Vaillant 937 storage combi

[Notapro]

Hello all,
I had to turn off the incoming cold main the other day and drain down the cold/hot pipes to and from the loft installed combi boiler to replace a leaking gate valve.
Slightly puzzled that the boiler seemed to be dumping its stored hot water into the cold supply that feeds the boiler.
This doesn't seem quite right to me. I can't see any obvious way of isolating the boiler or have I missed something? (I can't see any service valves).
Thanks for any advice.

 

[gas4you]

There should be an isolating valve on every pipe under the boiler except the HW outlet.

Are you sure it is not the storage tank PRV that is operating?

 

[Notapro]

Hello,

Definitely not PRV operating.

Maybe isolating valves hidden by plastic undertray.

Thanks.

 

[gas4you]

You have to pull the plastic under tray off to access the iso valves.

 

[Notapro]

Thanks, Dave.