Where's my hot water gone?

[Jenner]

We have a Truma water heater. Our habitation heating is working perfectly, as is our cold water supply. However when we try to fill the hot water tank, the water quickly disappears. Our immediate thoughts were that we have a leak, but there is no evidence of water leakage. Can anyone help?

 

[UKMotorhomes]

Without knowing which Truma and what van you have I would take a guess that a non-return valve has failed and is allowing the content of the hot water part of the Truma to syphon back to the water tank.
Andy

 

[Jenner]

Thanks Andy. We have now established that the water is syphoning back. The heater is a C3402/C6002 fitted on a 2003 Hymer 584. Do you know where the non return valve is situated?
Donald

 

[UKMotorhomes]

There's a schematic of the heater here: http://www.leisure-supplies.co.uk/productdetails.php?id=63
I can't see a non return valve indicated so there must be one associated with the water pump. I guess it's a submersible pump inside the water tank, someone who is familiar with Hymer water systems should be able to tell you exactly where the non return valve is. It will either be part of the pump itself, or more likely in the pipe coming from the pump.
Andy