Honeywell ST799 will control HC or HW but not both

22 Jun 2013
Reaction score
United Kingdom
Hello all,
I have a rental (gravity system, Baxi bermuda back boiler) on which I have replaced the mid position valve head as it was old and worn out.
Testing the system it works as it should, individually. But, If I set the controller to control the system, constant or timed, which ever I select first is the one that the controller operates.
Select CH then HW, room stat ON, Tank stat ON, the boiler and pump fire up, rads heat up. Room stat OFF, boiler drops out. pump keeps running, valve does not change over, Water does not heat.
Select HW then CH, room stat ON, Tank stat ON, valve changes over, boiler and pump fire up, tank heats up, Room stat has no effect. Tank stat OFF, Boiler and pump shut down. Tank stat back ON, boiler and pump fire up again???
Have I accidently pulled a wire in the connection box?
I'm sure I wired the new head in as the old was connected.
Has the controller given up as well???
Any advice appreciated.
Many thanks in advance.
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Had to read your post a few times ,and not totally sure I fully got the gist. In a nutshell ,if programmer is calling for c/ heating and also DHW ,does the MPV allow water to flow to both circuits ? If not, which port is open .
Assume both stats are calling for heat.
It depends on which I select 1st on the Honeywell controller. If I select (from both off) the DHW then the CH, the valve only opens to heat the tank, no CH flow.
If I select CH then DHW the valve opens to both, but the boiler cuts out on the room stat, doesn't switch over to heat the water only.
With only DHW or CH selected it works on the stats as it should.
post pics of the wiring , sorry but your posts make absolutely no sense
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I will go over to the property this evening after work, take pictures of the wiring centre and check everything from cold and note exactly whats happening.

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