All radiators are getting hot when we've only got HW on.

[BunkerMonkey]

We've been the house for just over 2 years. We've only just got around to figuring out this 'Y' plan system as we've never had one before. We've been using the immersion tank for hot water (using the electrical backup ££££) since we moved in, but with costs rising, I need to figure out where the fault is and get it sorted before winter ideally.

We've just turned on the HW part of the controller and run a test but it seems that all the hot water is going to the CH and only a bit is reaching the tank. The thermostat appears to function correctly when we turn it up/down.
I replaced the room thermostat last year as it was falling apart and didn't look safe.
I have done some research and it would appear that it is either the 3-way valve (Honeywell V7034a1039) or the controller (Honeywell ST4600A).

My question is if there is a hard and fast way to figure it out or do I need to get my electrical test kit and check the valve wiring to the box in the cupboard as per Honeywells diagnostic method?

Thanks in advance.

 

[ShaunCorbs]

3 way valve probably remove the head does the spindle move easily by hand if not try using a pair of pliers to move it back and forwards a couple of times

 

[rcpilot]

The "Y" plan valve is probably stuck in the CH mode they are pretty cheap so get a new one and replace the old one