Top of 2 radiators hot when system turned off (combi boiler)

[itm1960]

We've just noticed that 2 of our radiators (one in the hallway, the other in the downstairs bathroom) are hot at the top and cool at the bottom, despite the CH system being turned off. We have a combi boiler, so no hot water tank. The thermostat valve for the hallway radiator was turned off, but the pipe feeding it was hot.
I'm assuming that this radiator is linked to the hot water system for some reason, but I have no idea why, as I thought it was usually only bathroom radiators that were set up like that?
Is there any other possible explanation for this, and for why only the top of the radiator is hot? Would that be just sludge in the radiator?(the boiler was installed and the system flushed 10 years ago).

diverter valve failed in the boiler.

Yes I remember having this in the past it was some kind of valve that needed replacing.

Thanks for the replies. Would the valve failure also explain the fact that only the top part of the radiator is hot?

When a combi is trying to heat the tap water, none of the heat should go to the radiators. When the diverter valve begins to fail, some heat leaks past. The hot water going to the radiators rises to the top. It would take more time, and/or a greater flow, for all the radiators to get fully hot.

Ah OK thanks. What sort of cost should I expect to have the diverter valve replaced?

a few hundred pounds, probably.

Usually a local independent gasman, familiar with your brand of boiler, will be a good move. Probably whoever gives it the regular service. It needs to be a Gas-Safe registered fiter. Sometimes the boiler manufacturer has a list of local approved installers.

Advertising websites are a poor place to look, especially when they are disguised as recommendation sites.