Charging an Expansion Vessel

ChrisH said:

I agree that that might be the case, but then when the EV was pressurised the water would be expelled and would increase the system pressure, possibly above the 0.7 bar aimed for.

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Why would it be expelled? Where from/to? I don't understand what you mean.

Agile said:

Those others may disagree but if you want to do the job properly you must let ALL the air out with the system still pressurised, that indicates that the diaphragm is still intact and not leaking water to the air side

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No it doesn't! If you remove the insert from the exv fill valve, (say it's at the top, thinking of Ideal Responses because there are so many in trouble!) the air pressure within will go to 1 atmosphere. And sit there. The water itself doesn't (de)compress so won't force all the air out.

Agile said:

Then you have to open the system to atmosphere and LEAVE it open as you pressurise the air to about 0.9 Bar. Doing that will force remaining water out of the EXV to make it empty of water.

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Agreed - its what I say In the FAQ I believe. The difficulty is that bit about leaving the system open to the atmosphere. HOW? If you open anything lower than the boiler, it'll empty forever, as air goes in through the AAV. If you let 10 litres or so of water out, that's enough to empty any boiler's exv as far as it'll go (the diaphragm may be stretched halfway across), and also let you pump it up in one go. There will be a few litres of air in the system, which can compress as you empty the exv completely.
This does mean that there will be some air in the system. Some will go out of the AAV of course, but not all.

Another way to do it would be to pressurise the exv to say a couple of bar, bleed a rad until the gauge reads zero (EXV full of air) then let the exv pressure back to 0.7/0.9. Is this what someone else said?

Agile said:

...that indicates that the diaphragm is still intact and not leaking water to the air side

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Another way to test the integrity of the diaphragm is to see how long it holds the pressure applied to it. An air pressure test is a more demanding test than a water pressure one since air is much less viscous than water and will leak more easily.

ChrisR said:

ChrisH said:

I agree that that might be the case, but then when the EV was pressurised the water would be expelled and would increase the system pressure, possibly above the 0.7 bar aimed for.

Click to expand...

Why would it be expelled? Where from/to? I don't understand what you mean.

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I'm assuming that an EV with an air side charge of as little as say 0.1 bar and a water side pressure of 0 bar (a pressure differential of 0.1 bar) will have its diaphragm extended fully towards the water side wall of the EV, and vice versa. In other words I'm assuming that the diaphragm isn't very elastic and is quite floppy. This may be wrong!

If it's right, then any greater pressure on the air side will expel all water, and any greater pressure on the water side will expel all air, if the air (schrader) valve is open. I hope you can see that my earlier post makes sense based on this assumption.

If the diaphragm was more elastic and required a differential pressure of say 0.5 bar or more to push the diaphragm fully against one or other wall of the EV, then my earlier posts would be wrong. I don't really know how elastic diaphragms are. Does anyone or are we all making assumptions about this?

you must let ALL the air out with the system still pressurised, that indicates that the diaphragm is still intact and not leaking water to the air side

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ChrisR wrote

No it doesn't!

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Are you saying that when the bladder is perforated then no water will leak out through the shraeder valve if I had just opened the shraeder valve on my expansion vessel and connected the filling loop and forced the pressure up to about 4 bar say ??.