ch expansion vessels

[Stephen]

Hello,

I have a really old combi boiler and I think the expansion vessel needs replacing. What happens to them? Does the membrane eventually perish?

The vessel is at the back of the boiler in a really awkward place, so I was thinking of buying an external one. The manual says the fitted vessel is 8L but says if you add radiators and increase the volume of the system, a larger vessel will be required, which makes sense.

Now I do plan to add a few radiators and I also plan to download one of those spreadsheets to calculate what size radiators I need, so I may upgrade a few of the existing ones. Since a 12L vessel is only one pound more than the 8L one at tool station, does it make sense to buy the 12L vessel to give me some spare capacity?

I note that Screw fix sell a EV isolating valve with drain. Are these useful? Is the idea that you isolate the vessel and depressurise the CH once a year and measure the pressure in the vessel and top it up if necessary?

I know I need to fit the vessel as close to the boiler as possible on the return pipe. Does it have to be plumbed in 22mm or could I use

15mm?

I know there are two spreadsheets recommended here for CH sizing: one in Myson, what's the other and where can I find them?

Thanks, Stephen.

[newshound]

Yes

Not necessarily annually, but a kit does make checking and repressurising much easier. On my system I just fitted the valve and drain when fitting the external cylinder.

15 mm is fine, and I don't think it has to be particularly close to the boiler. Remember, it is not there to control water hammer, it is *just* for expansion.

[tabbypurr]

yes

The expansion vessel is to handle the change in volume as the water heats up. This is a slow process so there's no problem with connecting it via 8mm pipe. I know no reason it must be close to the boiler.

NT

[Jethro_uk]

Might be your lucky week

seems they're all reduced - <£20 can't be bad.

I fitted one a couple of years ago and never looked back.

(I'm still waiting for a genuine reason as to why they aren't fitted as standard. After all if those prices are *retail* then it would add a fiver to a full install of £5K ...)

[NY]

Yes. This allows water into the side of the expansion vessel that normally contains air. This means that the expansion vessel can no longer accommodate changes in pressure that are caused by the pump and by thermal expansion (because water is much less compressible than air), so the over-pressure valve dumps the excess pressure by venting to the drain. Once the water cools down, the baseline pressure is then much lower than it should be.

Hence a sudden loss of pressure which needs to be manually topped up and then disappears again next time the heating turned on is a sure sign that the expansion vessel membrane has failed.

[Halmyre]

You might get away with re-pressurising it.

[Andrew Gabriel]

The other thing is that the air leaks out, and may just need pumping back up with a bicycle pump. That happened to mine after about 16 years, but the membrane isn't leaking. You can tell if the membrane is leaking, as water will come out of the tyre valve used to pump it up - that won;t happen if the membrane is intact.

Connecting it near to the pump inlet reduces cavitation in the pump, reducing noise and increasing impeller life. This probably also requires a low impedance path to the air reserve, which an 8mm pipe is going to restrict. If you fit multiple pressure vessels, only one needs to be connected near the pump inlet.

If that's impossible, it's not a big deal - just the pump may generate more noise and impeller have a shorter life.

Other very remote possibility of connecting the vessel in a bad position is that if the boiler has a water pressure sensor to trip it out on a loss of system pressure, having the vessel connected a log way from the pump or sensor could cause a momentary pressure change at the sensor sufficient to trip the sensor at pump switch-on or switch-off, or if a motorised zone valve closes.

[Cursitor Doom]

How do you know that? The common symptoms are the pressure gauge heading north towards the red zone and the pressure relief valve spewing out excess water. Seen either of those signs?

[Chris J Dixon]

I have a condensing system boiler. Last winter I noticed that it seemed to need the pressure topping up regularly, but no leaks were detectable.

During the summer, where it was still in use to heat my unpressurised hot water tank, no topping up was necessary.

Back in the heating season the issue has returned.

In describing the situation it occurs to me that the difference is that the hot tank is next to the boiler, so is only raising the temperature of a small volume of water, compared with the whole radiator circuit, which latter would thus cause more expansion.

Chris

[Roger Mills]

Yes, it looks like your system has *some* expansion capacity but not enough to cope with the whole system getting hot. Re-pressurising the air in the expansion vessel will likely fix that.

Can you get at the air valve on the expansion vessel? If so, bleed enough water out of a radiator for the water pressure to drop to zero. Then check the air pressure with a car tyre pressure gauge. It needs to be somewhere in the region of 0.7 to 1.0 bar (10-15 psi). If it's a lot lower than this, top it up with a car tyre pump. then use the filling loop to re-pressurise the wet part.

[Stephen]

The vessel is at the back of the boiler and really difficult to get to. I had to buy an extension piece to screw into the schrader valve last time, so that I could reach it. No water came out of the valve and I did use a bicycle pump but that was over a year ago, so I guess the pressure has fallen again since then. I thought an external expansion vessel might just be simpler and easier to access.

Would a short length of 15mm be sufficiently low impedance?

Thanks.

[Stephen]

Some times there is water out of the PRV and sometimes the system needs repressurising via the water filling loop.

[Andrew Gabriel]

I did this. I measured the water extracted by running it into a

2 litre pepsi bottle. It took two cycles of bleeding water out and pumping up to get 2 litres out (and therefore approx 2 litres of air in the expansion vessel). I had expected to need to get more than that in an 8 litre expansion vessel, but 2 litres returned the system back to normal pressure range during operation.