Gravity CH system - one rad is getting incredibly hot?!

bucksman

With the boiler set for hot water only, the radiator in the bathroom is getting incredibly hot, and there appears to be less heat going into the hot water tank coil.  With this, there is lots of banging and air noise going on. Do you have any thoughts on this?  The expansion tank in the loft is normal.  Could this be a blockage, time to replace the pump (it is noisy) or perhaps the three-way valve is not operating correctly? TIA

Keep_pedalling

The motorised valves would be the first thing I would look at.

grumpy_codger

IIRC it's not uncommon to connect a bathroom radiator to HW heating loop (instead of CH one). That's because you want this radiator hot not only in cold seasons when CH is on.

bucksman

The radiator has been warm before, and I note that this used to be done to ensure towels could be dried, etc, in the bathroom.  The change to scalding hot is bizarre for this radiator.  Noted, the comment regarding the motorised valve (and thank you) ...is there a test I can do or is it best just to replace it - it has never been changed in the last 15+ years, to my knowledge. 

FreeBear

Had a gravity DHW & pumped CH system up until recently. During the summer, radiators upstairs would always get warm when heating the tank. Didn't have a motorised valve...

Motorised valves do seize up from time to time - There is usually a lever that you can move just under the head. If it doesn't move, then the whole thing probably needs replacing. Not a difficult job once the system has been drained down. Don't forget to add a bottle of corrosion inhibitor when refilling.

grumpy_codger

I think a fault with the motorised valve would affect all radiators, not just one in the bathroom.

And if it's the valve (I don't think so) the fault can be with the actuator, that can be much easier and cheaper to replace than the whole assembly. Even cheaper is to replace the motor of the actuator if it fails.

bucksman said:

The radiator has been warm before, and I note that this used to be done to ensure towels could be dried, etc, in the bathroom.  The change to scalding hot is bizarre for this radiator. 

TRV? Not sure if it has to be there and how it works if the radiator is connected to HW loop.

Noted, the comment regarding the motorised valve (and thank you) ...is there a test I can do or is it best just to replace it - it has never been changed in the last 15+ years, to my knowledge.

If it's a three(/mid)-position valve its actuator has to have a small lever indicating the regime - H(eating)/M(iddle)/W(ater). Just use your palms to feel the temperature of the valve's two outlet pipes. It's obvious which one heats the water in the cylinder. The other one is CH.