Anyone have any ideas or suggestions?
We have an old-fashioned conventional gravity fed hot water system, with large (28mm) flow and return pipes coming off the boiler going up into the hot water tank which is in the airing cupboard upstairs. As far as I can tell, there are no valves or pumps, not even a thermostat on the tank. The hot water in the cylinder just heats up when the boiler is on as hot water rises and goes into the primary coil inside the tank, then returns to the boiler.
The problem is that when the boiler is on but the central heating is off ie the room thermostat is turned down, we have no hot water. Only when the heating is on does the hot water in the tank heat up. Previously it would be hot with only the boiler on, ie no rads. There is no way to divert from heating to hot water, ie if boiler is on, then water should be hot.