Heatslave pressure drop (not the normal

[Taffy195]

Oil heatslave 3 years old
Heating and hot water water working fine and been on constant for about 4 weeks , no problem
Turned off the heating and leave hot water on 24 -7 and pressure is on zero the following morning ,
Top up pressure which only takes a second or 2 to reach 1bar and set away heating and water 24-7 and pressure stays fine for weeks until we go back to water only and drops to zero overnight
Any thoughts

 

[Burnerman]

As you know, the boiler pressure refers to the heating circuit only.
At this stage, I'd look at the expansion vessel pressure, which needs to be empty and set to around 1 bar air.
John

 

[Taffy195]

Tapping on sounds hollow , not a dull tap which normally sounds like water in it .,
out of curiosity do new expansion vessels come pressurised as we’ve not long been in the house and although the boilers only 3 years old the previous owner said he had replaced it for some reason

 

[Burnerman]

An expansion vessel has water in the bottom half, and air in the top......the two being separated by a rubber diaphragm.
They don’t last forever! If you depress the valve on the top and water squirts out, it’s likely to have failed and they are not repairable.
They will usually have air in them when you buy one, but top it up with a bike pump or whatever through the valve on the top. Between 12 and 14 psi is a reasonable starting point.
Rust build up in the water supply pipe and the vessel inlet are the usual issues, plenty of inhibitor in the system helps a fair bit.
John

 

[Taffy195]

it was on 9 psi when checked , I’ve set at 14psi so will turn off the heating tonight and see if it drops .
Coincidentally I found a receipt in the boiler manual and the vessel was bought 14 months ago .
Will let you know how it goes