I 've had a similar problem, changed the heater water valve, and all was ok. What I would do is when you next get that problem, check the pipes from your water heater valve in the plenium before blaming the aircon. The valve was closed for heating but passing hot water into the heater matrix, I opened up my valve which was a year old I found one of the rubber seals had split, so am looking for a supplier of those seals, after all their only rubber but used in a lot of similar 2 way automotive heater valves , but have not found one yet, the valves are made in Mexico I believe, but can't find the rubber seal supplier.