Honeywell ST 699 Programmer Unit malfunction

Ronjazzer · Dec 2, 2022

I have an ST699 control unit on my central heating system (some 15 years old) which has developed a strange fault. After running normally, when the required room temperature is reached it cuts out as usual but immediately loses electrical power, with both the time display and the red lights going out. I have then to switch the power supply off and turn it on, whereupon the unit fires up. Once again, when it cuts out it loses power and I repeatedly have to go through the same procedure. Is this a known fault and, if so, can it be rectified or do I need to replace the unit?

Basher · Dec 2, 2022

If you have Googled ST699 problems, you will have seen many strange behaviours of these programmers when they get old. There is a backup battery inside to keep the clock going in the event of power failure, and that has a life of typically 10 years.
Your issue might be related to that, but I feel it's likely something else as well. The battery may need unsoldering and a new one soldering in, which may or may not help, so I would suggest a new programmer would be a much more certain and reliable fix.

Ronjazzer · Dec 2, 2022

Basher said:
If you have Googled ST699 problems, you will have seen many strange behaviours of these programmers when they get old. There is a backup battery inside to keep the clock going in the event of power failure, and that has a life of typically 10 years.
Your issue might be related to that, but I feel it's likely something else as well. The battery may need unsoldering and a new one soldering in, which may or may not help, so I would suggest a new programmer would be a much more certain and reliable fix.

Thanks for that. I presumed it's probably something to do with the age of the unit. I'll probably take your advice and replace the unit.

Basher · Dec 3, 2022

Ronjazzer said:
Thanks for that. I presumed it's probably something to do with the age of the unit. I'll probably take your advice and replace the unit.

Replacing Honeywell ST699 Programmer with ST9400c

Hi, I recently replaced a 27 year old Honeywell ST699 Programmer with a new ST9400c. Whilst system delivers Heating and Hot Water, it will not deliver Hot Water when HW only is selected (despite green LED being illuminated) i.e. boiler does not fire. My wiring changes were as follows: ST699...

www.diynot.com

There's a table in post #4 in the above thread that shows how to transfer old to new connections, also a link in post #6 shows the wiring for each.