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Tim Cooke
Tim Cooke,
Category: HVAC
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Experience:  Owner at Self-employed
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I have a a Honeywell ST799 Programmer on my heating

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I have a a Honeywell ST799 Programmer on my heating controls. It was fitted about 9/10 years ago and has just failed two days a go. I have purchased a replacement and opened the old one to replace the front (programmer) and found serious overheating damage. On the basis that nothing changed on the wiring side I would like your opinion. Can I attach a photo to this chat?
JA: Do you have a multimeter or voltage tester? What colors are the wires?
Customer: No tester. Damage covers first three terminals on the left as viewed from the open front, brown, blue then black is the third
JA: How long has this been going on with your Honeywell wiring? What have you tried so far?
Customer: Programmer has been operating untouched and without problems for 10 years. Only just opened it and found the damage after it went off yesterday
JA: Anything else we should know to help you best?
Customer: No. Can I attach a photo?
Customer: replied 6 months ago.
On closer inspection it is the first terminal on the left with the brown power cable that has melted and the back plate. The next two terminals are melted but it is collateral damage. The power supply is portected by a a 5 amp fuse.
Customer: replied 6 months ago.
Now have a Fluke Volt Alert Tester and have changed the 5 amp fuse. All terminals OK except the extreme left side one. Tester on brown wire to that terminal shows 15mm before terminal as dead and then rest of the exposed wire as live

Hello, I see the photos and the melting/burning.

What would you like to know or do? Are you planning replace the programmer?

Customer: replied 6 months ago.
I need to replace the programmer. I want to ask Honeywell for their advice in view that the existing device failed
Customer: replied 6 months ago.
I contacted Honeywell direct in order to report the fault along with the effect and also to ask advice regarding like for like replacement or if they recommend a change of model. Your response has me confused. Are you a Honeywell Technician?

I'm an independent heating engineer. This site is an independent site where technicians and experts provide advice and online assistance.

Regarding your programmer, the warranty expired a long time ago, but even if it hadn't it would be invalidated by the use of a 5 Amp fuse. Honeywell stipulates a 3 Amp fuse to protect the circuit. Burning a printed circuit board track is a commonplace results of using a fuse of too high a rating.

Regarding replacement, you can still buy the ST799. Clearly you need to replace the backplate, but it comes with a new one so that's not an issue. I can guide you in the method of installing it safely.

However, the ST799 isn't good value for money. For the same money, and less, you can get a more sophisticated programmer. You could even get one that comes with a wireless thermostat. I would need to check the price of that if you're interested.

Customer: replied 6 months ago.
Thank you for the advice, particularly regarding the fuse. We have an electrician coming in the morning, so no back plate replacement instructions needed thank you. Thanks for the upgrade suggestion but the system already has a wireless stat. I will finish with a rating above. Is that it? or is anything else required re payment for your advice?

OK I can see you have things in place. That's good :-)

Thank you in anticipation of your rating; it's thoughtful of you to be so proactive with it.

This post can stay open as long as you need it, even after your rating. So if you have any further questions during or after tomorrow then feel free to ask.

Tim Cooke and other HVAC Specialists are ready to help you
Customer: replied 6 months ago.
Thank you Tim