Is my expansion vessel too small?

[harrypotter]

I've got a 300L unvented indirect cylinder with a 22L expansion vessel. All installed in 2011. Just been having some issues with the expansion vessel leaking air and while reading online I also leant that it could be too small. Apparently it should be at least 10% of the capacity of the cylinder, so 30L. But at the same time it was professionally installed so who am I to know.

 

[Johntheo5]

The 22L EV isn't the cause of your air leak, I'd say the cause is its 13 years old.
If you heated 300L from 15C to 65C and assuming a 22L EV precharge pressure of 3.0bar then the final pressure will be 4.33bar, if you install a 30L EV the final pressure will be 3.9bar. Most UV cylinders have the expansion relief valve set to 6.0bar but some, probably older ones, are set to 4.5bar.

 

[mcmoby69]

Installed in 2011.
Ever had it serviced?

 

[harrypotter]

The 22L EV isn't the cause of your air leak, I'd say the cause is its 13 years old.
If you heated 300L from 15C to 65C and assuming a 22L EV precharge pressure of 3.0bar then the final pressure will be 4.33bar, if you install a 30L EV the final pressure will be 3.9bar. Most UV cylinders have the expansion relief valve set to 6.0bar but some, probably older ones, are set to 4.5bar.

Something weird is going on, I don't know. So the expansion tank now seems to hold pressure. No water coming out of the Schrader valve so I assume the valve was sticky or something.

However, yesterday morning I isolated mains, opened hot and cold taps and charged the vessel up to 3 bar. When the water started heating up in the evening I noticed the 6 bar expansion relief valve was letting water through the tundish. I got my pump out and the vessel was indeed closed to 6 bar. Again, checked no water from Schrader.


This morning I checked the pressure again - 4.3 bar. I isolated mains water and opened taps again and the vessel was at 2.5 bar. Seems fine as I was testing the Schrader valve a lot. Any ideas?

 

[Johntheo5]

The cold water PRV supply could be leaking past (no longer drop tight) and building up pressure, next time the heating is off just open a hot tap and drain off say 5L of water, shut the hot tap and measure the pressure at the schrader valve, ensure nobody draws off any HW for a hour or so, the longer the better) also ensure no reheating takes place during this time, then measure the (air end) pressure again at the schrader valve.

 

[harrypotter]

The cold water PRV supply could be leaking past (no longer drop tight) and building up pressure, next time the heating is off just open a hot tap and drain off say 5L of water, shut the hot tap and measure the pressure at the schrader valve, ensure nobody draws off any HW for a hour or so, the longer the better) also ensure no reheating takes place during this time, then measure the (air end) pressure again at the schrader valve.

That's the 3 bar pressure reducing valve on the inlet multiblock right?

So I did what you said. Drained a good amount of HW from the taps. I noticed it came out of the faucet quicker than normal then reduced down a bit. Measured the pressure on the Schrader which was 3 bar. Touched nothing for 2 hours or so, boiler stayed off, and pressure of the vessel is now 5 bar.

I might be misunderstanding something however. If the pre-charge of the vessel is 3 bar, that is with no water pressure the other side of the diaphragm, then when the cold mains comes on shouldn't it increase to 3 bar plus the mains water pressure?

 

[Madrab]

Unvented cylinders are a regulated appliance and they shouldn't really be played about with by anyone that isn't qualified.

My advice would be to get your system serviced by a qualified unvented engineer.

 

[Johntheo5]

That's the 3 bar pressure reducing valve on the inlet multiblock right?

So I did what you said. Drained a good amount of HW from the taps. I noticed it came out of the faucet quicker than normal then reduced down a bit. Measured the pressure on the Schrader which was 3 bar. Touched nothing for 2 hours or so, boiler stayed off, and pressure of the vessel is now 5 bar.

I might be misunderstanding something however. If the pre-charge of the vessel is 3 bar, that is with no water pressure the other side of the diaphragm, then when the cold mains comes on shouldn't it increase to 3 bar plus the mains water pressure?

No, the PRV if not leaking past should maintain the pressure at 3bar, the pressure will then rise a little when the 1.5L or so of expanded water enters the EV, you need to get a new drop tight PRV installed by a G3 qualified person.