Heating comes on when HW ONLY selected - suggestions?

[kudu]

Since moving to our house two years ago we have had the old problem of the CH coming on when the HW ONLY is selected and the thermostat is turned down.

I took advice and replaced the old Potterton EP2000 controller with a Horstmann and then replaced the 3-Port Valve with a Honeywell Sundial Y-Plan motorised 3-Port Valve and still no change and all three pipes to the valve still get hot.

Seco Services on a similar thread on here suggested it could be 'crud in the pipes' but when I swapped the valves around it seemed all clear.

I'm at a loss as to what it could be and am fearful (of costs) if it is a wiring fault somewhere.

Any suggestions/advice would be really appreciated. It's ridiculously hot this weekend and we've got the heating on too [/code]

 

[seco services]

so you have a brand new 3 port valve and its passing on the heating side when only hw on ?

who told you to replace the programmer ?

 

[kudu]

Thanks for responding so quickly.

yes the new Honeywell is behaving the same as the old one passing the hot water through the CH pipe when the HW is on.

An acquaintance suggested changing the programmer (he's a Corgi reg. plumber who although is a nice guy I don't think I'd let him loose in our home).

 

[seco services]

(he's a Corgi reg. plumber who although is a nice guy I don't think I'd let him loose in our home).

why not ?
who wired up the new valve ?
are all the rads heating ?