Combi boiler pressure HELP...

[nellers2001]

Hey all, sorry this is my first post but a bit stuck here really as I'm having a problem with my Worcester 28i Junior combi boiler.

It has been working ok since we moved in a year a go apart from a rad once had a few cold spots, suspected dirt in the system. It hasn't been run other than hot water since Apr 12.

I decided today I would drain the system as needed to fit a new vent plug on my daughters radiator and thought I would drain the system and fill with Sentinel x800 to clean it all as no record or cleaning or new inhibitor since 2004 (installation date). My plan was to then flush it through in a couple weeks and fill back up with Sentinal x100 inhibitor.

Anyway drained the system today, apart from the water in 2 rads downstairs as no drain points on the bottom. Poured in the Sentinel x800 and fitted my daughters new vent plug. Switched the boiler back on, used the filling loop to pressurise to a little over 1 bar and bled the rads. Pressure dropped as expected and topped back up to a little over 1 bar. Now I ran the heating and within 20mins the pressure was up to 2.5bar so I quickly turned it off (before the PRV if that's the correct term dealt with the over pressure). I let the system cool and it dropped back to 1 bar. So turned it on again and within 20mins back to 2.5bar so turned it off again.

Read on here about the expansion vessel, so once the system was cooled down, released the pressure in the system and took a reading on the expansion vessel of 0.45 bar (no water was being released). Topped that back to 0.95 bar and filled the system again to 1.25 bar. Bled the rads and turned on the heating. Again within 20mins the pressure rose to 2.5 bar so turned it off again.

Now I'm scratching my head really?! I don't really know what to do or check...do I just call a plumber in the week?

Sorry for the long post but thought that all the info may help. I'm a pretty competent person but don't know the next move?!

 

[MANDATE]

When the heating is on the pressure will rise about 1bar so you don.t have such a large problem. The PRV operates at 3bar so 2.5 bar is OK
I would have the air pressure for the expansion vessel set to 0.8bar and the water pressure to 1bar, expecting it to go up to about 2 or 2.2bar.
What is really important with the expansion vessel is that it must be empty of water when pressurised. This gives the max amount of air which can be compressed.
If vessel only has half volume of air then the resulting pressure due to expansion is doubled.

 

[muggles]

X800 should only be left in for a day at the most, it's not designed to be left in for a couple of weeks

It sounds like either the feed pipe to your expansion vessel is partially blocked, or your expansion vessel is blown. I'm afraid I don't hold with Mandate's assertion that a rise to 2.5 bar is normal, it isn't, 1.5 bar maybe but not 2.5

 

[mcmoby69]

if your EV is ok (diaphragm intact) ie no water from schraeder valve, then with a pressure rise like that i would suspect that you have an undersized expansion vessel.

how many rads do you have on the system, what size and are they double panels etc?

many installers do not allow for the water volume when installing combi boilers. only hot water output.

 

[nellers2001]

I checked with sentinel this morning about the x800 and they said the new stuff they do can be left up to 7days in the system so I'm okay there.

When I first pressed the valve a tiny amount of mist blew out for about half a second and no water at all after that.

Maybe a blocked tube to expansion vessel. Going to get a quote from a plumber as don't mind doing a bit of work but may be getting too involved for me!

Once system is cold the pressure drops and stays at a little over 1 bar so don't seem to losing pressure just goes up when heating on?

 

[nellers2001]

n

if your EV is ok (diaphragm intact) ie no water from schraeder valve, then with a pressure rise like that i would suspect that you have an undersized expansion vessel.

how many rads do you have on the system, what size and are they double panels etc?

many installers do not allow for the water volume when installing combi boilers. only hot water output.

Sorry forgot to answer you. The boiler worked fine with heating in Apr just gone but heating was first fired up again after I had drained and refilled it and uses the original expansion vessel behind the boiler.

We haven't added any rads into the system, we have 6 type 11 rads, one type 21 rad and a towel rail and that's in a 3bed 2 floor mid terrace house.

 

[MANDATE]

The water volume increases when water gets heated and the extra volume has to go somewhere. So it expands in the expansion vessel and results in a pressure increase of approx 1bar. Providing there is adequate air in the vessel to compress without going beyond the 3bar.
Too low a air pressure in the EV means the initial water pressure moves the diaphragm before heating is on, resulting in reduced amount of air for compression leading to a higher water pressure.
Too high a air pressure in EV means diaphragm resists the movement and increases the pressure before air gets compressed.
Try setting water pressure to say 0.8bar and it should rise to around 2bar when heating is on, which is not considered a problem.
Agreed a larger EV having more air would result in the pressure rise being lower.

 

[nellers2001]

Right after feeling like I wanted to smash my boiler up last night I had a sleep on it. Rang a central heating plumber who is coming next week but in the meantime drained the system to clear the x800 out.

When the system was empty I checked the pressure in the expansion vessel at 0.8 bar. Filled the system up, bled it and switched it on. Sure enough system got to 2.0 bar and rising so turned it back off but this time thought I'd check the pressure in the vessel while the system is at 2.0 bar. I'm no scientist but in theory when the system is at 2.0 bar you'd expect the pressure in the vessel to be higher than 0.8 bar as it should be taking some of the pressure from the system. Well when I checked it the pressure was still 0.8 bar!

So am I right in thinking it sounds like a blocked pipe between the vessel and system which means the pressure cannot transfer?

 

[MANDATE]

Yes! To operate correctly the volume of water increases and moves the diaphragm and the air gets compressed and its pressure rises.
So the air pressure and the water pressure are equal if the diaphragm has the same area on both sides. The air acts like a return spring so its pressure goes down as the water volume and pressure decreases.
Any blockage in the connecting pipe will have an adverse affect.
I believe most of these connecting pipes are flexible so could have twist in it or solid matter.

 

[nellers2001]

Right well to update this further:

Had a plumber out today and he serviced the boiler and said chances are the expansion vessel pressure is too low. He said before fitting a new one try increasing the pressure to 1.5bar in the vessel and see if that does anything.

Well I increased the pressure to 1.5bar in the expansion vessel after he had gone and turned the heating on, sure enough up goes the pressure...1.25bar...2.0bar...2.5bar...3.0bar...3.5bar. Well I chickened out and turned the heating off as it was still trying to go up and no water etc coming out the PRV outside.

Rang them back to tell them what had happened and they said, "yeah you definitely need a new expansion vessel".

But my question is this, why didn't the PRV release the pressure like it should do with the pressure reaching 3.6bar?

Could I be right about the blockage but wrong about the location? Could there be a blockage in the flexi braided pipe coming from the back of the pump to the PRV as this would stop the expansion vessel pressure increasing like it did in an earlier post, but would also stop the PRV from releasing the pressure out the system? If this was the case the PRV wouldn't even know the pressure was that high if there was a blockage before it and that would mean no pressure gets taken up in the expansion vessel either? Am I right in the theory here or way off the mark (I'm not a plumber but a very competent DIY'er and fix diagnose all my own vehicles too)!

 

[MANDATE]

Two things here!
Increasing the air pressure in the vessel achieves nothing.
If you put say 2bar of pressure in and the diaphragm is biased to the water side. The diaphragm can't move and compress the air until the water pressure reaches 2bar.
Too high a pressure is just as bad as too low.
I agree the PRV should have opened at 3bar, maybe it's stuck. have you tried opening it.
The connecting hose is only between boiler and vessel, it is not related to the PRV.

 

[nellers2001]

Thanks for your advice and help so far everyone!

Another update:

After refusing to believe the "plumbers" that the expansion vessel had gone I went about my own work this evening. I removed the braided hose that goes from the back of the pump to the plastic "T" manifold which feeds the expansion vessel and the PRV. Well it was blocked solid!!

I have cleared the blockage and the boiler is back together. Heating is on full now so will update again later to let you know what happens with the pressure!

 

[nellers2001]

Well the hearing had been on about 80% heat on the boiler for over 20 minutes and stayed steady at 1.7 bar on the system gauge. Turned the heat up to 100% and slowly crept up to about 1.9 bar. Checked the expansion vessel and was reading 1.0 bar when the system was de-pressurised and when the system was at 1.9 bar the expansion vessel read 1.75 bar so that is clearly working and taking the pressure off the system as it should!! No leaks from around the pipe I removed so fingers crossed...Happy days!!

Thank you so much to everyone who took the time (and repetition) explaining things and suggesting problems, you have been a great help and saved me over £100 for a non-existent faulty item!! ccasion5:

 

[ukrobj]

Glad to hear you got it sorted, great news. Sad though that the guy you had come around failed to spot a basic fault. Did you explain to him the process you went through in regards to checking the expansion vessel?

 

[nellers2001]

Glad to hear you got it sorted, great news. Sad though that the guy you had come around failed to spot a basic fault. Did you explain to him the process you went through in regards to checking the expansion vessel?

Yes everything was explained on everything I had done and everything that I thought so he could make an informed decision on what he thought the problem was, unfortunately he either didn't listen or didn't care!

All I know is that I will be making a complaint to the company. As far as I'm concerned the advice of "don't worry about it, your PRV will release the pressure if there is a fault" could have actually resulted in a blown boiler and the possibility of someone getting hurt as the blockage was before the manifold which connects the PRV and EV so neither would work. Annoys me even more as I asked him to take this specific pipe off to check it, and I wasn't worried about any extra labour costs of him doing this but he just ignored me and said it won't be blocked!! Idiot!