Diagnosing CH pressure loss

[McKendrigo]

Hi,

I moved into a new home around 5 months ago. One issue we had to deal with early on was the CH rapidly losing pressure and lots of air filling the system - the combi boiler was old and had sprung a leak, so it was replaced with a new combi.

However, I've noticed that it seems we still have a slow loss in pressure. If I top up the CH to, say, 1.5 bar (CH off), over the course of a few weeks it will gradually drop down to around 0.8 bar. A couple of radiators will also build up a bit of air that needs bled off.

I've more or less exhausted everything I think I can do to try and locate the mystery leak. I have:

1. kept an eye out for any leaks inside the house. So far I've not seen or found any damp patches on ceilings or walls.
2. Checked the radiators for leaks. I put toilet tissue on/under any radiators that I suspected might have leaks. So far, nothing. No sign of leaks on carpets or wooden floors under the rads.
   
3. I've been in the basement looking at the pipes that run under the floorboards. Put kitchen roll and buckets under any pipework that I suspected might have a leak. Again, nothing so far.

I don't really think there is much else I can do myself, apart from continuing to search in the hope I finally spot something. I'm at the point where I probably need a pro to help, but not sure what order to have things checked, so looking for advice. I guess things to look at next would be:

• Check the new boiler. I'd hope with it being brand new it should be fine, but I suppose I can't totally rule out a fault with the boiler or its installation.
• Check the radiators themselves. They all look fairly old, and given the state of the CH when we moved in it's probably safe to assume this has been a problem for a while. Could the radiators be getting pretty rusty inside? Could this cause a slow drop in pressure, even with no obvious leaks?
  
• Check the pipes - it's possible there might be a small leak somewhere fairly hidden that I can't find. Fortunately our home insurance seems it would cover leak tracing if it came to that.

Any advice about anything else I should check, or the best order to have things checked over by professionals, would be much appreciated. I'm guessing that since the boiler is covered by a warranty that might be the best place to start?

 

[Johntheo5]

You could check the boiler for leaks, ensure cold, top up to exactly 1.5bar, isolate boiler to ensure it cannot fire up then isolate flow and return under the boiler, overnight if possible, and see if any pressure drop.
You would need a gas registered installer to check the expansion vessel pre charge pressure because if incorrectly pre charged to 1.5 bar then the slightest leak will cause the pressure to fall very rapidly, if properly pre charged to 1.0bar then a filling pressure of 1.5bar will give a reserve of 1.6 litres (with a 8 liter E.vessel) which would have to be lost somewhere before the pressure falls to 1.0 bar, if it is correctly pre charged then losing 1.6 Litres in a few weeks doesn't look good. You will get some idea of the pre charge pressure if it rises very rapidly while charging to 1.5 bar.