1 radiator only coming on with hot water

Well, in this case, if you have a hot water cylinder, you will have a motorised valve, usually located near the cylinder, in most installations this is usually a three port valve as it has three pipes connecting to it. On top of the valve is a metal compartment which houses an electric motor. On the end of the motor’s spindle is a rubber ball which seals and opens the valve’s different ports. The position of the valve determines how hot water from the boiler flows through the system. There are three possible routes: -Through the radiators -Through the cylinder coil (heating water in the cylinder for the hot taps) -Both the radiators and coil simultaneously If your central heating comes on with the hot water, well, the valve is not sealing off the central heating port correctly. Hot water flows through your radiator when it should flow through the cylinder coil instead. I would call qualified plumber to sort out Hope this help

You do not say where the radiator is positioned On occasions the system may have been designed that way ie to keep a bathroom warm If not consult a plumber as others have suggested

This problem could be down to a few things, either a faulty motorised valve, or if it is just the one radiator getting hot when the hot water on then it could be how the system has been installed, on old solid fuel systems you had what was called a heat leak radiator connected to the primary pipework from the solid fuel boiler for the expansion of hot water inside the pipes. If the system has been upgraded to a bbu or heat only boiler and the radiator not taken off the primary circuit then this could cause one radiator to get hot in hot water demand only. This would require a heating engineer to investigate to solve the problem. Thanks

Always hard without looking but I would say it’s one of two things. Either it is designed to come on with the hot water with the pipe work coming from the primaries. Or it has reversed circulation which is caused by an incorrect installation. I suppose if the valve was slightly open this could cause it but unlikely and more rads would heat up ? Turn that radiator off…