Worcester Bosch green star 28i junior pressure problems

I suspect it is your expansion vessel - most of the symptoms suggest this.

The increase in pressure when the boiler heats up suggests the expanding water has nowhere to go - it would normally go into the exp vessel.

Since the water seemingly cannot go in there, it instead builds up pressure until the PRV opens at ~3bar and dumps the excess water. When your system cools down, the water contracts and - since there is less water in the system now - the pressure drops to zero.

You top it up, and the cycle repeats.

PRVs are very bad at leaking after they've been tripped - have you checked to see if it continues to dribble after the main whoooosh? That would see your pressure continue to drop.

That there is some air in the exp vessel ain't proof that it's ok - you need to check further (see below).

Lots of air appearing in a rad also points to a leaking exp vessel internal - the other cause of rad air is corrosion, but this would tend to be a slow build-up over weeks. I presume you bleed this rad? How quickly does it have air in it again? If it has significant air after chust a day or so, then I suspect that's coming from a ruptured exp vessel diaphragm (when the water side pressure drops right down, air can escape through from the pressurised air side of the vessel).

So you have an air pressure gauge and presumably a tyre pump? Cool. Time to test the exp vessel properly...

Press that Shrader valve. Like, press it, man. Hissssssssssss? Cool. Now keep pressing it until it stops...

Does it chust empty itself of air - or have you also got some water coming out towards the end?

If there's water, stop there - you've found the problem.

If there is no water, then you need to recharge the vessel, but first you need to ensure the water side is at zero pressure.

You do this by bleeding a rad (takes time) or opening a draincock if you have one. You stop when water is no longer being ejected under pressure.

Now pump up your vessel to 1 bar. Release the water side pressure again until it's zero, and see if the air pressure has dropped. If so, bring it back up to 1 bar.

Now you can top up the water side to 1 bar also.

See where that gets you.

 

Change exp vessel as valve or diaphragm could be gone, cut another vessel in somewhere if its easier

 

Hold yer hosses - not so fast

You fully emptied the exp vessel of air and no water came out? That's promising - it might not be a faulty vessel at all.

And the pressure climbed to only 1.5 bar max as far as you know? That's also promising.

But there's clearly water still being lost. When your PRV was opened before by the 3 bar pressure, there is a very good chance that it didn't close off properly afterwards - this is very common - so water is trickling out that pipe all the time. That's easy to observe - chust tie a poly bag over the 15mm pipe outlet.

There is a chance that everything is ok apart from this water loss, so possibly all you need is a new PRV - ~£40 for the part and less than an hour's labour. And whatever else might need fixing, a new PRV will be needed anyways.

If everything works after this, then a pressure increase to 1.5 bar when hot is fine and not an issue, and it should return to 1 bar when cold each time.

Having said that, I'm still concerned about the air turning up in that rad every day. How much is there - how long do you have to bleed it for?

A way to test if it's 'air' (from the vessel) or a by-product of corrosion is to catch some and stick a match into it... If it's a result of internal corrosion, then it'll likely be hydrogen - so will pop or squeak or burn in some way when you light it.

First, tho', please check the discharge outlet pipe to see if the PRV is letting by, and also confirm - NO water AT ALL came out of the Shrader valve, and you discharged the air from there FULLY?

 

Blimey!

Any water that goes in to a rad also comes out the other end - or else the rad would blow... That expanded water will go into the whole rad system to some degree (presumably the rads will swell a small amount!), but it'll return when it all cools down again and the pressure should go back to what it was before. If the pressure has dropped, then either water is being lost, or else air is being lost from that exp vessel.

You say you can 'hear' the water 'escaping' from the exp vessel? Are you sure you aren't hearing air hissing out of it - say the valve?

This is a condensing boiler? So there is also a 22mm plastic pipe heading outside and pointing in to a drain?

If so, keep an eye on that fellow too.

When your boiler is running as normal, you should get a slug of water coming out that condensate pipe every few minutes, but water shouldn't come out of it when the boiler is cold and not running. So please check that.

 

Blimey - that gets mentioned as a possibility by a few plumbers on here, but I've always thought of it as being so unlikely...

Pleased it's sorted - that's great news .

 

One small clarification if possible please. If ev /prv etc suspect and pressure rising to 3 bar then if heating off and only hot water being used will the same events occur?
Than you.

 

Yes because its the same water being heated to heat your hot water then its goes around the plate heat exchanger rather than the heating circuit.

 

Thank you Dave.