C/H pump not working unless HW is switched off

[masonmjs]

My system comprises a Kingspan 180L unvented hot water tank, Danfoss FP715si controller and a Bosch Worcester Greenstar Ri boiler, which works perfectly as far as I can see (and is serviced by BG every year).

The problem is that the CH pump will only start if I switch the HW to manual, if they both come on together (as programmed) the CH pump will not run with the HW on. The system uses a Center EHE0200157/U 22mm three-port, mid-position valve.

This problem started shortly after a new tank (as above) was fitted and actually did operate OK for a week or two but then it started to go wrong. Initially both CH and HW would come on OK but the CH would stop circulating after around 2 hours running and would only work if I switched HW to manual as above, now it won't even allow the CH pump to start.

HW is only heated via the boiler, I have not connected the immersion heater so it is not part of the problem and can be ruled out.

Can anyone suggest what could be wrong and how to check it please (I am a control systems engineer by discipline so hopefully I can follow guidance and test as appropriate).

Thanks in advance.

 

[plumbingdoctor]

Hi
Maybe your tempering valve doesn't work properly. this device can regulate the C/H of your hot water systems even with a multiple water supply