Pressure loss in sealed central heating system

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I have a sealed central heating and hot water system running off of a Alpha CDR18 boiler. It has been experiencing significant pressure loss (from ~1.0 bar down to 0.2 bar) after the heating has been on and the water cooled back to room temperature.

No water loss from the PRV (balloon put over the pipe) and the addition of 2 doses of sentinel leak sealer reduced the pressure loss for 2 days, but it has now returned. No signs of leaks in pipe work or from radiator valves.

When cold and the boiler off (with system pressure at 1.0 bar), pressure loss is negligible - 0.2 bar over 12 hours - surely it'd lose more if there was a leak in the pipe work?
Pressure loss of ~0.8 bar when boiler has been on for several hours (from 1.0 bar cold to 0.2 bar once cooled back down)

System pressure rises to 1.8 bar when boiler on and water hot. Holds this pressure for full time boiler is on and then falls to 0.2 bar when off and cooled.
Even if starting cold pressure is 0.2 bar, hot pressure still rises to 1.8 bar (according to the gauge) and the system works fine (rads piping etc).

Could this be a leak within the boiler heat exchanger (and loss of water out the condensate pipe)?

Only other options could be a leak in underfloor pipe work, or from the coil in the unvented cylinder?

Any advice appreciated. Want to rule out a boiler issue before I go looking for leaks. Anything I could try to diagnose etc.

Thanks!
 
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The boiler shouldn't work if the pressure is 0.2. The low water pressure switch should activate at 0.5.
If the gauge is rising between .2 and 1.8 I'd be checking the expansion vessel has an air charge. If there is no water dripping from the PRV discharge pipe work then it looks likely there is either a leak below the floor or the main heatex is leaking.
If you can't find a leak below the floor then you need to call in a Gas Safe engineer to inspect the inner workings of the boiler.
At the risk causing another leak you could isolate the boiler from the carcass using the built in valves below the boiler and leave it off for a day or two. If the pressure continues to drop whilst the boiler is isolated then you will know the leak is internal.
 
It's strange then, because the boiler will fire when the pressure is below 0.5 bar. When it does, the pressure will rise to 1.8 bar within 10 minutes or so and stay there the whole time the boiler is on.

Thanks for the advice regarding isolating the boiler - i'll arrange for a gas safe engineer to take a look. From your experience, from what i've described as the issue, do you think a pipe leak, or a heat ex leak is more likely?
 
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I've spoken to another plumber who said that if the main heat ex was leaking that water would be dripping freely from the boiler casing, rather than into the white condensate pipe.

Can anyone confirm if this is actually correct?
 
I've spoken to another plumber who said that if the main heat ex was leaking that water would be dripping freely from the boiler casing, rather than into the white condensate pipe.

Can anyone confirm if this is actually correct?

The heat exchanger has an outer skin that collects the condensate waste water in a sump at the bottom. Any water leak from the heat exchanger would take the same path.

With all due respect, not all plumbers have an in-depth knowledge of central heating appliances.
 
A pressure drop of .2 bar is 12 hours is NOT negligible, although i would suggest, the first port of call would be getting the expansion vessel checked,
 
There has been a balloon over the PRV outlet pipe for 2 weeks with no sign of water loss, therefore I don't think this issue has anything to do with the pressure vessel lacking charge.

Unless I'm missing something?
 
As advised by previous posters you need to recharge the expansion vessel. Or, if not comfortable, get someone competent in to fault find who should probably check that as first port of call.

//www.diynot.com/wiki/Plumbing:faq7

The prv is a safety device independant of the ex vessel but you have at least confirmed it doesn't appear to be venting.
 

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