Radiator heats up when hot water is on and heating is off

AndrewLivingstone

AndrewLivingstone

AndrewLivingstone

Associate
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26 Aug 2021
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2
Location
London
I've a weird issue. I've old vented cylinder boiler. One of the radiator, nearest to the cylinder, heats up when only hot water is on, heating is off. Not sure why only one radiator is heating why not all of them heat up? I got the 3 port valve and pump replaced but problem persists. I'm not sure what else could be the issue. Any thoughts?
 
Think this is fairly standard on old systems like that, ours did it before being replaced and when I asked the plumber he said it was common. Normally the nearest rad is a towel one in the bathroom so it isn't a problem, but ours was this and a bedroom so we noticed it.
 
It's likely that the radiator is simply on the HW circuit, possibly for a bathroom purpose as mentioned. It can be changed to be on the CH circuit but its important generally to ensure that there's always a flow path for water in the event that the pump is in running and all the radiators are either off or their TRVs (if fitted) have shut down. If the 3 port valve is mid or HW then then the cylinder is a path but for CH only there could be a problem.

Years ago this was achieved by leaving the TRV off one rad or by fitting a partial bypass loop (stop valve partly closed etc) on the boiler flow / return, but more commonly now by fitting an automatic bypass valve (ABV) which only opens once the pressure goes past a certain point.
 
Our house that is an old B&B has the towel rads running from the HW circuit. The rationale being that even in the summer you may want warm/dry towels.

Seems odd to me, but never bothered trying to change it.
 
Whoa, isn’t this bad? I’m not an expert, but wouldn’t that mean all the rust from inside the rad would come out the tap if it was on the Hot water circuit? And that it would not heat up with the other rads? radiator circuit is also meant to have a rust inhibitor added… there is also meant to be a ‘valve free’ raditor somewhere on the CH circuit so if the boiler kicks in, it can actually pump water round, otherwise you can damage an older/cheaper boiler.
 
nox_uk said:
Whoa, isn’t this bad? I’m not an expert, but wouldn’t that mean all the rust from inside the rad would come out the tap if it was on the Hot water circuit? And that it would not heat up with the other rads? radiator circuit is also meant to have a rust inhibitor added… there is also meant to be a ‘valve free’ raditor somewhere on the CH circuit so if the boiler kicks in, it can actually pump water round, otherwise you can damage an older/cheaper boiler.
Click to expand...

The water that comes out of the tap does not actually go through the towel rads... that water just lives in the immersion tank.

The towel rads are just connected to the same circuit as the heating element in the immersion tank.

The two lots of water never meet.
 
My towel rail is plumbed like this and I love it.

The rad has a valve on it, and I turned it off for the two absolutely hottest weeks of the summer but it's been on for the rest of the time.

Who doesn't love a warm towel? :D
 
Two of my radiators are the same as this (both bathroom ones). I had TRV valves fitted to them so I can turn them off in the summer. I generally keep them on most of the year though.
 
TheFew said:
The water that comes out of the tap does not actually go through the towel rads... that water just lives in the immersion tank.

The towel rads are just connected to the same circuit as the heating element in the immersion tank.

The two lots of water never meet.
Click to expand...

Aaaah, phew! As mentioned, not an expert hah! We don’t have that complexity here with a combi boiler!
 
TheFew said:
Our house that is an old B&B has the towel rads running from the HW circuit. The rationale being that even in the summer you may want warm/dry towels.

Seems odd to me, but never bothered trying to change it.
Click to expand...

My new-build house this year has the towel rails plumbed in so they come on with the hot water too :)
 
Didnt read other replies but a very
Common design fault

people who installed the system got it wrong

The best solution is to turn them radiAtors off during summer months
 
I had this and it was a stuck valve. The radiator system wasn’t closing off and was letting some flow through when the hot water was on - just enough to heat up the radiators closest to the boiler on the circuit.

Worth checking at least.
 
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