Sorry. Yet another boiler thread....

UKworkshop.co.uk

Help Support UKworkshop.co.uk:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Cozzer

Established Member
Joined
13 Jun 2017
Messages
1,083
Reaction score
1,504
Location
Derbyshire
The continuing saga of my Vaillant Ecotec Pro 28 boiler.

Mentioned before on this esteemed site, but I'd value your thoughts on this quandary...but please, no witty quips like "change your plumber" or "should've bought an X..."
Some general info : Fitted circa 6 years ago, had both PCB's replaced a few weeks ago following an unexplained electrical fault. We have never used the onboard timer/clock, and instead switch the central heating on/off using the Honeywell wall thermostat located downstairs. Obviously during the summer months the heating's not on, but we always have enough hot water for showers etc.

For the past 2/3 years we've experienced the dreaded F22 error, solved by topping up.
At first it was every few weeks, but currently it's literally every other day. Obviously we've mentioned this to our plumber - who's away for the next 3 weeks! Cheers! - in the past, and he suggested there might be a leak somewhere in the system. We've checked every radiator, and pipes (where we can see 'em), and there's no sign.
He also mentioned that the leak needn't be "huge", as it takes only a cupful of water to alter the status from working to not...

I know many of you know a lot about boilers, so perhaps these two observations of mine might help?

1) Every single case - hundreds, now - of F22 has happened when the central heating is off. Overnight, usually. It's never happened - so far, anyway - when the heating has been switched on.
2) The advice when topping up is to achieve 1.5 Bar, close the valves, watch the level. It'll probably dip a bit, so back to the valves and slowly add until 1.5 is shown.
I've tried this, countless times - no chance.
It takes barely a second on the valves for the display to show 1.7, and obviously climbs the longer you have the valves open. So no, I can't register 1.5.

Not sure whether these 2 elements are of any use...they're just things I've spotted.
 
and he suggested there might be a leak somewhere in the system. We've checked every radiator, and pipes (where we can see 'em), and there's no sign.
The only way to check for any leaks is to use a pressure tester, I can recomend the Rothenburger one which is probably what your plumber has or should have.

Have your checked the expansion vessel pressure ?

Now it might be a bit time consuming but you do not want to over pressure the boiler so I would look at isolating the radiator circuit, block one end and pressure test at around 4 to 5 bar, once at this pressure go round and look and listen at every radiator and around the house as often you will hear a slight hiss if something is seeping. Leave it at this pressure for an hour or so and see if it holds, if not then you might want to try some stop leak in the system, Fermox F4.

If this all holds pressure then your problem is in the boiler side and you need to start looking at this, because it is a combi there are more parts to leak and seep.
 
My former Wolf system (domestic hot water and heating) had a similar problem, and no one could find the leak. The problem gradually increased from once a month to once every other day. This was during the summer, so the boiler was working only for the domestic hot water demand. The plumber checked all of the radiators and pressure tested the system, but couldn't find anything to explain the constant loss of water.

Early one morning, I noticed water dripping at a steady rate from the pressure relief valve (PRV) directly into the drain even though the boiler was not heating at the time. While I was standing there, the underpressure error code appeared and a few minutes later the PRV closed. I refilled the system and the error code went away. The plumber installed a new PRV, and we didn't have any problems.
 
Mike and John Brown have hit my two thoughts, the other one can be if the system is boiling in use this can allow steam out the air release valve, however this is a very obvious loud knocking noise, so unlikely as not mentioned.

A good test is to pressure the system in the morning of a day and then take a pressure reading every hour. A slow depressurisation is likely a leak of some sort, a fast drop shows a different issue eg the PRV lifting on heating as the expansion vessel is shot.

Replacing the expansion vessel in a boiler can be expensive and I have previously added a separate vessel separate to the boiler as a cheaper option.
 
When I had a problem with an expansion vessel, a few years back, it was noticeable that it took a very small amount of water to repressurize the system. What you said about finding it hard to stop at exactly 1.5 bar could be a clue.,
 
My former Wolf system (domestic hot water and heating) had a similar problem, and no one could find the leak. The problem gradually increased from once a month to once every other day. This was during the summer, so the boiler was working only for the domestic hot water demand. The plumber checked all of the radiators and pressure tested the system, but couldn't find anything to explain the constant loss of water.

Early one morning, I noticed water dripping at a steady rate from the pressure relief valve (PRV) directly into the drain even though the boiler was not heating at the time. While I was standing there, the underpressure error code appeared and a few minutes later the PRV closed. I refilled the system and the error code went away. The plumber installed a new PRV, and we didn't have any problems.
I don't know if it's true, but a plumber once told me that once the PRV had vented, it would never seal properly again. Could be ballcocks, I'm not a plumber..
 
I don't know if it's true, but a plumber once told me that once the PRV had vented, it would never seal properly again. Could be ballcocks, I'm not a plumber..

I think the plumber is mistaken, but I'm not a plumber either. The PRV is supposed to open as needed and then close.
 
It sounds like your expansion vessel needs inflating with air usually between 12-18 psi. Without the expansion vessel there is nothing to compensate for excess pressure and it could then trigger the PRV which I believe operate at about 3bar. it's not uncommon for these to stick open when triggered simply because they never usually open in service.

Gerry
 
Last edited:
In the UK the PRV has to be a factory fitted component for the hot water cylinder, it also opens on over temperature as well. For the heating side it is just a simple release valve. I have seen systems with issues where all was fine but then someone has had some changes such as radiators and then the expansion vessel is undersized for the new layout and water gets discharged.
 
I think there is a bit of confusion regarding the PRV valve. One is a Pressure Reducing Valve and the other is a Pressure Release Valve more commonly called a Safety Valve. Boilers have SVs whereas unvented hot water cylinders have PRVs.
 
I had a similar F22 problem on my daughters Valilient Ecotech Pro over a few weeks. It was the expansion vessel failing which eventually started to drip slightly on a Saturday. I told her it would be OK till Monday when I could get a replacement. It started leaking badly on Sunday so she had to turn it off. I replaced the expansion vessel on Monday and it promptly blew the fuse on the mother board. I then had to get and replace the pump after having to diagnose that it was actually the pump that was faulty, blowing more fuses in the process. Water had got into the pump despite it being IP rated for splash proof.

I did extensive reading on the F22 fault at the time and the advice given above is correct the main causes apart from water leaks in the system are:
Expansion vessel not pressurised correctly
Pressure release valve leaking
Expansion vessel diaphragm leaking so it does not work.

As had been said the rapid increase in pressure when filling is indicative of a problem with the expansion vessel which was one of the symptoms with my daughters, which I only realised in retrospect.

The fact that the plumbers merchants had the expansion vessel in stock would indicate it is not a rare occurrence.
 
Top up the pressure and turn off both heating valves under your boiler to see if the pressure still drops over a period of time, this should confirm if its the boiler, expansion vessel or radiator system leaking. It can be very difficult to find a small leak on the radiator circuit when a small amount of water can soak into carpets or house structure but not be that obvious. Pressure increasing quickly when topping up does tend to indicate there is little expansion room left in the vessel.
As mentioned above tape a plastic bag over the PRV outlet to contain any blow off. Although most boiler manuals say to test the PRV occasionally On manufacturers courses they always told us not to touch them as they won't always reseal 100%.
On our Vaillant 831 the expansion vessel failed under the guarantee (3 years old) and the Vaillant engineer who came out was trying to convince me that it was fine and the leak must be on our 40 year old system. I explained I had just retired from the job after more years than Id care to admit to so he reluctantly took it out (an easy job on these boilers) and it weighed a ton :) because it was full of water. He left the old vessel with me so I cut it open to look inside and was surprised how thick the diaphragm actually was but it did have a small 1/4" nick in it.
 
Crikey! Thanks a lot, chaps!
Some interesting stuff in there!
I thought MikeK's earlier posting was so similar, I could've written it, so I grabbed my copy of the brochure and studied the description of the appliance diagram.
PRV...
I can see a Pressure Gauge...I can see an Expansion Relief Valve....I can see a By-Pass Valve....but no mention of a PRV?
 
I can see a Pressure Gauge...I can see an Expansion Relief Valve....I can see a By-Pass Valve....but no mention of a PRV?

I called it a PRV, but the correct term in the manual for the old Wolf boiler is Sicherheitsventil (safety valve). In the manual for the new Viessmann boiler it is Überdruckventil (pressure relief valve). Potato...puhtahto. :)


This is similar to the Wolf part, and the plumber charged me €45 for parts and labor to change it. The female threaded outlet had a short pipe with a 90-degree bend that ended over a drain.


Sicherheitsventil.png
 
It been a while (approx 5 years) when I was last a plumber/gas engineer, now a picture framer but my thoughts.
Someone mentioned that if the PRV opened it might not reseal completely, this is correct as debris can be present on the faces of the rubber seal and the brass faceplate it seals against. Expansion vessels do need pumping up usually after approximately 3 years but could be sooner, the correct way to do I this is isolate the flow & return under the boiler ( assuming they turn ok and fully close) you then empty the boiler via the drain off not the PRV ( or risk of contamination and failure to close properly) you then connect a foot pump or decent bike pump with a pressure gauge to the expansion vessel valve and pump up to about 0.8 bar or amount stated in manual, leave the drain off open while you do this as the pumping up will force water the has accumulated in the expansion vessel out. Disconnect pump,Then close drain off and open flow / return valve and you may have to top pressure up to manufacture value (think it was about 1bar) the PRV is designed to open at 3 bar if a over pressure state occurs. I seem to recall sometimes pressure could be lost by crossover in the domestic hot water plate heat exchanger due to cracks internally in the unit.
 

Latest posts

Back
Top