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Honeywell ST9400C programmer - component faults or something else

Discussion in 'Plumbing and Central Heating' started by Lucid, 1 Jul 2020.

  1. Lucid

    Lucid

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    I'm not a heating engineer so please forgive if I get some of the terminology wrong.

    There's a problem I'm trying to troubleshoot. I have a CH/Water system with a conventional boiler, hot water cylinder, pump, 3-way motorised valve, and a gas boiler. The boiler is an Ideal Icos HE18 (18HE?). The programmer is a Honeywell ST9400C. There's also a wireless room stat for the heating, but at the moment I'm just working on getting the HW side up and running.

    The system is roughly 12 years old. We stuck with a hot water cylinder as my wife preferred to keep that after experiencing slow/low pressure HW supplies with family and friends who had combis installed.

    Here's the problem: I put a nail through a pipe while refixing so loose flooring. (I know, I know). I had to shut the system off and drain it. This is probably the first time it has been off and drained since installation.

    I refilled the system and bled the rads and checked the drain valve fitted over the boiler. All looked good.

    Switched the system back on (restored power at the system isolation switch) and found that the programmer was buzzing and the display going off then on then off then on and so on. Sounded like the relays weren't latching correctly.

    I opened up the programmer (see pictures) and replaced the relays and large capacitor with identical value components. Relay now does not buzz (that's good), but still getting this power cycling on/off. Obviously something is still not right.

    It looks (and feels like) the pump is running, and from what I can tell, the motorised value is still operating.

    I need to continue troubleshooting, but what to look at next is the question.

    Does this still sound like a problem with the programmer. If so, are the heat marks corresponding to the resisters and diodes a clue? Do I replace those components?

    Is there something else I should be trying first?


    I can solder, use a multimeter, and follow schematics. I have checked the wired connection points, but since none of the wiring has changed, and these issues only surfaced after the system was off for a while, then I'm not convinced that this is simply a loose wire problem unless someone can tell me different.

    Thanks in anticipation.

    Chris


    front cover.jpg rear cover.jpg board_display side_heat marks.jpg board_heat marks.jpg board_componentside.jpg R13 resistors.jpg
     
  2. oilhead

    oilhead

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    Remove the programmer from the base, using links, prove the integrity of the system, if all ok, but and fit a replacement programmer.
     
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  4. bernardgreen

    bernardgreen

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    I do not have the circuit diagram for the 9400



    I suspect the power supply section of the 48 volt supply is a capacitor+diode and it might be that the two capacitors have failed

    upload_2020-7-1_18-49-24.png

    After a certain age some of these will remain "good" only as long as there is a voltage to polarised them but when un-polarised the dielectric surface deteriorates.
     
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  5. DP

    DP

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    Replace the programmer.
    Also get the boiler serviced as indicated in the manual. Anything less is not a service
     
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