Honeywell ST9400C programmer - component faults or something else

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

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

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



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.

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

I have a similar situation with this model of programmer, what did you do to fix in the end?

Apologies, late to this party. I have the same heat mark and warm to the touch spot on my ST9400C. This is Deja Vu for me, given my other posts on a Manrose extractor fan circuit, where a resistor is used to bring down the voltage for the timer circuit, then the heat given off by the resistor eventually blows the nearby capacitors. Another bad/cheap design driving premature failure and unnecessary electronic waste. I would say if you can no longer feel heat when placing your finger on the casing heat mark, the resistor is gone and could be replaced to fix. If the resistor is still getting hot, then a nearby capacitor has popped instead. The latter was the issue with my extractor fan, replacement capacitor fixed it.

When my ST9400c inevitably fails and a capacitor or resistor replacement fails to repair, anyone able to recommend a good replacement heating and hot water 7 day timer, that either doesn't have a hot resistor to bring down the timer voltage, or has one sufficiently ventilated or located far away from any capacitors on the PCB? Thanks!

PcxP said:

Apologies, late to this party. I have the same heat mark and warm to the touch spot on my ST9400C. This is Deja Vu for me, given my other posts on a Manrose extractor fan circuit, where a resistor is used to bring down the voltage for the timer circuit, then the heat given off by the resistor eventually blows the nearby capacitors. Another bad/cheap design driving premature failure and unnecessary electronic waste. I would say if you can no longer feel heat when placing your finger on the casing heat mark, the resistor is gone and could be replaced to fix. If the resistor is still getting hot, then a nearby capacitor has popped instead. The latter was the issue with my extractor fan, replacement capacitor fixed it.

When my ST9400c inevitably fails and a capacitor or resistor replacement fails to repair, anyone able to recommend a good replacement heating and hot water 7 day timer, that either doesn't have a hot resistor to bring down the timer voltage, or has one sufficiently ventilated or located far away from any capacitors on the PCB? Thanks!

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You really should start your own thread, @Lucid is a well known member of this forum and we know he is into electronics, as normally posts on the TV page.

In the main simply not worth repairing, as to what to replace with, well the ST9400C is a simple programmer, same job could be done with one of these except they look hideous, so today we want a programmable thermostat so instead of simple on/off, it can set different temperatures through the day. A simple programmable thermostat is cheap, around £35 but wireless or one which also does domestic hot water (DHW) cost a little over a £100, likely the cheapest is Hive.

Since I got it wrong, and selected Nest Gen 3, which does not work very well, I can see your problem, but the problem only exists as I want individual room control, if I lived in a open plan house Nest would be great.

The old Danfoss 3060 I show was in this house, and I had to decide how much to spend and how far to go to improve my system, and you have the same problem, how far to go with the improvements?

The introduction of the so called energy saving light bulb, changed what we needed to control our central heating. Tungsten bulbs give off inferred heat so one one turned the lights on the rooms felt a degree or two warmer, so a thermostat set at 18ºC was ample as in the evening the lights made it feel like 20ºC, but with no inferred with the lights, we need to lift the evening room temperature, likely the LED bulb wastes energy as now the air temperature is 20ºC so losses increase, however since electric is around 5 times cost of gas or oil, it still makes sense to fit LED bulbs.

This is my schedule, the thermostat is set to heat up more and more as the day goes one to get around the fact it is monitoring my hall not all rooms. At the moment DHW is not on a schedule, as my heating is C plan I can't turn off DHW, it is always one when the CH is running, I did in the summer heat DHW three times a week for ½ hour each time, but since fitting solar panels it is electric heated in the summer, it costs 9p a kWh in lost payment on the feed in tariff, so cheaper to heat with electric.

But this shows my point, each person will have a different situation, and what suits me as some one home 24/7 with a large house, with solar panels and oil fired central heating may not suit you.

The Moes range of thermostats is about the cheapest with internet control, the BHT-002 BC is around £25 which is very cheap, but that leaves you to find some method of heating the DHW. Not an insurmountable problem, a simple timer could be used, on the other end of the scale you could fit EvoHome Wiser etc, and control each room independent.

However the main point is it is unlikely to be cost effective to repair the programmer, like mine they are destined for the museum, they are no longer fit for purpose.