?
If you did the membrane can't be perforated .

Yes that's correct, I got the pressure up to 1.6 bar with the drain valve open, left it for 20 minutes, then deflated it back to 0.8 bar to bring back into service.

 

[terryplumb]

Fill the system ,bleed air from all rads including the one used as a makeshift EV ,fill to 1 bar.
And run c/ heating. See how you get on .

 

[Johntheo5]

Very good then, just open the schrader valve

I think I get you but not 100%.

If I press the shrader valve now, I would expect some air to come out first (the air Ive just put in, which will be at the top of the vessel). Then the water that was in there before, if it hasn't drained away, might also come out.

If that water keeps coming, the diaphragm is holed I agree.

But then I will have let all the pressure out so I will need to repeat my draining and pumping up procedure again to refill it with air?

And also the vessel would have drained down with my system, so the wet side should be full of air too now (the vessel is upright so air will rise to the top and stay there?)


Yes that's correct, I got the pressure up to 1.6 bar with the drain valve open, left it for 20 minutes, then deflated it back to 0.8 bar to bring back into service.

This is just what I would do, with system full and pressurised, open the schrader valve, if the diaphragm is Not punctured then the pressure will die off very quickly and prove without doubt that the EV is OK