Overflow head scratcher !

[gassafe]

Bung the cold feed temporarily if you want to watch it come over the vent!

 

[Sparkgap]

First thing I'd look for is to check mixing valves for stuck check valves, only takes a slight weep through if you've got mains on one side and tank on the other. As for overflow coming up expansion pipe, won't happen unless you've got a lot of pressure blasting it up! :smile5: