Central Heating Losing Pressure ONLY when pump is off

[andysi1234]

My boiler loses pressure within 24hrs, but ONLY if the heating pump is OFF. When the pump is ON continuous, it doesn't lose any pressure (at least for several weeks). I have tested this multiple times and it is very repeatable.

Obviously this leads me to suspect the pump, but I cannot see an obvious leak and I cannot understand what exactly would be happening to cause this scenario? Could it be something other than the pump that would cause it to happen this way?

I discovered this when turning the pump off for the summer. Not sure how common this type of setup is, but when we moved into this house, the system was setup this way so we haven't changed it; there is no master thermostat, instead every radiator except the bathroom has a smart TRV. In the winter the pump is usually set to continuous but in the summer, I turn the pump off as we don't want the bathroom radiator on either.

 

[foxhole]

Pump is off when the heating stops, cold joints leak easier than hot , so could be anywhere .

 

[HERTS P&D]

Do you have a concrete ground floor?

Andy

 

[andysi1234]

No, we don't have concrete.

 

[CountryFan]

What make and model of boiler? Some boilers can add pressure when the pump kicks in, eg the Vaillant ecotec pro. I've had this with our customers when they say it's constantly losing pressure, but the reality is they're falsifying the boiler to come on by means of the pump.

 

[andysi1234]

It's a Grant Vortex Pro combi.

 

[CountryFan]

Check the safety discharge pipe from the prv. Usually copper to outside

 

[andysi1234]

I recently had the PRV changed as it was leaking slightly.