Baxi Combi Instant 105e Daily Pressure Issue

[FreezingFreddy]

Hi there, I'm new to this site, and wondering if anyone can help me.
I had a Baxi Combi Instant 105e fitted 3 years ago and everything has been working fine until about 2 weeks ago. I've occassionally had to top up the system pressure to between 0.5 and 1 bar idle, which rises to 2 - 2.5 when heating water or ch.
What's happened is that the pressure has repeatedly dropped while idle to zero (or just above), and although topping it up at this point holds while it is idle, once the ch comes on, within 30mins or so, the pressure has risen to 3 - 3.5bar. When it cools down again, this drops back to zero again.
I've checked the pipes for leaks and all is fine.
I've bled radiators and all is fine.
I've checked the plastic expansion?? pipe (from the boiler to the waste), and when the above happens, I get about 1/2 pint of water through it.
Im assuming that what ever water I top up with is basically expanding and coming straight back out the system, but don't know why.
I can live with it at the moment, as Im getting ch and hot water still, but guess it's not good for the boiler, and it is very noisy when it first starts up.
Any ideas on if there is a simple fix for this, or if I need a plumber round, and how much Im looking at for a fix?

Thanks.

 

[FreezingFreddy]

Think Ive realised I've been monitoring the wrong pipe. The plastic one is for Condensate, right? I'll check the copper one for PRV.

Also, having had some time to search this site properly, looks like there is a lot of posts relating to Baxi Combi's having to have their expansion vessel replaced. I'll look at that also.

Looking at the instructions (installation & maintenance) there are no details on the expansion vessel in terms of where the 'Schrader Vavle' is. Can someone point me in the direction of instructions for this? Cant find it in the FAQs.

Thanks

 

[powell30]

Almost certainly the expansion vessel is kaput. I assume no leaks on system or from the prv valve.? Unfortunately, it is a job for an experienced repairer.

 

[bellairaphon]

try pressing the repressurisation valve on the expansion vessel think u will have to take the bar at the top off take cap off n press see if air comes out or water if nothing could just need pressurising. if so u sound quite knowledgable. take the case off n open the prv till it dipurses all the water out then kept it locked open n attach a foot pump n pressure up to 1 bar see how that goes. but like ya man said if its the expansion get someone in.

 

[FreezingFreddy]

Thanks guys. I've checked the expansion overflow pipe, and when I top up the pressure and leave the boiler cold, there is no water released at 1bar, but when the ch is on and the pressure rises to 3bar, the water is released. Hence it seems the prv is working correctly (set to 3 bar), and so it seems the expansion vessel is the issue.
I actually started to try to get to this, but have given up and will get someone round to look at it, as it seems you have to strip the whole boiler to get to it!
I just wanted to check that although I need to get this fixed asap, can I actually carry on using it by topping up without damaging the boiler as long as its not long term (1-2 days until I get the time)?

 

[bellairaphon]

if u look over the top of the boiler u will see it then mate

 

[FreezingFreddy]

Thanks again - spot on.

I've actually just had the BG man out to look at it. No water coming out of the vessel valve, and there was some pressure in there. He repressurised it and retried the boiler and same problem (water pressure going to 3 bar and prv triggering).
He tried again, but this time closing off the taps for the ch into the boiler and releasing the pressure in the system throught the prv manually down to atmosphere. This time when he pumped it up (using a foot pump) it worked.
Seems like the diaphram was intact in the vessel, but some sort of blockage was preventing the ch water expanding into it. These actions must have loosened the blockage.
So far so good, but will monitor to see if it comes back. Could it be sludge/corrosion in the system?