With the gas service valve isolated, the gas test holds. Once the valve is turned on it drops even though the controls aren’t calling for heat/ hot water. Can the main gas valve be locked open?
It’s an old VC GB 242EH GC-No.41-044-07
Thank you.

Phil Billingham, May 2023

It's unlawful to work on gas appliances unless you are Gas Safe certified. Get a competent engineer in.

Dghu, May 2023