Help - falling boiler pressure but no sign of a leak???

Hi

We have a Remeha Avanta Plus condensing boiler. We recently moved to the house (3 months ago) and have noticed the pressure gradually drops and requires regular (weekly) topping up to restore the pressure. This has worked fine for the past few months... until now.

When I open the filling loop to top up the system the pressure no longer increases, it sits at 0.6 - 0.7 bar. Simultaneously water flows (not drips) from the small copper pipe on outside of the adjoining wall (I'm not sure what the name of this short pipe is or what it does precisely).

Any ideas? Would really appreciate some guidance.

Thanks
Roy

It's a pressure relief valve, if it is discharging at a pressure as low as that, it's faulty.

Solution - a new pressure relief valve is needed.

Working backwards, consider how the PRV comes to be leaking in the first place.
Well its set to about 3bar and in a satisfactory system it would not open.
It's well known that once opened they don't always seal properly and replacement is the best option.
Now why did it open initially? The expansion vessel provides space for the extra volume of water that's created when water is heated and expands.
The vessel is initially charged with 'air' to about 1bar, like inflating a car tyre and this charge is sometime gradually lost.
This results in a much reduced volume of 'air' so when under compression a much higher pressure is reached in theory. In practice the PRV acts as a safety valve and then fails to re seal correctly.
So while you attend to the PRV, its an ideal time to test and re charge the vessel, otherwise the problem will re occur.

MANDATE said:

Working backwards, consider how the PRV comes to be leaking in the first place.
Well its set to about 3bar and in a satisfactory system it would not open.
It's well known that once opened they don't always seal properly and replacement is the best option.
Now why did it open initially? The expansion vessel provides space for the extra volume of water that's created when water is heated and expands.
The vessel is initially charged with 'air' to about 1bar, like inflating a car tyre and this charge is sometime gradually lost.
This results in a much reduced volume of 'air' so when under compression a much higher pressure is reached in theory. In practice the PRV acts as a safety valve and then fails to re seal correctly.
So while you attend to the PRV, its an ideal time to test and re charge the vessel, otherwise the problem will re occur.

Click to expand...

Hi, Hope you don,t mind me hijacking this, but have identical problem, vokera combi.checked prv, and pressure vessel, both ok.Had this problem ages.looked for leaks, everywhere, cannot find any. Topped up to 1 bar tuesday, back to 0.5bar today.It seems to not get much lower than this.Tried isolating boiler in summer, upping pressure, and seemed ok, so pionting to system leak.any ideas would also be helpful.sorry OP for intruding.

Hi Joe!
As you've not found any system leaks as such, I would think its highly likely you have not got any.
In any case you wouldn't want to be lifting floor boards, only to find the problem was elsewhere.
So unless there is evidence of a leak (and also taking the easier option) I would want to ensure the expansion vessel and the PRV are functioning correctly.
You say they have been checked and are OK, so I have to question how you checked them.
It's well known that PRVs once opened may not seal again properly. Dirt gets trapped on the sealing face and allows water to escape. Obviously the amount that escapes will depend on the pressure and the time period.
Cleaning the PRV is an option, but not as good as replacing it with a new one.
If a PRV is replaced, perhaps its better not to operate it by hand, otherwise you could be back to square one.

A plastic bag secured to the discharge pipe will collect water if that's where it coming from.
The root cause of this type of problem stems from the expansion vessel.
You set the water pressure at say 1bar and it rises to 2bar and that's OK providing the vessel was full with air.
But with a vessel already half full with water and only half the volume of air, then the pressure doubles.
See what happens now, It's now 1bar rising to 4bar. PRV kicks in at 3bar.
So its not just checking the air pressure alone. The requirement is also for the vessel to not contain any water and I believe this is what misleads us.

MANDATE said:

Hi Joe!
As you've not found any system leaks as such, I would think its highly likely you have not got any.
In any case you wouldn't want to be lifting floor boards, only to find the problem was elsewhere.
So unless there is evidence of a leak (and also taking the easier option) I would want to ensure the expansion vessel and the PRV are functioning correctly.
You say they have been checked and are OK, so I have to question how you checked them.
It's well known that PRVs once opened may not seal again properly. Dirt gets trapped on the sealing face and allows water to escape. Obviously the amount that escapes will depend on the pressure and the time period.
Cleaning the PRV is an option, but not as good as replacing it with a new one.
If a PRV is replaced, perhaps its better not to operate it by hand, otherwise you could be back to square one.

A plastic bag secured to the discharge pipe will collect water if that's where it coming from.
The root cause of this type of problem stems from the expansion vessel.
You set the water pressure at say 1bar and it rises to 2bar and that's OK providing the vessel was full with air.
But with a vessel already half full with water and only half the volume of air, then the pressure doubles.
See what happens now, It's now 1bar rising to 4bar. PRV kicks in at 3bar.
So its not just checking the air pressure alone. The requirement is also for the vessel to not contain any water and I believe this is what misleads us.

Click to expand...

Hi Mandate, I did as you said, disconnected pipe from PRV and put receptacle under it, in a plastic bag.No leak at all.Topped it up to 1 bar, 3 days later, no water in receptacle/Bag.Also no rise in pressure, when CH is on, 24/7 at present, 0.5 bar cold, rising to 1 bar hot, 50 degrees, lowest boiler setting.BUT, lost 0.5 bar = ??? water0.5 litre???I may add that, whenever I have had Homeserve out to Repair it,once this year,twice last, they always drain boiler using PRV, and it used to leak, but no more.To the expansion tank, as this problem has been over years,I drained boiler to check tank, and pressure,No sign of membrane leaking, and I topped to 1 bar with pump.If water was going into tank via membrane, it should be full by now, the time I have topped up over time.??I could only do boiler test cold, topped system to 1.5bar, turned off valves CH, 24 hours, no loss??As I cannot find a visible leak, and topping up now above 0.5bar pointless as within 2-3 days it just goes back to 0.5bar, which then, somehow, it doesn,t seem to go much further,but very slowly.It appears to lose it when hot, so could be inside boiler, evaporating, slight corrosion around heat exchaner, which was why Homeserve came, new expansion vessel DHW, small one burst. Bit syntac, sorry.Wary of doing anything invasive at present, need CH/DHW.Like the OP, it,s finding the leak, without ripping home apart, now.Need to locate it, even if I change boiler, which I am considereing, next year;;Appreciate any more help.

when I was faced with similar problem, there was evidence of the PRV having opened and not re sealing correctly.
With regards to the expansion vessel, the 'air' pressure was tested when water pressure was zero and found to be low.
I considered the schader valve to be the most likely place where some air loss would have taken place. (valve replaced)
I pumped up the vessel to 1bar and tried the heating. The water pressure went from 1bar to 2.9bar. So not what I expected or wanted.
I then realized the volume of 'air' must be much reduced and my pumping up had not moved the diaphragm back making the air volume larger.
It was only when I discharged water through the PRV that I knew the diaphragm membrane give the maximum air volume. I then replaced the PRV. problem solved.
There is one other item that could leak. That is AAV (auto air valve).
It's supposed to allow air out when filling the system, but not water.
I believe it operates with a loose cap, but it can be tightened for testing purposes.

MANDATE said:

when I was faced with similar problem, there was evidence of the PRV having opened and not re sealing correctly.
With regards to the expansion vessel, the 'air' pressure was tested when water pressure was zero and found to be low.
I considered the schader valve to be the most likely place where some air loss would have taken place. (valve replaced)
I pumped up the vessel to 1bar and tried the heating. The water pressure went from 1bar to 2.9bar. So not what I expected or wanted.
I then realized the volume of 'air' must be much reduced and my pumping up had not moved the diaphragm back making the air volume larger.
It was only when I discharged water through the PRV that I knew the diaphragm membrane give the maximum air volume. I then replaced the PRV. problem solved.
There is one other item that could leak. That is AAV (auto air valve).
It's supposed to allow air out when filling the system, but not water.
I believe it operates with a loose cap, but it can be tightened for testing purposes.

Click to expand...

Hi Mandate, If I think you are referring to a problem I had last year was boiler leaking masses of water from bottom left.Homeserve came, eventually, and float in vent had stuck.This may be aav the same vent you mention.He put new float in, with needle on top, and solved probem that.s when I checked exp vessel, no water in boiler, checked bike valve, no obvious leak of air 0.7 bar. ,seems you have had similar problems.great boiler, even if 16 years old.just this nagging leak.Over these years I have become intimately involved with it that I,m reluctant to change;so easy to work on.weather permitting I'll strip cover off and recheck expasion tank, although 99% sure it,s ok.PRV I know is ok now, although I did have trouble with it seating, took ages for it to settle down.just have to take boards up by looks of it.Thanks anyway.Cheers