Honeywell ST9400C central heating programmer fault problem fixed

About 2 years ago we had our central heating boiler replaced and a new Honeywell ST9400C programmer was installed with the new boiler. This worked fine for about a year and then we found that although the radiator circuit came on via the timer, the water heating (to the hot water cylinder) did not. The green LEDs came on for both circuits however. I found I could still press Extra Hour on the h/w side and it would then heat the hot water. It just didn’t want to operate hot water and space heating together.

I was busy with other jobs and did not feel confident about getting a heating engineer in, so I just used the Extra Hour button work-around. I bled the system of air & re-pressurised it; this made no difference. I wondered if some sort of fluidix effect was in play where the water decided to go round one circuit & kept going on that one circuit.

Summer came & the hot-water worked fine on the timer, with no space heating necessary. When that stopped working I checked out the ST9400C. It was 2 years since installation.

I found this is a Atmega 3290V microprocessor based controller. It has two 48V DC relays which provide switching to switch the mains Line out to “OFF” or “ON” outputs (presumably for controlling open/shut on zone valves). One pair of outputs is for the h/w circuit, the other for radiator space heating.

I discovered that when one circuit was on, the relevant relay only received 21V DC, well short of 48V. With both circuits on, there was only 16.5V. Presumably the available power had been reducing over time so that at first there was only enough to trip one relay and then eventually neither relay would switch.

There were two aluminium-can electrolytic capacitors near the diode bridge & I replaced these with capacitors rated 63V. This did not fix the problem. I tested the big blue mains series film-type capacitor. It did not seem to leak, measured with a DVM & it measured 0.1uF on a bridge. I was puzzled. I did not understand the markings on this capacitor so I did some internet research. I discovered the capacitor should be 0.68 uF. So perhaps it was leaking and failing under mains voltage?

I replaced this capacitor with a similar 0.68 uF item, but with a higher voltage rating & re-tested the ST9400C. The voltage to the relays was well up, better than doubled and they switched over fine. So that problem was fixed!

The previous boiler & mechanical clock controller had worked fine for 30 years. That did not contain any capacitors though. The fancy Honeywell controller lasted a year before going faulty. Maybe the capacitor was poor quality?

Older, simpler, AC devices are being replaced with more complex devices which have chips & processors requiring DC supply and so more components. The processors run software.

An aeroplane now is a computer network which happens to fly.

This is all fine if the additional risks of complexity are off-set by sound design & maintenance so that stuff works. The risks may increase with the internet of things. I hope I don’t get to the stage where the car won’t start because the fridge is having a network flame war with the toaster!

Douglas Adams foresaw this sort of situation years ago when the spaceship Heart Of Gold was distracted from defending against a missile attack because Arthur Dent had requested a real cup of tea from the vending machine. He was ahead of his time, that man.


14 thoughts on “Honeywell ST9400C central heating programmer fault problem fixed

  1. Graham,

    Whilst your comments about the advance of technology brought a wry smile and some concurrence, I have to say that without the IOT I would not have happened upon your fantastic blog!
    Not only are you an excellent EE you’re also extermly philosophical- hats off to you sir!


  2. bought a brand new programmer and yes its doing this fault
    any idea if another programmer may cure this tried drayton and that’s the same


  3. Good post, I have just diagnosed exactly the same issue in mine. Funnily enough, I recently fixed an oil tank level monitor with the same type of failure. It (and the Honeywell) use capacitive dropper psu designs using an X2 mains rated capacitor. These self-heal when they develop failures due to over voltages and transient spikes, but this involves losing some of the plate area so the capacitance reduces a little every time the capacitor fails. Eventually the capacitance is too low to permit enough current to flow and the unit fails – I could see the backlight dimming on my Honeywell as soon as one of the relays switched on, so that was an instant clue.

    There is a YouTube video showing a temporary workaround by turning the backlight off, but the only real fix is either a new capacitor or a whole new unit.


  4. I have an issue where hot water does not work. Led light is on and I can hear boiler sound, but hot water not coming. Do you think it’s same issue and solution



  5. I also have had low voltages due to my 680uF X2 capacitor reducing to 200uF over about 3 years . The same loss of capacitance then happened with my replacement capacitor again after about 3 years. So I am now on my 3rd capacitor. I know it’s only a half hour job to renew the capacitor but rubbish design. Is there a permanent fix? The X2 capacitor is rated at 305v


  6. Dave E, lots of mains domestic devices use these series capacitors these days. You might try a capacitor with a higher voltage rating to better cope with mains spikes.
    The real solution is to remove the capacitor entirely and have a proper iron-cored mains isolation transformer which feeds AC into the unit at enough of a voltage to show 48V DC on the relay solenoids.
    This solution would also allow for proper earthing of the negative DC rail inside the unit; with the current design, the unit is not properly earthed, but as all the external parts are polymer insulators, this is not so important. Flooding the unit with water could be potentially dangerous however and may not trip a consumer unit RCB, so don’t try this at home!


  7. Graham – it was only after I had experienced this same problem and fixed it that I happened upon your blog post above. My timer was much older and although the symptoms were slightly different, the underlying fault was the same. I documented my experience on my personal blog at Personally the process was useful as it gave me an excuse to fully understand the heating system at our house.

    Anyway – just wanted to say hi and that I’ll keep an eye out for your call sign, perhaps on 40 (I’m in East Sussex) albeit with a fairly rubbish antenna at the mo.


    Ciaran Brady – G8ZSN


  8. Hi Graham Thanks for your great diagnosis and fix information. I have just repaired my THR860S
    Honeywell controller with a new capacitor. WORKS PERFECTLY NOW.
    Thanks for taking the time to share this valuable information.
    Cost me £2.79 for 2 x caps, so still have a spare.
    Cheers Steve S


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s