On 14/03/2018 12:15, Richardmerri wrote:
Hello all... My Honeywell ST799 Programmer is problematic! I can no longer use HOT WATER alone, but only the CENTRAL HEATING programme switches on the boiler. Also, when I attempt to programme in the time for either the HOT WATER OR CENTRAL HEATING the times change for both simultaneously! Has anyone else come across this 'delinquent' behaviour? Or can anyone suggest a fix? Any advice will be much appreciated... Richard.

You need to tell us a bit more about your system. Is it fully pumped, or
gravity HW and pumped CH? Does it have any motorised valves? If so, how
many and what type (two port or three port)?

I could be wrong, but as far as I can see from the data sheet for your
programmer, it only has one timer, so you can't have different times for
HW and CH. You can independently set each to on, off or timed but, when
timed, the both go on and off at the same time. Is this not correct?

My hunch is that you probably have a fully pumped S-Plan system with two
2-port motorised valves. If the boiler isn't coming on for HW, this
usually means either that the HW zone valve isn't opening - or that it
*is* opening but not enough to trigger its micro-switch to switch on the
boiler. [I've just had this problem with my CH valve].

When the boiler runs for the CH, does the HW also get hot?
--
Cheers,
Roger
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