Unvented HW cylinder dumps HW

Use it by hand (against the wall{protected}). It only takes a few pumps.

Reply to
<me9
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I wouldn't have thought so.

Reply to
Ronald Raygun

No, that's on the heating side, not the hot water side. The heating system pressurisation is a different kettle of fish and handled by the boiler. The pressure you're having trouble with is the hot water pressure - the secondary, not the primary.

Reply to
Skipweasel

There are two distinct pressurised hot water circuits, each with their own expansion vessels and pressure relief valves. One circuit is for the water which circulates through boiler, radiators, and the heating coil inside the HW cylinder, and its EV and PRV are usually located inside the boiler housing. This is the water which has your inhibitor in it.

The other circuit is for the hot tap water, and this is the one where your problem lies. Its EV is typically located within the HW cylinder. These two circuits (heatING water and heatED water) are completely separate (you don't want your inhibitor coming out of your HW taps) and are independently pressurised. Your tap water is at the same water main pressure as your cold tap water, and my understanding is that the only reason that your hot water circuit needs an EV to absorb volume changes is that there is bound to be a non-return valve somewhere (probably on the HW cylinder's cold water inlet) to prevent back-flow from the hot water circuit back into the cold circuit (the hot water circuit is at times tepid and is potentially a pleasant breeding ground for bugs which could swim back into the public supply and contaminate the whole neighbourhood). If it weren't for this NRV, it would be a simple matter just to let the expanded heated water to "push back against" the cold supply, and there would be no pressure problem. This is similar to the state of affairs in a vented system, where expanding heated water pushes back on the cold supply, which causes the level in your cold water tank to rise a bit, and this is of course isolated from the public supply by virtue of the cold water tank float valve outlet being clear of the water surface.

Reply to
Ronald Raygun

You see what volume the expansion vessel is? At most six times that much, since you said it was supposed to be at six bar.

(Since the diaphragm means it will only be about half full of air when you are done, and it isn't totally empty of air now, it'll be quite a bit less than that in fact.)

Reply to
Alan Braggins

If you got a small air volume, that's good. It probably indicates the thing is not ruptured. It's just lost it's ait volume, and over 10 years, that's not bad. Just pump it up and see how it goes.

The 4 bar is a pre-charge pressure: ie the pressure in the bladder without eny external water pressure. So shut off the cold water inlet to the cylinder, then open the lowest hot tap. Since there's no air volume, it will stop running fairly quickly.

Pump it up unter these condtitions.

Shut the tap, and turn on the cold supply.

No, the symptoms you describe are totally normal for this failure mode.

Reply to
Ron Lowe

If you got a small air volume, that's good. It probably indicates the thing is not ruptured. It's just lost it's ait volume, and over 10 years, that's not bad. Just pump it up and see how it goes.

The 4 bar is a pre-charge pressure: ie the pressure in the bladder without eny external water pressure. So shut off the cold water inlet to the cylinder, then open the lowest hot tap. Since there's no air volume, it will stop running fairly quickly.

Pump it up unter these condtitions.

Shut the tap, and turn on the cold supply.

No, the symptoms you describe are totally normal for this failure mode.

Reply to
Ron Lowe

no.

shut off mains supply, and open two hot taps, one upstairs and one down.

Reply to
The Natural Philosopher

Nope, what the others said - the CH side is not (or at lease should not) be connected to the domestic hot water side at all.

Reply to
John Rumm

Update...

Well the expansion vessel was completely devoid of any pressure at all, but at least there was no water leaking out so no probs with the diaphragm. I then discovered a slow (air) leak on the Schraeder valve so have replaced the valve core, and it seems to be holding OK so far.

Wasn't able to get more than 2.4 bar into the vessel using my footpump against the wall at chest height - likely to be a problem? Bloody hard work towards the end (yes the system was depressurised as direct!) - should it be that difficult? Will it matter not getting to 4 bar?

I'd like to have replaced the whole valve not just the core. However: on top of the vessel, the valve body is held in place by a retaining nut (just as it would be on a bicycle tyre). It was easy to loosen the nut, but it was clear to me if I removed it completely I'd just lose the valve body for good down inside the expansion vessel! So didn't go there... are these things supposed to be serviceable? Can the vessel be dismantled to get at the innards, or will it be a bin job for the whole vessel if my new core still doesn't work?

(It's a Varem 19-litre white - potable - vessel by the way, in case that helps)

David

Reply to
Lobster

Has your system got a pressure limiting valve on the cold feed? If so what is it set to? Mine is set to 3.5 bar - and my expansion vessel is also supposed to be pre-charged to 3.5 bar - so that water will only enter it when it expands due to being heating.

If the vessel is pre-charged to less than the supply pressure, some of its capacity will be taken up by the static cold pressure before it starts to get hot. That may or may not matter - suck it and see! If you have insufficient expansion capacity (remember, you started at zero!) the PRV may continue to discharge when the water gets hot - but I doubt whether that will happen.

If the valve has the same thread as a car valve, you can simply fit a screw-on metal cap containing a rubber washer - which will seal even if the valve core doesn't.

Reply to
Roger Mills

Try a bike pump - most of them are Schraeder these days. A fiver for a cheap one.

Andy

Reply to
Andy Champ

Getting beyond 40psi on the cheap ones can be hard work though... A posh stirrup pump style one would no doubt do it easily if you had a long hose.

Reply to
John Rumm

A decent schrader valve "cap" will hold the pressure if fitted tightly even if the valve weeps

Reply to
cynic

Bet it's easier than a foot pump up against the wall... and anyway, I run my bike tyres _way_ higher than 40 PSI.

Andy

Reply to
Andy Champ

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