Honeywell T4R thermostat and st9400c programmer - who's the boss?

24 Jun 2015
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United Kingdom
Mum had a new conventional central heating system fitted last year, all working superbly but I have a couple of questions.

There is one Honeywell T4R wireless thermostat, and a Honeywell st9400c programmer in the airing cupboard.

The hot water is set to 'Auto' on the programmer, and comes on at the specified times. The central heating is set to 'Once' on the programmer, and uses the scheduled times set at the thermostat (not the controller) to decide when the heating comes on. All working OK.

My questions are: if she wants to turn the central heating off for good soon (well, until autumn), should she turn the programmer central heating switch to off, and leave the thermostat on? Or turn them both off? If the programmer CH is set to off, but thermostat is still on, will it turn the boiler on anyway?

Also, what is the difference between CONT (continuous) and ONCE in this set up? I'm assuming AUTO means it would use the programmer's time schedule, not the thermostat..

Apologies for the long-winded post!

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For the CH to come on both the 9400c and the T4R have to be scheduled to be on with the T4R calling for heat, therefore turning either off should stop the boiler from firing for CH.

On the 9400c CONT means that the timer schedule is ignored completely whereas ONCE means the 1st "on" point and the last "off" point are the only events that the programmer will obey.

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