Wondered if anyone had used a leak sealer additive. I was thinking about the offering from Sentinel. I have a customer whose system very slowly looses pressure. It's a vented hot water / unvented primary. The pressure gauge on the expansion tank drops to around.75 bar from 1.5 bar over approximately 7 days (when cold). The system has been installed for just over a year. There are no obvious weeping joints or fittings.