Just back at my Mum's house after she's been away for a couple of weeks due to isolation, and found the hall ceiling on the hall floor when I walked in. Fantastic. Turns out the hot water (not the CH) had been left on 24/7. The cupboard with the (vented) hot water cylinder was flooded out with water dripping from the roof of the cupboard. Dripping water was tepid not cold, so it's not a leak from the cold water tank in the loft (also checked and no evidence of moisture up there). The tank is boxed in so I haven't been able to do full investigation yet, but my best guess is that the water in the tank has been boiling away for the last couple of weeks, and this is all steam condensing on the soffit and dripping down into the cupboard. All the towels and sheets are destroyed. Odd thing is, shouldn't the thermostat on the water tank (set to 40, oddly) signal to the boiler to shut off even if it is on 24/7 once the set temperature has been reached? And even if that fails, shouldn't the cylinder just vent into the feeder/overflow tank in the loft rather than spewing steam over the airing cupboard? I need to take the boxing apart and see what's happened, but I'd be grateful for any early thoughts on what might have happened here? We had a total cowboy in 15 years ago who I suspect has crossed the CH and the HW circuits, plus wonder if his controls have been bodged, but then it's run for 15 years ok on a timer, so lost on me! Any suggestions gratefully appreciated so I can know what to have a look at. Could the thermostat have failed and the vent pipe be blocked/installed incorrectly? But even then how the hell would steam (assuming that's what it was) be getting out of the HW cylinder? Do they have some sort of emergency pressure release valve? Ah the joys. Spending the day putting the ceiling back up before Mum comes back to find it all in pieces.