Charging an Expansion Vessel

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Hi

I have read that to charge a CH expansion correctly that I have to drain off the CH water until the pressure makes sense so that you can read simply the air pressure in the expansion vessel.

I understand the reason but I am loathed to drain some of the CH water off as I have just managed to get all the air out of the system.

Do I have no option but to drain some of the water or is there perhaps a rule of thumb than can be applied ?

Cheers
 
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Isolate the boiler from the ch system then drain just the boiler.
 
open bleeding screw on a rad and drain of half a litre at a time til pressure is below 0.5
 
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Unfortunately the information given in the FAQ is incorrect.

There is no point in trying to drain off 10 li as stated.

Also before starting ALL the air should be let out of the EXV to ensure there is no water in the air side. However, that is not necessary on the types with the air valve at the lowest point.

Tony
 
The point of the exercise is to improve the situation for a diy-er who has a problem with draining the system, not to produce a manual of the most perfect way of starting from scratch theoretically.
As long as the system has enough pressure to start safely when cold and enough compensation to stay under 2.5 bar when fully hot, OP is happy.
Worst case scenario he can do it twice without getting his knickers in a twist.
If you want it done REALLY as it should, the only advice would be to call in a RGI.
 
You need to ensure that there is no water in the water-side of the expansion vessel before you re-pressurize it.

Domestic pressure gauges are usually cheap and nasty and it is inadvisable to rely on them. The only way you can ensure you have got 0 pressure on the water-side of the expansion vessel is to drain it.

If you want it done REALLY as it should, the only advice would be to call in a RGI.

An RGI is qualified to work on gas, only gas and nothing but gas.
Many RGIs have acquired knowledge and experience of heating installations, but this is not a part of the ACS qualification.
A lamentable number of RGIs are clueless about heating systems.
 
You need to ensure that there is no water in the water-side of the expansion vessel before you re-pressurize it.
Not so. As long as the system pressure is below the precharge pressure that you want to achieve then it will be OK. For example, if you want to pressurise the EV to 0.7 bar, then release the system pressure to achieve say 0.5 bar or less. Then pump up the EV to 0.7 bar, checking that the system pressure remains at or below 0.5 bar (if the system pressure has risen then release more water until it is back to 0.5 bar or below).

You will then have a fully charged expansion vessel since the rubber diaphragm will be pressing on the water side of the EV. Only when the system pressure starts to rise above 0.7 bar will it begin to push the EV diaphragm away from the water side.
 
You need to ensure that there is no water in the water-side of the expansion vessel before you re-pressurize it.
Not so. As long as the system pressure is below the precharge pressure that you want to achieve then it will be OK.

Which is a different way of saying the same thing.

If there is some water in the expansion vessel, it will be at the same pressure as the air charge.

If there is no water in the expansion vessel, but in contact with the diaphragm, it can be at a lower pressure or the same pressure.

You can come unstuck if you use the existing system pressure gauge to tell you that the system is at a lower pressure. They very often don't have a current calibration certificate. :eek:

If you drain it, then you know it's at atmospheric pressure.
 
Agile said:
Unfortunately the information given in the FAQ is incorrect.

There is no point in trying to drain off 10 li as stated.
Why don't you say why you think so? If I think you're right I might even change it! But I think you're wrong.

Agile said:
Also before starting ALL the air should be let out of the EXV to ensure there is no water in the air side. However, that is not necessary on the types with the air valve at the lowest point.
You can't "LET all the air out"!! If the valve is at the top you'd have to FORCE it out by opening the valve then pressurising the system to push the diaphragm fully into the air half.
They often have a little water on the air side, without giving problems.
 
You need to ensure that there is no water in the water-side of the expansion vessel before you re-pressurize it.
Not so. As long as the system pressure is below the precharge pressure that you want to achieve then it will be OK.


I disagree, If the system gauge is showing any pressure at all, then the exv isn't empty of water. It might only have 100cc of air in it.
 
Ben said:
open bleeding screw on a rad and drain of half a litre at a time til pressure is below 0.5
Disagree. If it were full of water, you only have a 0.5 litre expansion vessel after your procedure!

The point of the exercise is to improve the situation for a diy-er who has a problem with draining the system, not to produce a manual of the most perfect way of starting from scratch theoretically.
Why boither giving incorrect advice when the only difference is the quantity of water let out? :confused:
 
I disagree, If the system gauge is showing any pressure at all, then the exv isn't empty of water. It might only have 100cc of air in it.
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. That is why I said that the system pressure would need to be checked again once the EV is repressurised.

Of course it's simplest to let the system pressure discharge to zero, and that is what normally happens. But if for some reason the EV needed repressurising while the system pressure remained at say 0.5 bar then it is possible to do it that way.

With reference to the FAQs, I don't see why it should be considered necessary to drain a further 10 litres once the system pressure is down to zero. What if there is no water in the EV? Then where is the extra 10 litres going to come from?
 
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

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.

Only by doing it properly will you end up with the system operating optimally.

If you want to do a bodge job then just pump the air up to 1.5 Bar and go to the pub and get drunk! If its not your own then charge the customer £65 for a bodge job done in 10 minutes.

Tony
 

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