Worcester Greenstar 28i, pressure gauge

[amfisted]

This issue is pretty well covered right across the internet, but the answers given tend to be ambiguous so I'm hoping for a clearer diagnosis here. :0)

My problem concerns the pressure gauge on my 7 year old Greenstar 28i. On occasions I've topped up the system because the pressure has been low, but it's never strayed into the red when in use. The other day when I put the heating on for the first time this year it shot up into the red zone, way over 3 bars. When cold it drops to normal, around 1 bar.
I've bled all the rads, I've checked the overflow pipe outside but there's no apparent drip from it when the boiler is operating, and although the inside of the pipe is wet I'm uncertain as to whether that's from an overflow or from the rain. The boiler mains feed is definitely not open.

I've read that the expansion vessel might need pressuring, which judging by some of the chatter on the internet is a fairly simple procedure; but I don't want to fiddle with stuff I'm not familiar with and risk making things worse. Recently I did some work in the bathroom, including fitting a new toilet, and for a while afterwards I noticed a banging noise from within the boiler when the hot taps were turned on and off. No idea whether that might provide a clue to the nature of the problem?

Question is, how should I proceed from here, because presumably if the pressure is that high when the heating is on, it's likely to damage boiler internal parts and/ or pipework?
Cheers

 

[MANDATE]

Pressure is not supposed to go beyond 3bar because the PRV opens at that pressure.
If expansion vessel is full or even half full with water there will be insufficient air to compress which means the pressure would go well beyond 3bar if it wern't for the PRV.
The above happens when the air pressure is not checked at regular intervals and it gradually drops.
The air pressure needs to be approx 0.8bar, but just as important the vessel must be empty of water.
There is also the possibility the connecting hose to the vessel could be blocked making the vessel non functioning.
There is lots of information regarding expansion vessels on this site so suggest you do search and read.
There is one thing certain, when pressure does drop the water has gone somewhere, so its either out through the PRV or you a leak. my bet is PRV

 

[amfisted]

Thanks for signposting me to the other info on the site about EVs.

I pressed the Schrader (sp?) valve on top of the EV and water spurted out. I kept my finger on it till all the pressure was gone, then following instructions seen elsewhere drained all the pressure out of the water system, got my footpump and put about a bar of pressure in the EV. I pressed the valve again and this time there didn't seem to be as much water coming out, so I pumped it up to 1 bar again and put the cap on.

I topped up the water to about 1 bar, bled all the rads and turned the CH on. The pressure gauge got to about 2.5 bars and stabilised; bit higher than I would have liked but well out of the red.

Question I have now is this: should I consider the EV knackered, what with it containing water on the air side? It seems that on this boiler there's a helluva lot of work to do to get to the EV, but I also read somewhere that if you install an external EV in the pipework it'll do the same job.

Should I just monitor the pressure for now or start to look for a more permanent solution?

 

[MANDATE]

It don't look good with water on the air side of the diaphragm.
I suppose it all depends how how quickly it takes to pass through the diaphragm.
I would have gone for air pressure 0.8 and same for water, thinking the pressure with heating on may be a little lower.
One major issue is that once the PRV has been opened it rarely seals again properly so you end up still with pressure dropping, this time with heating off and over a longer period of time.
Daughter had expansion vessel problem and a straight exchange would have meant removal of boiler from wall. So that was left in situ and an additional installed. With the new one the connection was with copper pipe, so flexible hose to get blocked.
Also be aware the capacity of the vessel must not be too small.
A large volume of water in the system means more extra water after expansion.
So air in an oversized vessel results in the water pressure not being as high.

 

[amfisted]

Thanks again.
I've seen the external EVs for sale online for about thirty quid, so price wise it wouldn't be an issue and I'm OK with making basic pipework connections. I did fit a shock arrester in the pipework feeding my washing machine to prevent water hammer, but I assume this would have to be something different? I read somewhere that you should insert a T piece into the piperun and fit the EV on that; but I don't know whereabouts in relation to the boiler it should be positioned; so that info would be helpful too. :0)

 

[ianmcd]

If you could actually re pressurise your boilers existing expansion vessell then this suggests there is no problem with it the connecting pipe hose may be blocked but you have to leave an open drain when re pressurising the expansion vessel

 

[amfisted]

Thanks, Ian; and Mandate. I've topped up the EV today and the CH ran at a gauge pressure of about 2.5 bars, which is in the green zone but a tad higher than it should be. When cold the gauge is showing just under 1 bar.

I'll check over the next few days to see whether the pressure in the EV is maintained. However, the main issue seems to be the fact that water came out when I put my finger on the valve pin which means, I'm told, that the EV diaphragm is knackered.

Stupid question this may be, but how can I tell which is the flow pipe from the boiler and which is return, so that I can identify, if necessary, where to put an external EV? Presumably the one that gets hot first is flow...

 

[ianmcd]

The flow is the hottest when the CH is running, if putting an external EV it should go on the return, a little bit of water from the shraeder valve is not unusual doesnt mean the EV is faulty , if you have pressure in the EV then I would doubt if it needs replacing , more likely a blocked feed hose/pipe

 

[amfisted]

Thanks again. So how do I deal with a blocked feed hose/ pipe? Is it possible to disconnect it and clean it out, or would it have to be replaced? How easy is it to access the pipe on a Greenstar 28i?

 

[amfisted]

I've now put the matter in the hands of my home service cover provider.

Their engineer came today, immediately went to check the EV and pumped it up to 1.5 bar--which he said was the correct pressure, in contrast to what I've read here and elsewhere--and told me that if the pressure gauge continued to rise into the red they would have to fit an external EV. So I'll be monitoring the pressure and probably calling them again.

Thanks all for your advice.