Hot water turns on when heating is on

[worrybynature]

Hoping someone can help. We have just bought a house with a Worcester Bosch boiler in it, that seems pretty good, energy efficient etc. However, for some reason whenever I put the heating on, the hot water heating light comes on too! It will not let me run the heating without having the hot water on as well. It does let me have the hot water on and not the heating. Any ideas?

How old is it?

What colour is the hot water cylinder, and can you see a thermostat strapped to the side with a flex coming off it?

Please describe your timer. Do you think it might have been fitted by someone incompetent?

Is the hot tap water scalding?

Thanks for getting back so quick piglet!

I'm not at home at the moment, so can't answer most questions until tonight. However, the boiler was apparently put in 2004. Hot water cylinder is yellow. Not seen a thermostat (actually I should really look, as wanted to turn it down). Timer was probably fitter by previous owner, as he was quite handy. I will let you know the make of the timer when I get in.

Our Worcester Bosch boiler came with a seven year warranty, so if yours had one it may just be in guarantee - this would have been declared to your solicitor on purchase. Obviously you need to be sure it's the boiler at fault and nothing else, as the guarantee will only cover for that.

Ok, found the thermostat. Yes the water is scalding. The timer is a Siemens RWB29

might not be a fault, our boiler will do hot water on it's own, hot water and heating but it won't do heating on it's own, but then our boiler is old.

Most likely your three port valve is faulty. Call your mender.

Huh? If you have gas central heating don't the radiators get hot by pumping hot water into them? So you can have the hot water on but not the heating, or you can have the hot water and heating on. You can't switch the heating on without hot water.

Since at least 1980, yes you can.

Every boiler I've ever had works like this!

It doesn't have to. At the very least, even if you don't have separate timers, there should be a thermostat on the cylinder which closes a motorised valve once the ccylinder is up to temperature, and prevents the boiler sending more heat to it.

The room thermostat similarly prevents more heat being sent to the radiators once the rooms are warm. The boiler and pump only run when either the room stat, or the cylinder stat, are calling for heat.

This prevents overheating and waste of energy.

Solid fuel boilers are slightly different as they cannot be instantly switched off, so the heat has to go somewhere or they would get too hot.

Boilers up to and including the 1970's usually had gravity circulation by convection to the cylinder. This is inefficient in energy but can be modified at a cost. There are still some around like that. The summer gas bills are needlessly high due to the wasted heat.