Replacing a CH pressure vessel - anticipation of problems to ensure adequate preparation!

[RJS]

The diaphragm in the pressure vessel for my CH system has failed - water dr ips from the Shrader valve when it is opened.

Pictures are here:

4443376800/

So, what should be a simple job is on the cards for next weekend. Doubtles s I will be unable to find an identical replacement so will have to create some pipework between the new PV and the existing over-pressure valve. Tha t aside, the following (possible) problems come to mind:

[1] How do I grip the PV such that I can unscrew it from the OPV without da maging the valve and/associated pipework? I wondered about using a big oil filter strap wrench, i.e. a ratchet strap round the PV and gripping the OP V with a suitable spanner. Note that the threads on the PV appear to be r usty. [2] Should I replace the OPV as a matter of course because it has been open ed to reduce the system pressure when I realised that the diaphragm had fai led. [3] It looks to me from the photo that the PV is lowered into a notch on th e wall bracket which may mean that [2] is a redundant question as I may hav e to cut the OPV valve pipework to remove the PV.

Do any other suggestions or helpful tips spring to mind?

Ta

Richard

[Broadback]

I had to replace mine, it was too much trouble to find a replacement that would fit in the boiler. As we had adequate space we fitted and external one, so just one junction and a bit of pipe to the new expansion tank.

[harryagain]

Pictures are here:

So, what should be a simple job is on the cards for next weekend. Doubtless I will be unable to find an identical replacement so will have to create some pipework between the new PV and the existing over-pressure valve. That aside, the following (possible) problems come to mind:

[1] How do I grip the PV such that I can unscrew it from the OPV without damaging the valve and/associated pipework? I wondered about using a big oil filter strap wrench, i.e. a ratchet strap round the PV and gripping the OPV with a suitable spanner. Note that the threads on the PV appear to be rusty. [2] Should I replace the OPV as a matter of course because it has been opened to reduce the system pressure when I realised that the diaphragm had failed. [3] It looks to me from the photo that the PV is lowered into a notch on the wall bracket which may mean that [2] is a redundant question as I may have to cut the OPV valve pipework to remove the PV.

Do any other suggestions or helpful tips spring to mind?

Ta

Richard

You should just be able to unscrew the two compression joints on the flexible pipe and the relief valve. Holding the PV will not be an issue, it is such a large diameter holding with your hands will probably be enough. It will be quite heavy (full of water.)

[RJS]

Thanks Harry,

Are you suggesting that I remove the PV with the relief valve attached and separate them ex situ? It's only fitted to 15mm pipe and I'm pretty certai n that if I turn the PV without providing any support to the relief valve I will twist its supply pipes. Yes, the filling loop can be removed.

Indeed the PV sounds very heavy when tapped and I will drain as much water from it as possible via the Shrader valve.

Ta

Richard

[harryagain]

Thanks Harry,

Are you suggesting that I remove the PV with the relief valve attached and separate them ex situ? It's only fitted to 15mm pipe and I'm pretty certain that if I turn the PV without providing any support to the relief valve I will twist its supply pipes. Yes, the filling loop can be removed.

Indeed the PV sounds very heavy when tapped and I will drain as much water from it as possible via the Shrader valve.

Ta

Richard

If you're worried about the weight, drill a couple of holes in it (top and bottom) and drain the water out. It's knackered anyway.

[newshound]

I'd be inclined to disconnect the three pipes, then lift out the vessel plus valves, then disassemble. There are flats on the four way coupling, that will probably unscrew with an adjustable spanner while someone else grips the vessel. And, as you suggest, I would replace the pressure relief valve while I was at it. The vessel looks like one of the standard sizes, presumably you have checked BES which I have always found to have an amazing range of hardware at good prices.

[RJS]

r drips from the Shrader valve when it is opened.

57644443376800/

tless I will be unable to find an identical replacement so will have to cre ate some pipework between the new PV and the existing over-pressure valve. That aside, the following (possible) problems come to mind:

t damaging the valve and/associated pipework? I wondered about using a big oil filter strap wrench, i.e. a ratchet strap round the PV and gripping th e OPV with a suitable spanner. Note that the threads on the PV appear to be rusty.

opened to reduce the system pressure when I realised that the diaphragm had failed.

n the wall bracket which may mean that [2] is a redundant question as I may have to cut the OPV valve pipework to remove the PV.

Hi

Picked up the whole thing - PV and relief valve and filling loop in one pac kage, i.e. no option on the relief valve etc, from Drakes this morning. B it damaging to the wallet but at least I got it from a real person who knew what I was doing.

One interesting question that they couldn't answer was whether I could use a flexible hose to connect the PV to the relief valve - the new PV will hav e to be mounted remotely from the relief valve. Thoughts?

Cheers Richard

[harryagain]

Hi

Picked up the whole thing - PV and relief valve and filling loop in one package, i.e. no option on the relief valve etc, from Drakes this morning. Bit damaging to the wallet but at least I got it from a real person who knew what I was doing.

One interesting question that they couldn't answer was whether I could use a flexible hose to connect the PV to the relief valve - the new PV will have to be mounted remotely from the relief valve. Thoughts?

Cheers Richard

The important thing is that there are no isolating valves between the relief valve and the system. Someone might leave it closed which might be catastrophic.

[RJS]

Regarding my question about using a flexible hose to connect the PV to the system: I contacted the suppliers of the PV and asked them if I could use o ne. Their answer is that it used to be permitted but it isn't a wise thing to do because the temperature of the water emerging from the boiler can be above the design temperature of the rubber in the flexible hose. As the w ater temperature rises and falls with the heating cycles the hose could be exposed to boiler-temperature water eventually causing hose failure.

[Roger Mills]

Doesn't that depend on how far the PV is from the boiler - or are they saying that a flexible hose *anywhere* in the primary circuit is a bad thing?

Incidentally, what *is* the design temperature for the hose?

[RJS]

the system: I contacted the suppliers of the PV and asked them if I could u se one. Their answer is that it used to be permitted but it isn't a wise t hing to do because the temperature of the water emerging from the boiler ca n be above the design temperature of the rubber in the flexible hose. As t he water temperature rises and falls with the heating cycles the hose could be exposed to boiler-temperature water eventually causing hose failure.

Hi Roger

The paperwork with my PV says that it, i.e. the diaphragm, can cope with a boiler temperature of 120degC and a cycling temperature of 70degC (I think that those are the right words). The bod at the PV suppliers - I'll check it's name when I go downstairs - suggested that the flexi-hoses are rated f or 70degC. The problem is, he suggested, that boiler temperature water _ca n_ or _could_ flow into the PV thus exposing a flexi hose to water at whate ver temperature it emerges at from the boiler. It's a cautious suggestion to not fit a flexi, that's all. I imagine that the perceived risk increa ses as the PV is mounted closer to the boiler. Mine is about 1.5m from th e boiler.

Richard

[Roger Mills]

Fair enough. You wouldn't normally run the boiler at anything like 120 degrees (unless the system runs at some enormous pressure to prevent boiling) but you may well have primary circuit flow temperatures of around 85 degrees.

[RJS]

Hi Roger

Revised answers having read the paperwork:

It's an Altecnic 25l robokit extra.

Maximum operating temperature = 120degC Maximum continuous operating temperature at the diaphragm = 70degC

Well, I've re-filled it and run both CH and HW for a couple of hours. No gushing leaks - just a weep where the 3/4 fitting screws into the underside of the relief valve fitting. I might just ignore this because the thought of dismantling everything today fills me with horror.

It may just need time to settle, i.e. fully purge the air but I'm not convi nced that the pressure isn't slowly dropping. The usual answer to this is that there is a leak. However, all my pipework is at first floor and above so I would expect to have seen evidence of water leaks over the years. Ma ybe it's a fickle system as it has always shown a very slow pressure loss!

I haven't added any of Mr Fernox's products yet as I'm going to re-drain th e system (tomorrow) to add a couple of valved spurs for an additional rad t hat hasn't been sourced yet.

Cheers

Richard