Hot water turns on radiators

[R_Manchester]

When the hot water is set to on, the radiators also warm up.
Two 2-port valve / cylinder/gas boiler system.

Seemed possible likely that the valve to the heating was sticking open (partially?). This valve was replaced the problem remains.
What could be other possible causes?
When the heating is on, the hot water does not warm up, so I don't believe pipes have been crossed. (We have had various building works done involving changes to the central heating and at the time believed this had caused some 'dirt' to get to the valve.)

 

[fowlerboi]

Turn the hot water on let it heat up and then stick your hand on the closed side of the heating 2 port valve. If its hot its the valve.

I expect its not, i think its more likely to be reverse circulation. What changes have been made to your heating?

 

[R_Manchester]

Turn the hot water on let it heat up and then stick your hand on the closed side of the heating 2 port valve. If its hot its the valve.

I expect its not, i think its more likely to be reverse circulation. What changes have been made to your heating?

The closed side of the valve doesn't heat up.

Please can you explain what is 'reverse circulation' and what we need to do about it.


Prior building works - substantial kitchen extension involved moving the boiler and adding radiators, so mistakes possible.

 

[fowlerboi]

In the diagram below shows a typical S-plan (2x two port valves.)

I've labeled the flow and return next to the boiler but your boiler may have them the other way round.

The big arrow at the bottom pointing to where the cylinder return tee's in is part you've likely got wrong.

The cylinder should always be the last thing back to the boiler to prevent it circulating backwards through a radiator. I expect you will have another tee in that section of pipe work which is the return for a radiator