i've come home tonight to a cold house, all the radiators are cold except one which is in the same room airing cupboard / hot water tank. Hot water from the taps is still boiling. It was working fine this morning. if i open the airing cupboard, the pump is red hot to even touch...is that right?? it's making a noise every so often...does sound like its struggling but not sure. a few weeks ago it was quite noisy but this seemed to have stopped? any ideas?? I've read about opening up vent plug above pump..is this recommened? In one DIY book it says...slacken nipple in flow pipe from pump?? any ideas