Worcester Bosch Greenstar 29 CDi Classic ErP - new boiler losing pressure when heating off

A warranty job? That depends on the cause - eg a failed exchanger inside (in which, yes.)

First, there are some Qs and tests you can do.

Ok, I'm guessing you top up the boiler when it's a zero? You take this to - what? - 1 bar? Cool.

Now, what happens to that pressure reading when the boiler is turned on and gets to hot working temp? And then when it's turned offski at night? And then in the morning?

(Lots of possibilities going orn, but let's start with chust that )

 

Ok. For the pressure to drop that much, we are talking about probably a cupful or two of water - quite noticeable. Ie - not a drip from a rad valve. So your rads ain't the issue I don't think.

That boiler is a high-efficiency type, so has a condensate drain pipe coming from it which is 22mm dia plastic. It'll also have a 15mm copper pipe coming from it which is the safety discharge pipe - this only let's out water when the pressure goes over 3 bar due to a fault.

Both these pipes head outside and usually point down in to a drain - can you ID them?

Cool.

Ok, the copper pipe should NEVER have water coming out of it. If it does, there's yer problem. You can check on this by taping a small clear bag around the outlet and seeing if it catches water.

The plastic pipe WILL have slugs of water coming out at regular intervals when the boiler is running, but this should come to a halt pretty soon after the boiler shuts orf, and no water should come out whilst the boiler is not running. Do NOT tie a bag around this pipe! But, if you know the boiler ain't going to be used for a good few hours, you could place a jar under it to check - but you need to shield off rain.

Keep topping up the boiler to 1 bar or just above - I wouldn't go as high as 1.5, but no issue if you do.

Is it an 'analogue' gauge - can you see what the pressure is at all times? If so, what is the HIGHEST it goes to when hot?

 

When water is heated, it expands - a lot. This would normally cause a sealed system to, well, explode. Or to well explode.

Except it has a safety valve to prevent that.

It also has a thing called an expansion vessel inside the boiler which is a tank with a rubber membrane in t'middle and will compressed air on one side and the system water on t'other. The idea is that the expanding water is forced in to that tank and pushes the rubber dia against the air, compressing it even further. The diea is that the pressure should remain fairly constant betwixt hot and cold and not vary much at all.

The fact that your system pressure is fluctuating as you describe suggests that the exp vess ain't doing its job properly.

It would therefore be particularly interesting for you to monitor the 15mm copper 'discharge' pipe outside. Can you do that tomoz?

 

Get the installer back to fit your condensate correctly - it should be a minimum of 32mm externally (including just before it passes through the wall to outside). He also needs to fit an external air break to it before entering the rainwater down pipe. It's all in the installation manual he didn't bother to read.

At the same time he can check the heat exchanger isn't leaking.

 

Fair points by T'Morg. If that condensate pipe freezes -as they are prone to do - then your boiler will pack up.

Yes, that copper pipe is the safety discharge. It points back at the wall so that it doesn't spray passers-by with scalding hot water if the system goes awry

If you could monitor this, that would be useful - this covers the most likely cause of pressure loss, so worth doing (and is usually pretty good news too as it's normally a straight-forward repair)

The plastic - condensate - pipe ain't going to be monitorable since the end disappears, so forget that one

Analogue gauge - that's good 'cos it gives an on-going readout.

(The initial pressure drop is something I've seen in my sis-in-law's boiler too - I suspect it's due to the pump coming on and causing a slight drop in pressure in the boiler as it 'sucks' the water oot and round. When the system then heats up, the pressure climbs due to expansion.)

Anyhoo, I suspect your exp vessel ain't as charged as it could be, and ain't doing the pressure stabilising job as fully as it should.

 

'Gurgling' usually means air. That could possibly be from a perforated expansion vessel (the rubber membrane allowing the compressed air in to the system water) or perhaps it's just air that's in the water you keep topping it up with. Try bleeding the noisy rad and see what comes out.

The EV comes pre-charged, and usually keeps its pressure for years. If it fails/loses pressure then you can expect the system pressure (gauge) to fluctuate wildly, but certainly increase when on - which yours isn't really doing. The classic outcome then is that the pressure hits 3 bar which opens the safety valve to discharge the excess, but then the safety valve doesn't shut properly afterwards so continues to leak - ie loss of system pressure.

But, if you have no water in that plastic bag, it ain't that...

So, it's now down to either a leak within the boiler (usually not good...) or one on the rad circuit (not good if it's under the floor...)

There is a way to check, but the boiler needs keeping off. Under the boiler you will find all the pipes joining it. Two of these - both copper and both 22mm - are the CH 'flow' and the CH 'return'. These are the pipes that go to your rads. (Your boiler is a 'combi', isn't it? You don't have a hot cylinder?)

Ok, what you do is, make sure the boiler is pressurised to at least 1 bar. Then turn off the main power switch to the whole CH system so it CANNOT come on. Identify the flow and return pipes underneath, and you'll find small taps that can isolate them - perhaps it needs a screwdriver to give it a quarter turn (at the moment the slots should align with the flow - turn it to right-angles to the flow).

Make sure it's the correct pipes - look up your manual - on-line if need be.

Then you simply leave it for as many hours as you can - at least for as long as it's taken for the pressure to drop in the past.

Ok, if the pressure doesn't now fall - and if it does as soon as you open one of these valves again - then the leak is in your rad system, pipework etc.

If the pressure falls when both these valves are still shut, then the leak is within the boiler, and chances are it's the main exchanger - the single most expensive part .

Why am I -ing? Because almost certainly you can oblige WB to replace it under warranty.


(Make sure you fully open both valves again before you turn on the boiler...)

 

Don't anyone do a dry pressure test on CH pipe work before connecting to a boiler nowadays, sometimes old ways are the best.

 

Phew...

 

I suspect - as you do - that it'll be under the ground floor for the reason you said - you would surely have noticed an internal leak by now.

No, isolating individual rads won't help - unless it was the actual rad wot was leaking.

Anyhoo, come back with the result from opening the valves back up.

 

Had a quick look. Depends on what cover you have. If you have central heating there is a good chance. If you have bought this cover as a result of a mailshot or campaign, and the boiler was NOT fitted by British Gas, I would tell them as little as possible - don't tell them about trying to trace faults, plumber trying to find faults etc. Just say "had a boiler fitted a while ago. All has been fine, and now its broke. Please come and fix it". If you give them any other oinfo it'll just give them reasons to try and wriggle out of sorting. Think you'll still have a bit of a fight. Cynical about insurance? Moi?

If you locate a pipe section that is leaking in due course, don't necessarily think about ripping up decoration and floors to access it. Sometimes it is just easier to route a new pipe section to bypass the damaged section and leave it in place.