Need help with the faq on repressurising an expansion vessel

[JohnB47]

Hi. My first post here.

In the faqs section, there are great instructions on repressurising an expansion vessel but I'm not sure about one point. One part says:

"Drain enough water from the system to top drop the water pressure as read on the boiler gauge, to Zero. Close the drain cock. "

Then later it says:

"When you get to 10psi, undo your drain cock again. If there was still water in the PV, your air pressure will force it out. If that happens, recheck the pressure and pump more, and repeat until undoing the drain cock doesn't give you a spurt of water. "

The thing is, surely water is just going to come out of the drain cock at this point, as you wouldn't have fully drained the system. So is it that you should just get a steady flow of water, as opposed to a spurt? And anyway, wouldn't you expert a sort of spurt, because you've effectively increased the pressure on the system side by pressurising the air side of the diaphragm 15 psi?

Thanks.

 

[Burnerman]

When you add air to your expansion vessel, the rubber diaphragm that separates the two halves of the vessel tries to compress the water in the central heating circuit......effectively it bulges towards the water inlet.
By releasing this water pressure, its possible to get an accurate air pressure reading inside the EV. Hence a small amout of water will escape at the point where the system is opened - be it a drain cock, radiator valve et al. Once the pressure is released, any spurt of water stops.
The system won't drain itself because it is sealed, and air would have to be introduced somewhere for the water to flow out.
John

 

[JohnB47]

Thanks for that.

I see what you mean but then how will water be drained if I follow the instructions:

"Find a drain cock on the pipes somewhere, or on the boiler. DO NOT use the pressure relief valve, or it WILL leak afterwards. If you can't find one you can use a radiator bleed point, but that's in a pinch, because you'll be letting several litres of water out.Drain enough water from the system to drop the water pressure as read on the boiler gauge, to Zero. Close the drain cock."

Pehaps it should say to open the bleeder on an upstairs rad first - to let air into the system? Then close it off when doing that second bit where you watch for a spurt?

Ah, hang on. Water will come out initially because its under pressure. But when pressure reaches zero, the flow will stop, apart from that possible spurt. Is that it?

 

[Gas2Air]

That guide is good, but not fool proof. I wouldn't advise anyone to do it by a rad bleed. Basically you can't pump an expansion vessel up on a closed/sealed system. The expansion you give via foot pump will, if the hose or pipe isn't blocked to EV, will push water out, sometimes a lot. So best way is a drain off, close to boiler or within the boiler.

 

[Burnerman]

I usually crack open the flexible braided water connection to the EV - so long as its nicely available!
John

 

[Agile]

John, you are not draining the system!

Merely reducing the pressure to zero.

There are some ways it could be improved and I would be willing to do that.

Tony