Pressure loss

[monty9120]

I have a Worcester Bosch 28i junior boiler

Last month or 2 I’ve had to top it up a tad every few days nothing drastic

Have checked all connectors on radiators and boiler where I can get to

Expansion vessel has been checked and is holding pressure fine

I have noticed if the boiler is off and goes cold and it fires up I hear a rush of air and the pressure drops a tiny bit like 0.1-0.2 bar

I fitted all new radiators last year and has always been fine with no air bubbles only recently I have noticed this issue

I only have to top up the same amount each time so it’s not getting any worse

I have no dampness on ceilings at all

Any ideas

 

[monty9120]

I have checked the pressure relief pipe too and no leaks from there

 

[MeldrewsMate]

The pressure drop when the boiler starts may be just the pressure being depressed dynamically when the boiler pump runs, so nothing to be worried about there.
Do you have pipework under the floor? If joints there are weeping it can be difficult to find.
Have you checked around the rad valve glands, both TRV and lockshield ends, for dampness?
Now the warmer weather is here it is easier to try turning off the heating and see if the pressure drops over a few days.
You could try disconnecting the condensate discharge pipe and divert it into a dry bucket. Does it discharge over a few days (of course you must turn off the hot water as well as the heating to do this properly).

Another test is to isolate the heating system from the boiler for a few days. Does the pressure drop? If not, does it drop when you re-open the isolation valves?

 

[monty9120]

Thanks for the reply I have gone round all connections and can’t find any leaks

I always get air in one radiator upstairs which is normal all others are fine

It makes no difference how often I top it up I get this rush of air which I’ve never had before

 

[monty9120]

I checked the auto air vent no leaks but I did notice the cap on the top was screwed shut

Conflicting info online weather it should be tight or not