Boiler pressure rising to 3 bar when hot and dropping to zero when cold

[LittleJoe]

Hi,
I have an Alpha Intec 28S System Boiler and over the past few weeks the system pressure kept dropping slowly and needed re-pressurising every week up to 1 bar. I then noticed that the system boiler pressure was rising to 3 bar when the heating was full on up to temperature. I had also noticed that the PRV outlet had discharged at some point as there was some browning stains on the roof below the outlet outside.
Overnight when the heating is off, the system pressure has been dropping to zero and the boiler wouldn't fire up in the morning.
I called out my Gas Safe plumber who actually fitted the new boiler originally about 5 years ago when he relocated it from the downstairs utility room into the loft.
He has replaced the expansion vessel and PRV with new ones. (Although he didn't actually feel there was anything wrong with the old expansion vessel as some air came out and no water)
He was convinced that this would solve the problem. However, it didn't.
He came back yesterday and removed the pipe connecting the CH flow side to the expansion vessel believing that it must be blocked. It wasn't and decided to fit a second new expansion vessel on the very slim chance that there was a fault with the first new one. Still hasn't solved the problem. Its had 3 expansion vessels now.
He has removed the filling loop to see if there was any pressure passing (getting) into the system via the loop. I am testing this at the moment by cooling the system right down, setting the pressure to 1 bar and then firing the boiler up to temperature with the filling loop disconnected to make sure there can be no pressure introduced via the filling loop valve.
My plumber is very experienced and professional and he is quite speechless and stumped as to what the problem is !!
The heating system is indirect ( I think thats what its called !!) ie. the system boiler heats a coil in the Domestic Hot Water tank. He is now wondering if the coil has failed, the DHW cylinder vent could be blocked, and my shower pump is introducing some pressure back into the boiler flow system. He is grasping at straws at the moment.
I have got faith in my plumber but I thought I would post this problem here just in case anyone has any ideas !!
My concern is that my wife and I are going on holiday tomorrow for 10 days and my 21 year old son will be on his own. I don't really want to leave him with this problem unsolved.
Any thoughts, suggestions or ideas are welcome !!

 

[Hot&Cold]

get a second (onsite opinion) its an easy repair for an alpha experienced operative who can actually do repairs,not just shove a probe up a hole

 

[LittleJoe]

Sorry but not helpful at all. I don't know why you bother.

 

[rochdalegas]

Its not a boiler i usually work on so not familier with all its foibles. Did he check the new expansion vessels pressure after fitting? To make sure there was enough charge in it. Or just assume it to be at the correct charge. Its a very unlikely set of events that would lead it be the shower. Check everywhere for a small leak a favourite is around the radiator valves. I think you are dealing with two seperate issues here a small leak and not enough expansion room when the heating is on. What is the size of the system (ie number of rads)

 

[LittleJoe]

Hi, Yes he did check the pre-charge pressure in the vessel. I think you are on the right track when you say you think there might not be enough expansion room. This is the conclusion we have come to today !! With the system up at temperature and the boiler pressure gauge at about 2.6 bar, I checked the pressure with a tyre pressure guage on the expansion vessel and it was at 2.5 bar. This meant that the vessel was getting pressurised and there were no blockages. I released some pressure from the vessel to bring it down to approx. 2.0 bar. I then switched off the boiler and let it cool for several hours. The boiler water pressure dropped to about 0.5 bar. I re-pressurised it to about 0.9 bar so that the boiler would fire up..
After an hour the water pressure was at 2.6 bar and didn't rise to 3.0 bar and activate the PRV as it had been doing.
So.. we think that the Alpha Boiler's 8 litre expansion vessel is too small for the number of rads in the system which is 16. Some large high volume vertical radiators were fitted in the kitchen a couple of years back which have probably pushed the expansion vessel over the edge.
So we will probably fit something like a 15 to 18 litre external expansion vessel. We now feel fairly confident that this is the problem.
Regarding leaks, we are pretty certain that there are no leaks in the system. The only leak was when the PRV blew off at 3 bar I think !!
Thanks for you suggestions which were definitely in line with where we were going.
When we have fitted the external expansion vessel I will post an update.
Many thanks for your help. Much appreciated.

 

[rochdalegas]

A stop gap procedure would be to half empty a radiator to allow for expansion would confirm your suspicions also.

 

[John D v2.0]

I hope he didn't charge you for all those replacement eplacemen vessels! 8 litre is for a relatively small system, depending on your system design temperature you might need 1 litre of expansion for every 10 of system volume. If you have some time you can estimate the volume of your system by looking up similar rads online and adding a pipework allowance. But it seems clear the existing is too small.

 

[oilboffin]

Rochdale has the right idea it's what I'd do for a stop gap I'm puzzled that it's been ok up to now could be a small blockage somewhere causing expansion probs.Bob

 

[oilboffin]

Just seen you have 16 rads you need at least 24 litres of expansion.Bob

 

[John D v2.0]

Rochdale has the right idea it's what I'd do for a stop gap I'm puzzled that it's been ok up to now could be a small blockage somewhere causing expansion probs.Bob

I'm puzzled too, but could have had air trapped somewhere in the system and only bled out recently? Or Ch target temperature is higher now.
OP do either of those sound possible?