All the caravans I have had the water is pumped into the tank via a non return valve via the fresh water inlet housing each time a hot tap is turned on.
If you look at the pipe work inside you should see blue or clear pipe connected to the crystal water pump housing. This has a 'T' connector to feed blue pipe to carver tank then continues on to feed the cold taps.
The hot water outlet from the tank is a red hose that feeds the hot taps when tap/s turned on.
Operation: Turn on the pump switch. Turn on a hot tap and when water flows without interuption (air) you then know the tank is full and its then ok to either use gas to heat or as mine I have a choice of a mains immersion in the hot water tank.
Each tap that is turned on operated the water pump.
The cold water feed in some caravan/motorhomes have a pressure valve rather than microswitch taps.
It's a white unit with knurned adjustment knob on top.
bob, August 2012
