Our radiators are working fine but we haven't been getting any hot water for about a week now...not good for showering! The water is hot for about 30 seconds then starts to run cold. Our boiler seems to be working fine and the hot water tank upstairs seems to be hot. Our house is about 18 years old and we don't appear to have a cold water tank in the loft. There are two pumps in the airing cupboard linked in to the hot water tank. These are Grundfos pumps and I'm assuming that perhaps one of them has failed but they both seem to be whiring away fine. The immersion heater sounds like its heating the tank when we switch it on - but still no hot water. Cant really afford a plumber at the moment. Any ideas anyone?