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PostPosted: Sun Sep 24, 2017 10:39 pm 
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Hi Experts,

Issue: Honeywell ST9400C does not fire boiler when in Auto mode but works when Continue mode. For water, it works in both modes.

Background.
I had it working for over 3 years now. Recently 2 port motorised valve gone faulty and got heating engineer out. He replaced it, removed the air and got it working in continue mode. I thought it will work in timer. I left it for a day and it did not fire. So got him back again and he checked, found no issues with boiler as it worked in Continue mode.

I thought it must be some issue with ST9400C, so left it as it was not that cold. Now checked today and it does not fire boiler even in CONT mode.

so not sure if it is an issue with ST9400C or something else. Water pump and motorised valve works. Water works as said above, boiler fires. Just not the central heating

any pointers?

Regards,
Taral


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PostPosted: Mon Sep 25, 2017 8:02 am 
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As the valves and other controls don't know whether the timer is on auto or continuous, I would assume it is a fault with the 9400.

Fairly common actually from my experience.

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PostPosted: Tue Sep 26, 2017 9:22 pm 
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Hi Dave,

Thanks. Checked it today, in continuous mode it does fire the boiler but again not everytime. Today I started in continuous mode, it did not fire. I turned it off mode and put continuous mode again and it fired the boiler. it is not firing boiler when in AUTO at all.

So looks like ST9400C issue?


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PostPosted: Wed Sep 27, 2017 7:55 am 
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Looks like it, but I would want to check things out with a multimeter before I replaced it.

Are you safe and confident enough to check 240VAC and know how to use a multimeter?

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PostPosted: Thu Sep 28, 2017 12:51 pm 
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Indeed Dave. Luckily I used to play with Digital MM when I was doing my degree in Electronics and Communication. I am planning to remove the programmer and will check. All functions are working for water and heating except Auto.

Water
It works in continuous, once, auto mode

Heating
It works in continuous mode (again not all time), does not work in Auto and once mode


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