Rads heating on HW circuit

[Marty1966]

Trying to get to the bottom of this issue - A couple of radiators get absolutely boiling when the HW circuit is on which I don’t want.
Have been told it could have been plumbed that way as was the trend to always have a hot rad to dry towels without having CH system on in summer.
Above the boiler there is a 155mm pipe valve which appears to link Primary flow to the return … is this the cause and solution ? Can upload a pic if helpful
Thanks

 

[Marty1966]

That should have read a 15mm pipe with a valve ..

 

[muggles]

Pics are always helpful

 

[Madrab]

pipe valve which appears to link Primary flow to the return

Is this in between the flow and return to the HW cylinder, or is it out on system before the zone valves? FYI, the term primaries, when it comes to a Central Heating system, are the main pipes supplying the Hot Water cylinder.

 

[Marty1966]

I will upload a few pictures later when I’m free. It appears to be before both zone valves
Thank you

 

[fixitflav]

That should have read a 15mm pipe with a valve ..

I would hope so

 

[ianmcd]

I would hope so

I was so looking forward to the pics of the 155mm pipe

 

[fixitflav]

I was so looking forward to the pics of the 155mm pipe

So was I

 

[oldbutnotdead]

The 15mm link pipe with valve is probably a bypass (so when all TRVs, motorised valves etc are closed there is a circulation path for the hot water still in the boiler.
Very common on older systems (often required) was to have a radiator connected before any motorised valves and fitted with conventional valves only (not TRV) again to dump excess heat from the boiler. Favourite was bathroom rad.
You'll need to trace your pipework, valves etc to determine whether it's a design feature or reverse circulation caused by poor design

 

[Marty1966]

Thank you, it is the bathroom rad that’s heating up so sounds like you could be absolutely correct. I took some photos before the new floor went down so I’ll dig them out

 

[Marty1966]

A close look at that photo shows a 15mm flow and return pipe going to the bathroom radiator and not Tee-ing off the CH loop. Think that confirms your suspicions ‘Oldbutnotdead’
Is the solution therefore to join the bathroom pipework into the main CH loop after the zone valves and have all rads on the one system ? Worth mentioning- There are three rads throughout the house that have conventional rad valves thereby allowing that safety element.

 

[oldbutnotdead]

Have a read of the manual for your boiler, see if it requires a bypass of any sort (some do, some don't). If yours doesn't then you've got lots of options- move takeoff for that rad to the heating zone is one. If yours does need a bypass then you can install a bypass valve (link between f & r with a valve set to open typically at 0.2 bar) and then do what you like with the rads. Or you could look at smart radiator valves as well as (or instead of) the heating zone valves.

 

[Madrab]

Need to understand whether you have a Y plan or an S plan system too. One needs a bypass, one doesn't (as long as there is a rad that is always open).

 

[Marty1966]

Thank you both, Madrab Its an S Plan system with the two separate 2 port motorized valves, one for CH and one for domestic HW.

 

[Madrab]

A close look at that photo

Sorry, may just be me but I can't see a photo?

You now need to determine if the feeds to those radiators are prior to the 2 port valves or connected to the HW primaries after its 2 port valve. That and as suggested, check whether your boiler has an internal bypass or not. Even then it's always better to add an external bypass as far away from the boiler as possible (within reason), before the radiator's 2 port port valve, which that (Gate - red handled?) valve may be providing.