Hi, a great choice of Expanda. I'm not an expert but this is what I think. The tank will automatically fill if you have mains water connected or water in your tanks, and the 12v switch on. You should also be able to run cold water from the mixer tap on hot (ie through the HWS without having heated the water). So I'd connect the water etc - and then see if cold water comes out when the mixer is on hot. There is also a pressure-relief valve (I think that's what it is) on the water tank, same as a home HWS. Its a good indicator of water in the tank. Good luck