Central Heating System loses pressure.

[ColinJames]

I have an Ariston Clas HE EVO System boiler. Over the last few months, getting on for a year, the sytem loses pressure. Not regularly or constantly. It was working ok for weeks, months and then when I get up in the morning pressure would be a zero. This is getting more frequent. I repressurize, check everyday and there would be no change then suddenly, zero. I never seem to catch it half way. The pressure regulator has 1.4 bar when the system is at zero and I tied a bag onto the relief valve discharge and it is empty. I can't see any leaks from the boiler. It takes minimum input to repressurize. It is an old house, radiator pipes are under the floors and hard to get to. I've had a quick look and can't see anything.
So question is how do I find the leak location? Although the boiler looks ok, the fact that the everying works, then suddenly the pressure drops and recharging is a minimum amount, makes me think it is something getting switch on/off and leaking at one position. The zone valves are under the boiler and easy to see and there are no leaks there. When the central heating was on, all radiators were hot right to the top, no air trapped. And if it is the radiator pipes, is there a quick way to identify where the leak is?

Any suggestions welcomed.

 

[denso13]

And if it is the radiator pipes, is there a quick way to identify where the leak is?

You could isolate sections of the pipework but that will involve capping/turning off different areas.

 

[ColinJames]

Thank you.
With the layout of the house, that would be a big job.

 

[Mister Banks]

Presumably in this weather you are only heating the HWC therefore the radiators are isolated at the motorised valve. This suggests that the "leak" is somewhere in the boiler or the hot water circuit.

 

[ColinJames]

Thank you Mister Banks. Yes, that's my logic...although my logic has failed me before.

 

[denso13]

Thank you Mister Banks. Yes, that's my logic...although my logic has failed me before.

They aren't completely separate, they'll both be connected to a common return so either could be leaking.

 

[CountryFan]

Could be the expansion vessel or automatic air vent if fitted.