Diagnosing a failed expansion vessel

Dear Experts,

I have convinced myself that recent problems with my central heating are due to a failed expansion vessel. I think I can resolve this by fitting an external one, but before I do that it would be great to know if others agree with my diagnosis.

I've lived here for a couple of years. The boiler is I think about 15 years old -it's a Volkera Linea 28, and is the first combi boiler i've lived with. I have always had to top up the pressure periodically. Since turning on the heating in the autumn, and especially more recently as it has got colder, the pressure have reached 0 overnight and i've got up to a cold house. Something must be done!

My first guess was of course a leak, but there is nothing obvious above floor level. Googling revealed how a failed expansion vessel would cause the pressure to rise as the system heats up and vent through the pressure relief valve, resulting in zero pressure when next cold and failing to start. So i looked for the pressure relief valve and found that it is plumbed into a pipe that just dangles behind the kitchen units and empties into the space under the floorboards! I removed that and put a bucket under the value. Now, if i provoke it by pressurising to 1 bar when cold and turning up the boiler thermostat, i see the pressure reaching 3 bar and water in my bucket.

As an experiment, i drained a small radiator and left it with its main valve off and lockshield valve slightly open. The idea is that the air in this radiator acts as an improvised expansion vessel. This actually seems to work, up to a point; having pressurised to 1 bar cold and turned the boiler thermometer to max, the pressure now rises to about 1.3 bar and there is nothing in my bucket.

BUT, having run for 24 hours the pressure is below 1 bar. There is nothing in the bucket, so maybe there is a leak somewhere...

I attempted to pump up the in-boiler expansion vessel, but it seems that the pressure is already high enough (or even a bit over pressure - it's hard to read on a cheapo tyre pressure gauge that goes up to 10 bar). There does not seem to be water leaking from the expansion vessel valve.

So, am i right that the pressure vessel is the problem?
Should i do more to "fix" the present one before i resort to fitting an externl one?
Will fitting an external one be as straightforward as i hope?

If i do go ahead, can anyone comment on:
- does the orientation matter?
- does the position (above, below) the boiler matter?
- would it be OK to use a flexible hose to connect it?
- should there be a valve in series with it?

Many thanks for any feedback.

Phil.

Those boilers are renowned for a blockage in the pipe that leeds to the expansion vessel. Get an engineer in to have a look. If this is the problem its a 30min fix. Dont try to unblock it yourself please

TCCHeating said:

Those boilers are renowned for a blockage in the pipe that leeds to the expansion vessel. Get an engineer in to have a look. If this is the problem its a 30min fix. Dont try to unblock it yourself please

Click to expand...

Ah yes, it does have an s-bend shape that looks like it could accumulate crud. Thanks for the suggestion.

Would could possibly go wrong if i tried to poke a stick down the pipe?!!

Ha ha, if your going to try it yourself, just cover any electrical components whatever you do

And prepare for a black shower

And don't say I didn't tell you so!

TCCHeating said:

Ha ha, if your going to try it yourself, just cover any electrical components whatever you do

And prepare for a black shower

And don't say I didn't tell you so!

Click to expand...

yes please try to do this yourself and then you will appreciate why you pay proffessionals

If you're near me give me a call once you've soaked the pcb, I'll give you a quote to fix it!

Dear All,

So I ignored your advice and did it myself

I started by entombing most of the boiler in cling film to keep the electrics dry, and improvised a washing machine hose with a right angle bend to catch the water and direct it into a bucket. I depressurised both the boiler and the air side of the expansion vessel, and cautiously unscrewed the hose. It all went to plan and I caught all of the mucky water in the bucket. Phew!

Now to clean the pipe. I massaged the hose and put some water back into the system through the filling loop, hoping that it would come out through the disconnected hose, flushing out the crud. But even when pressurised to 3 bar, water only trickles out of the pipe. Although some crud did come out, the pipe now seems to be basically clear; I have just fed a piece of plastic down it and it passes without any friction.

Am I right in thinking that water should be gushing out of here when the system is pressurised? Or is there some subtlety that I have missed? It would be great to hear that what I am seeing is expected and that I can just reassemble it and expect it to work. My fear is that the blockage is a bit further down, requiring more dismantling....

Thanks for any suggestions!

Because we are dealing with some people who don't take our advice that's why we don't give any advice on DIY gas work.

The blockage can be anyway between the boiler and the inside of the EXV.

Tony

OK, so as soon as I had posted that message I realised that the hose could easily be removed by pulling out a clip at the other end. I then found a blockage right at the boiler end, and having cleared that it all works as expected. It is now reassembled, and I am waiting to see if the pressure remains stable as it warms up...

endecotp said:

OK, so as soon as I had posted that message I realised that the hose could easily be removed by pulling out a clip at the other end. I then found a blockage right at the boiler end, and having cleared that it all works as expected. It is now reassembled, and I am waiting to see if the pressure remains stable as it warms up...

Click to expand...

A couple of points!
You've not mentioned pressurising the vessel with 'air' so it would be nice to know what pressure you set it to? and also what was the water pressure at the time.
Also why are you waiting to see if the pressure remains stable as it warms up.? Water expands when heated and the extra volume has to go somewhere which means the expansion vessel. The existing air gets compressed and the air pressure rises and so does the water pressure.
So it won't remain stable. It should rise and it should drop back when cooling down. The variation in pressure should be around the 1 to 1.5 bar.

Yes, as above. When you originally pumped up the EV, did you depressurise the water side beforehand?

Also, when the problem is solved, you will obviously need to route the blow off pipe for the pressure release valve through a wall to the outside. Not only for safety and because whoever installed it must have been wearing spurs and stetson. The pipe's end needs to be visible to give warning that there is a problem with the boiler/system.