3 year old boiler dropping pressure to 0 repeatedly

[scaredpassenger]

For context, it’s a three year old Alpha combi, a newly installed system, rads pipes etc all brand new at the same time. Only a small two bed flat, we’ve had the place apart and checked all pipes for leaks, found absolutely nothing, no drips from the boiler itself, no moisture in the property or wet carpet around any piped area.
We have British Gas home cover and they attended and just said it was the expansion vessel, replaced it the next day, and since then have had pressure loss completely every few days (been going on for about 5 weeks now and BG have been out twice more unable to find a fault, and charged us each time for the privilege).

The only thing I’ve been able to see is that the expansion pipe externally is almost like a running tap for the first 3-4 minutes of each cold cycle for the boiler, it drains onto the roof of a double entrance flat roof below us and will completely cover the roof in that time.

any thoughts? Thanks

 

[foxhole]

Maybe pressure release valve, what pressure do you set at boiler. ?

 

[Exedon]

When you say external expansion pipe I presume you mean pipe from pressure relief valve?
Assuming your topping up system to around 1.5 bar and expansion vessel is working correctly you should have zero water leaking outside!
Faulty prv or expansion vessel first things to check.
Also ditch B G and find a decent local service engineer.

 

[scaredpassenger]

Sorry it’s the prv pipe yes, topping the system up to 1bar, we’re in contract to BG for another 3 months, it’s rented out so part of the deal.

 

[scaredpassenger]

On the alpha it’s a pressure relief and condensate pipe in one (don’t know if that’s normal or not)

 

[Stuckinarut]

On the alpha it’s a pressure relief and condensate pipe in one (don’t know if that’s normal or not)

It’s allowed, however, it could be condensing rather than the prv.

 

[gas112]

why are you paying them for each visit its a reattend within a few weeks and should be free
So when it drains onto roof when first turned on does the pressure drop as it does this