Honeywell st699 clock issue

31 Oct 2017
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United Kingdom
I have a Honeywell st699 programmer controlling hot water / heating. It is an older model with the brown backplate. Recently the time on the clock has been randomly changing sometimes twice per day. I will reset it to the correct time and realising the boiler hasn't fired up later I will check the time on the clock and it will be hours different. Once I reset the time the boiler fires up. I know a power cut will reset the time but this is totally random. I assume a new unit is required So my question is can I fit a newer version st699 to the existing backplate. Would changing the unit battery have any bearing. Many thanks for any assistance.
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the battery is only supposed to be for backup in case of power failure. If yours is very old there may be a problem such as tarnish on the contacts. If you can see them you could polish them with a nylon toothbrush and a smear of WD40

I have one and the papers say it has a rechargeable lithium battery with an expected life exceeding seven years. I can't see the battery type number. They say the programmer fits onto a standard backplate

I hardly use mine now as I have a programmable (room) thermostat, but it would be inconvenient if yours has failed as you describe.

I see the instructions say the battery is not replaceable
The battery is only there as a back up if the power fails, they will work happily with the a faulty / discharged battery until a power cut when they will then loose their settings. Unless you have been having power cuts, then replacing the battery won't help.
If the unit hasn't kept time during a power cut for some time the battery could well have started leaking all over the circuit board and begun causing damage, seen 2 like that now.
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Many thanks so far, the battery was an afterthought, I didn't think it would affect it, I will certainly give it a clean, just wondered if the newer models are a straight swap without having to change wiring. Just hope it is a unit failure and nothing sinister. I too have a room thermostat but this unit serves a different purpose, many thanks.
Yes they are a straight swap and done in seconds.

Altgough I like to change the backplate also as it's neater and prefer the matching backplate. Simple one for one wiring and should still take less than 10 minutes to do.

Thanks for the reply, not sure how old the unit is, been there at least 13 years so probably had it, just hope it's not a power supply issue.

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