DIY isolation of unvented cylinder

[jhs14]

Hi all

I've agreed to do a simple job for a friend on their plumbing system - they have a weeping joint on the domestic hot water, so a simple isolation of the system and replace the joint. They have a boiler downstairs feeding an unvented cylinder in the loft. I didn't see an expansion vessel so I assume the cylinder is a megaflo or similar with an internal expansion.

There's no isolation between the feed from the unvented cylinder and the leaking joint, so in order to do this job I'll need to isolate the feed to the cylinder and effectively de-pressurise the cylinder.

Easy I thought - but since then I've been reading about how unvented cylinders are G3 appliances and shouldn't be touched by unqualified persons, so I just wanted to check that what I'm proposing to do sounds OK. I think I'm just paranoid from the rare news articles of these cylinders going badly wrong - but I'm not going to be touching any of the controls or pipework relating to the actual cylinder so I think it is just paranoia.

So my plan is:

- Isolate the gate valve on the cold feed to the cylinder
- Open a hot tap downstairs and wait for the pressure to drain off
- Repair the leaky joint
- Reopen the gate valve on the cold feed to the cylinder
- Wait for hot tap to stop spluttering
- Job done

And that should be it... Do I then need to follow the instructions for recharging the internal air gap in the cylinder through the TP valve (which again I understand is a job designed for the user to be able to do?) or is that unlikely to be necessary?

Thanks,

 

[muggles]

Yes that's fine (although if it really is a gate valve, which is designed for low pressure systems only, I wouldn't hold out too much hope of it fully isolating the cylinder). Always worth recharging the air gap on these while they're drained down - it should really be done as part of the annual service, but I bet your friend has never had it serviced!

 

[ianmcd]

it shouldnt have a gate valve to isolate the cylinder, just turn off the mains stop cock coming into the property and as @muggles says good idea to reinstate the bubble while drained down

 

[jhs14]

Thanks - yes from memory I'm sure it was a gate valve - I'll just use the stopcock then.

It's a fairly new cylinder and I think he does keep it up to date with servicing so no worries there

To clarify - charging the airgap when the system is already depressurised (not drained down) should simply be a matter of quarter-turning the TP valve above the tundish until it stops gurgling?

Thanks again

 

[ianmcd]

The instructions will be on the cylinder

 

[fezster]

To clarify - charging the airgap when the system is already depressurised (not drained down) should simply be a matter of quarter-turning the TP valve above the tundish until it stops gurgling?

Thanks again

Correct, but also opening a hot tap LOWER than the cylinder whilst doing it, to allow water out and air in. As per instructions on the side of the cylinder.

If like me, your cylinder is downstairs, you have to drain the cylinder down to below TPRV to achieve this.