Suspect expansion vessel "problem" with Worcester Bosch Greenstar

[larkim]

Noticed the other day that pressure was at zero on the boiler. Hadn't checked for ages, so just assumed we'd lost a little through one small leak which we'd kept an eye on over the years on an old radiator.

Re-pressurised to 1.5 bar or so and moved on.

However, next morning noticed that it had fallen again to zero. Confused and dazed (it was Christmas) I simply topped up again.

However, it is now clearly falling back to zero when cold, and when warmed up it is getting back to a more sensible pressure (in the "green" zone on the pressure display).

Having googled, it seems the expansion vessel is a likely culprit, and we've not spotted any leaks at all anywhere else in the system (even the old one is bone dry, and probably has been for quite a while now).

I'm not keen to get stuck in with gas / water (partly as I'm not allowed and partly as I recognise the danger and damage I could do!), so two questions - firstly, if the EV is knackered, what is the impact? And secondly, if the EV is knackered, what's ballpark replacement and fitting cost likely to be (e.g. how many hours labour, materials cost etc etc)?

Or third question - in fact, is this something I should be trying a DIY solution for?

(I think the boiler is a Greenstar 27cdi but could be wrong)

[Bob Minchin]

I watched a youtube video on this the other day. Earlier models have the EV mounted on the wall frame behind the boiler which means the whole boiler has to be taken off the wall to get to it. Newer ones have it inside the boiler cabinet and you can see it as soon as you open up the boiler.

If you have room, a simple, cheaper, easier solution is to fit a second EV somewhere in the circuit and leave the defunct one in situ.

Lots of EV kits on Ebay from respectable plumbers merchants

[Roger Mills]

If the expansion vessel *has* failed, and it is inside the boiler casing, it's often easier and cheaper to abandon it and fit another one externally - or even leave a radiator only half full in the short-term.

But first of all, check that it actually *has* failed. It may simply need re-pressurising, or the pressure relief valve may be leaking water to the outside world. [Come back if you need more detail about exactly what to do.]

[John Rumm]

It may just be it needs pumping up. You would need to find the filling valve for it (it will be a shrader valve as you would find on a car tyre), and give the pip in the middle a prod. If nothing comes out, then I would try representing (foot pump etc). If water comes out, then the internal diaphragm has failed. In which case add an external EV and ignore the internal one unless its particularly easy to get at and change.

Yup, there are plenty of DIY options, even if you decide no to bother doing anything with the internal EV. As Roger said, you can just leave some air trapped in the top of a rad as a "get you going" fix for the moment.

[newshound]

My similar (relatively new) Vaillant has started showing this behaviour, and I already have an external vessel (from the previous boiler), repressurising didn't fix the problem. So I suspect I have an unidentified leak (I don't think it is from the pressure relief valve but I havn't done the plastic bag check yet). So I am thinking it may be time to add Fernox or similar leak sealant via the Magnaflo.

[larkim]

Boiler is about 7 years old, Greenstar 27CDi. Not sure if that helps to diagnose where in the boiler the EV is located.

If there is no working EV in the system, what's the worst (or most likely) "bad" thing that can happen before I get it fixed? Will it overflow naturally, or will something disastrous happen somewhere in the system?

[John Rumm]

The service manual will tell you where it is - but it should be fairly obvious by inspection. The only significant implication of its location is the difficulty and hence cost of getting at it if you were having it replaced. Often painted red if it helps.

There will be no way for the system to accommodate the expansion of the water as its heated. So that will force the emergency pressure release valve (PRV) to operate, venting system water outside via the blow off pipe. Next time the boiler cools there will be inadequate pressure to restart. Hence you get into a cycle of needing to top up the water every day. That in turn depletes the corrosion inhibitor and that leads to more system corrosion. It might also cause a failure of the PRV valve.

As mentioned before, if you need a temporary fix, partially drain a rad and then reseal. That will trap a pocket of air that will work as an EV for the time being.

[larkim]

Excellent, thanks for the clarity! Will take a proper look tonight.

[Roger Mills]

If there's no pressure in the system, the boiler may refuse to operate.

If you top it up manually when the system is cold, the pressure will rise when it heats up such that the pressure relief valve opens and lets some out. Then, when it cools, the pressure will fall again . . .

Under normal circumstances, the pressure rise would be absorbed by the EV with no loss if water.

Apart from the inconvenience of having to keep topping it up, you are introducing fresh oxygenated water each time and diluting any inhibitor in the system, thus increasing the risk of corrosion.

[Jamie]

replying to John Rumm, Jamie wrote: Does anyone know what pressure the diaphragm should be when checked with a gauge or pumped back up?

[Jamie]

replying to John Rumm, Jamie wrote: Does anyone know what pressure the diaphragm should be when checked with a gauge or pumped back up?

[Brian Gaff]

Interesting that this thread started in January, but posting via this broken web based system seems to completely break the threading of messages here in Usernet, which somehow defeats the point of having it connected to the system in the first place! Brian

[John Rumm]

Its normally filled to around the same pressure as the minimum operating pressure for the primary water... So often around 0.75 to 1 bar.

That way at minimum water pressure the bladder is just starting to compress - and you then have (most of) the full volume of the vessel available for expanding water.

Note it needs to be pumped up while the pressure is released on the wet side (so bleed an upstairs rad until no air or water comes out).

[John]

My combi is doing this Worcester . If repressured daily For a few weeks Will this corrode the rads?

[Vir Campestris]

It's possible this means you have a water leak somewhere, in which case where is it going?

It's 3 years since anyone else posted on that thread. HoH has its problems.

Andy