Boiler losing pressure

[StuartRobins]

My boiler is loosing pressure regularly. I have to top it up at least once a week. When the pressure falls too low the boiler stops working until I top it up. This means I'm forever bleeding the rads! I have a maintenance contract for it and they have been out about 8 times. They have replaced the PRV twice and also the expansion vessel. There is no water being thrown out of the boiler. Each time they find something that might be causing it but the last time they found nothing so have said it must be loosing water elsewhere. How do I go about finding where it is.

Thanks
Stuart

 

[moggridgeplumbing]

Assuming you have checked every radiator and valve and all visible pipework and joints and drain off points. Are there any signs of water stains showing? Then if you don't have any water stains on your ceiling then the leak must be coming from underneath your floorboards on the ground floor. If so it's anyone guess - try to imagine how the pipes are run.

 

[MANDATE]

Don't understand why you're ' forever bleeding rads '
Bleeding rads will make the pressure go down and topping up is needed to re-establish the pressure. You could try the following.

(a)Set cold pressure to 1bar, then switch heating on and see pressure rise to 2bar then when heating is off see pressure falls back to 1bar

(b)Or set cold pressure to 2bar without heating or hot water on, also secure plastic bag on PRV and observe for reasonable period to see if pressure drops and what you collect in bag, Re-set pressure to 1bar before using heating or hot water.

The most usual problem for pressure loss, is a incorrectly charged expansion vessel. This results in a higher than designed pressure rise and the opening of the PRV.
Also PRVs once opened rarely seal again properly.

If the expansion vessel is charged to 0.8bar with water pressure at zero and functions correctly, you could try.

With boiler off, Close the 'flow' and 'return' valves (if you have them)
Set cold pressure to 2bar and observe over reasonable period.
If pressure does not drop, problem in heating circuit
If it does drop it can't be the heating circuit, so must be boiler related

 

[StuartRobins]

Thanks for the responses. The reason I am always bleeding the rads is they fill up with air as the pressure drops. Once Ive bled them I top up the boiler again and wait. There are no signs of water stains anywhere.

I am no expert so can i check:

When you say put a bag over the prv do you mean the pipe outside the house? as im not sure what the prv looks like!
I normally put the pressure at 1.5bar and it does increase when I put the boiler on but I dont think by 1 bar.
The last time the engineers where here they used a foot pump to inflate the diaphram(?) , i assume this is the expansion vessel? then they came back and replaced it.

Stuart

 

[moggridgeplumbing]

Don't understand why you're ' forever bleeding rads '

I was assuming as the problem had already been looked at by previous engineers then the reason being to bleed rads was because air being drawn into the system perhaps via an AAV as pressure dropping because of leak.

 

[grahamtheplumber]

are you just venting one radiator.the air in the system is produced by electrolosis in the system.the gas forms and gas can be compressed where water cannot,therefore the pressure drops.if its just one rad try turning it off at both ends and see what happens.

 

[StuartRobins]

Hi Graham,

Thanks for the quick reply. It tends to be the two towel rads I have. I assumed it was because they are the highest in the house and the air gets forced round?

Stuart

 

[grahamtheplumber]

you only normally get air drawn into an open vent system when the pump is on the return.the pump causes the pressure in the system to drop below atmospheric pressure and a loose joint can suck air in.with a pressurised system this shouldnt happen.combi boilers have the pump on the return.
when a set the pressure up on a sealed system you need to see what minimun pressure the boiler requires.then add 0.1 bar for each metre the highest point of the system is above the boiler.
if the boiler needs say 0.8bar and the highest rad is 4m above the boiler you have 0.8+0.4=1.2bar cold pressure.if you have a tall house and only set the boiler to 1bar this can cause the top rads to come under less than atmospheric pressure and possibly suck air in.if the gas is caused by corrosion it will be hydrodgen,if sucked in will be air.check for a strong smell.

 

[MANDATE]

Hi Stuart! Yes I should have said PRV discharge pipe outside.
The expansion vessel will have a schrader valve suitable for a foot pump or a bicycle pump.
The idea is to get the maximum amount of air on the 'air' side of the vessel but not at a pressure that would stop compression taking place.
The new vessel when purchased may have 2 or 3bar pressure but can easily reduced to the manufacturers standard fig. (usually 0.8bar)
The most important point is making sure there is no water taking up space where air should be.
So the requirements are:
opening a means of discharging excess water.
Water pressure to read zero
inflate or adjust air pressure to 0.8bar
Closing the means of discharging excess water
Topping up with water to 0.8bar (well just over say 1bar)

Now the vessel contains all air and no water. (best conditions)
Ideally the cold setting pressure for water would be 1bar, whereas with 1.5bar some compression of air has already taken place before heating is on. I just think rising from 1bar to 2 bar is preferred to 1.5bar to 2.5bar.
Because 2.5bar approaching the PRV opening pressure of 3bar

 

[StuartRobins]

Hi Graham,

The engineer who came told me to set the pressure between 1.5 and 2 bar. I have followed his advise so far. I would say you were right at about 4 metres so 1.2 bar would be correct.I certainly have not noticed a strong smell. Where does this leave me?

 

[grahamtheplumber]

pressure is lost 3 main ways.through the pressure release valve to outside.
by leaks through pipe/fittings or leaking rad valves or due to gassing in the system.the thing i would try is isolate the towel rails where the gas collects to confirm or rule out them being the cause.also try some leak sealer in the system.

 

[StuartRobins]

thanks, I'll try it and let you know.
Stuart

 

[moggridgeplumbing]

Wow boys! -lots of tech stuff! You and I know but don't try and confuse. Keep it simple - Still think it's just a leaking compression joint.

P.S. next time you bleed the rad Stuart, put a match to the bleed valve and see if it pops. Hydrogen if it does.

 

[StuartRobins]

yes, a bit techy for me! If its a compression joint will the leak sealer have a chance of fixing it?

Thanks
Stuart

 

[moggridgeplumbing]

Always worth a try. Fernox Leak Sealer should cure it. You will probably know if its worked within a couple of days.