Hot radiators in summer

This topic has 7 replies, 7 voices, and was last updated 23 June 2026 at 12:13 by JasonB.

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23 June 2026 at 10:37 #853069
Gary Wooding
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@garywooding25363
I have an Intergas CH boiler in the garage, and a Honeywell wireless room thermostat in the living room. The room thermostat is switched to manual with the target temperature set to 7 degrees C. Last night, in the heatwave (we had the fan on in the bedroom), the bathroom radiator was hot (not burning hot but much hotter than warm).

Any ideas why?
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23 June 2026 at 11:24 #853077
Nicholas Farr
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@nicholasfarr14254
Hi Gary, I’ve had a similar situation, but not to that amount of difference in temperature, none the the less though, the room temperature was hotter than the much lower temperature the room stat was set too, and even though I turned the room stat down a little more, the boiler was still heating the radiators, so I just set the room stat to off altogether, this doesn’t stop the hot water side from working, but I don’t know why the radiators were getting hot when they shouldn’t have, maybe I’ve a faulty room stat, but it’s been working okay all through the cold weather.

Regards Nick.
23 June 2026 at 11:26 #853080
Dave Halford
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@davehalford22513
There’s a restrictive link between go and return, sometimes plumbers use the bathroom rad. There will have been an instruction never to close the valves on that rad.
23 June 2026 at 11:27 #853083
David Jupp
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@davidjupp51506
One possibility of probably many. Had the boiler been heating the domestic water? a sticky or passing diverter valve could allow radiator(s) to heat up.

Our own CH even dumps hot water into the radiator circuit after achieving HW target, rather than just shut down whilst still hot.
23 June 2026 at 11:50 #853086
Peter Cook 6
Participant
@petercook6
Assuming, as David J suggests, you are using the boiler to heat the domestic hot water it may not even be a leaking diverter. Depending on the pipe routing, the return flow from the water heater may use the same return pipe as the the bathroom radiator.

What you are seeing is possibly convection flow from the shared return pipe back up into the bathroom radiator.

I have a landing radiator – next to the cupboard containing the hot water tank – which suffers from the same problem.
23 June 2026 at 12:05 #853089
Nicholas Farr
Participant
@nicholasfarr14254
Hi, I have a small radiator in the hall, which is the only one that doesn’t have a thermostatic control valve on it. I wouldn’t think my diverter valve is faulty, as it is relatively new, changed on the annual service before the last one, next service is due soon.

Regards Nick.
23 June 2026 at 12:12 #853099
Colin Heseltine
Participant
@colinheseltine48622
All my central heating boilers over the years have had a direct connection to the bathroom radiator. This usually a direct or ‘unvalved’ connection to the bathroom radiator (or a heated towel rail) primarily to act as a boiler bypass. It ensures the boiler always has a safe path to dissipate heat and safely release pressure when the rest of the house is turned off. As a by product your towels are dried.
23 June 2026 at 12:13 #853104
JasonB
Moderator
@jasonb
Very common to have bathroom radiators or towel rails on the HW loop to the boiler. This means your towels get dried all year as the rad comes on when hot water is being made.

Mine are plumbed that way as are many of my clients systems that I have worked on. These days the bathroom rads should have individual RTVs so you can shut them off if you don’t want your towels to dry in summer.
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