Hot water problem (wiring?)

[Mattt]

Got a Y plan system with unvented cylinder - use a Tado but wiring is standard to the controller.

Issue I have is that if CH is off then HW will work fine turning on and off as needed.

If CH is on, HW will come on and off but won't come on again.

The wiring at the cylinder seems fine as the dedicated 2 port valve will open and close each time and cylinder thermostat will click on and off to drive it - I believe the issue is somewhere around the 3 port valve.

Any ideas?

[bazjude2998]

In my experience the fault usually lies with the motorised valve ie the motor itself,the head which contains the motor can usually be changed easily with no need to drain the system down.The head is just a simple motor which slots onto a key way which in turn rotates the valve into its various positions .The other problem being the valve sometimes seizes in which case the system needs draining down and the valve needs replacing .On some valves on the back plate you will see a small lever,you should be able to with a bit of force manually open the valve this giving you a good indication as to whether it's the motor or valve at fault.If ther,s no lever take the head off and operate the valve or better still have the motor tested.Regards Baz

[Pheo]

I had some problems with this, turned out it was mainly the overheat thermostat tripping incorrectly - when my loft was cold the temp differential seemed to cause the problem

Thermostat change solved the issue.

[Mattt]

I'd considered both of those, I forgot to say I changed the whole valve and actuator assembly a year back to eliminate that as a cause.

As the HW works fine when CH is off I don't think it would be the stat, I can make the system turn on and off on demand by turning stat temp up and down.

I think it must be something with the wiring, as it only happens when both are on - next step is to rip out all the wires and redo it with a Honeywell wiring centre if I can't easily identify that one particular wire is in the wrong place.

Current wiring is a bit of a mess so can't easily put up a diagram unfortunately!