Y plan rads heating up when hot water on

[Subby]

Hi all. I have a problem with the rads heating up when there is call for hot water only on a small 5 radiator y plan system with regular boiler, grundfos alpha 2l pump , nest 3rd gen stat and drayton 3 port.
When the thermostat calls for heat the valve switches over, boiler fires and pump runs, the rads start to heat up a little , it does not appear that the 3 port valve is passing as the heating pipe at the valve stays cool. Is there some other way that heat could be leaking to the rads? The rads are piped up with 10mm.

 

[leesparkykent]

Reverse circulation on the return. Is the cylinder return The last T before boiler?

 

[townfanjon]

Bypass anywhere ?

 

[Subby]

Yes there is a bypass near the pump (gate valve)

 

[Subby]

I've got a feeling the cylinder return is not the last return, it may even be the first (the furthest from boiler)

 

[king of pipes]

You have a similar problem as Dempseys post the same action is needed to cure the problem regards kop

 

[leesparkykent]

Heating Off Rads getting warm

 

[Subby]

Thanks I think his problem is zone valve passing, my problem appears to be "reverse circulation" as the cylinder return is not the last tee before the boiler.
I am trying to get my head around this reverse circulation.
Am I right in thinking if the cylinder return is not the last tee, then hot water flows through the coil, down the cylinder return, then through the return of a radiator further down the line out through the flow and then around the other rads through the flow pipes, out the returns back to the boiler . Is this what is happening?
Why does this only happen if the cylinder return is not the last tee?
As a side note, Are bi directional trvs a bad idea on this type of system as they allow the water to flow backwards through the radiator?
Just trying to make sense of it

 

[king of pipes]

If you are confident the valve is working as it should you could try fitting a non return valve just after the return connection from the cylinder where it tees back in the common return back to the boiler , this would stop heat creeping down the return pipework and warming the radiators when the hotwater is heating cheers kop

 

[Subby]

Thanks will try that

 

[SimonG]

If you are confident the 3 ways not passing then just shot a non return in as kop said.