Central Heating repair guide Honeywell Timer Problem with Back Boiler ST9400C

A look into why another Honeywell Timer caused problems with a 28 year old Baxi Back Boiler.
Fitted timer just now.
Pressed Hot Water, I heard one click
Press Central Heating whilst Hot Water was on, heard second click.
Turned all off
Pressed Central Heating, heard 2 clicks less than 1 second apart.
Job Done.

Or Buy a new one from B&Q for £25, they work really well.

And

[chinesegambler]

I had the same problem, the timer would click on the water side but not the heating. After watching this video a few times, decided to give it go. Ordered the 0.66 305v from ebay and changed it. Now I get to hear the "click" on both the water and heating side and it all works. Only time would tell if it lasts., albeit no cap test and some very dodgy soldering. A few quid saved for now, so thank you very much John B.

[iancraig]

Thank you for this. I had the same problem which took a month to find because one day the heating came on and another day it wouldn't. It seemed so random. Hot water always came on. I didn't find out what was happening until I turned the hot water off and the heating switched on! The valves looked OK. The boiler was serviced and it worked because hot water wasn't being demanded. It's one of those problems that can make you go bonkers.

In the end, a two minute job to replace the st9400c and it all worked again! So

[petechatten-berry]

I know its some time on from when this video was released but it really helped me out as I had the same issue. Got the capacitor from eBay for £2 instead of a new controller for £80 👍🏻

[Jestey]

Many thanks John.


We have a standard 'Y' plan system with a 3 way valve. A few days ago we had no hot water and following a quick check I was convinced that the Honeywell 3 way drive unit was the culprit. I replaced it and all appeared OK, but then the next morning, no hot water again.
The readings I was getting on the 8 way wiring block, were very confusing. One minute a terminal would show 240v and a couple of minutes later it had gone. I checked all of the wiring and everything was correct.
Then I discovered your video and the penny dropped. Sure enough if one relay was energised the second wouldn't switch, and of course the voltages were only present for the circuits controlled by the first relay. Testing confirmed the 680nf capacitor was actually 350nf.
Radio Spares EPCOS 680nF Polyester Capacitor PET 305V ac ±10% RS Stock No. 874-1456 cost £2.12 for 5 (minimum order) including VAT, and free delivery. I now have 4 spares, which should last me out. So 40p against Screwfix 's price of £96.59, for a Honeywell ST9400C programmer. The relays now operate with a reassuring click and the system operates as designed.
In conclusion I owe you big time. Cheers Noel.

[ukmwg]

Just fitted a new capacitor as described in the video and all working again OK. Thanks for taking the time to make and upload the video, I'm surprised to see such poor design from Honeywell.

[daveylad]

Nice info on the problem with electronics of the ST 9400c Only one heat exchanger on a Baxi Bermuda though 👍.

[stuartmorris]

Very smart. I have one of these showing the symptoms you describe.
I should point out an important safety issue. The choice of capacitor is very important. It must be X rated, which means it can be connected direct to low voltage mains and if it fails is less likely to fail in a ball of flame. It must be X2 or X1 rated as these can handle higher voltage spikes. The capacitance value is important because this will dictate how much power is drawn from the mains. excess power is dissipated as heat so a larger capacitance will create more heat and probably lead to failure. A lower capacitance will not provide enough power to power the circuit.
Another thing I should point out is if something else in the circuit has failed and is drawing excess power then replacing the X2 capacitor will not help. It is for this reason I'm not going to repair mine.

[koffibanan]

Great bit of information, thanks! Although I've never seen a back boiler, I don't think they're common at all in Belgium. Almost all systems are pressurized combi boilers. Cheaply made to fail electronics however, seen lots of those :)

[jam]

Thanks for the vid, John. I have a ST9100A which is the same from this aspect. 48V relay is only getting 20V. What a cheap and nasty circuit. How many electrical appliances are thrown out every year due to a bad dropper cap I fear to imagine (same thing happened on my freezer, which I also fixed - with that the dropper cap was voltage under rated for AC). My unit is about 7yrs old.

[piclife]

The caps age by slowly reducing capacitance. I tested many in this circuit arrangement and depending on the quality of the supplied capacitor the effect maybe very fast, like months or, may last many years. I experienced a nightmare batch problem with a product where all units started failing in a few months. Cost the company (not Honeywell) a fortune to replace them all. Take care to fit a really good quality cap from a reputable supplier.

[Itsrach]

Thankyou for this video. Really helped us out 👍🏻

[davidfisher]

Hi John, really great video with good explanation. My problem with this unit is intermittent output to the boiler. Boiler runs for awhile then stops - no voltage. The capacitor on this unit measures .864 microferads (the one on your video was .68 microfarads.) Could this capacitor still be the culprit in this case? May just have to replace the unit.

[rjt]

Hi John, great video and wondered if you could help me with my problem. I have the same programmer and if I put the heating or
The hot water on a hear a click for both but for some reason the heating doesn’t actually come on but the hot water does. I’ve tried them both separately but just doesn’t seem to want to get the heating side on. At the min I’m having to put the heating on via the water but move the 3 port valve over. Could it still be the capacitor not drawing enough power even though I hear the click? Thanks

[Alfapash]

I have one of these and it sometimes switches the CH on when it shouldn't be on. Although its set to auto, all times for on and off set, it changes to once or Continuos on its own. No problems on the HW. Its a vaillent boiler not a combi and only 18 months old. Any ideas, its driving me mad.

[stewartbrown]

The comments may be accurate on the electronics, but less so on the heating, the boiler has a single heat exchanger with connections for both pumped and gravity circuits.If gravity circuits are used for the hot water, the control will only provide heating if the hot water demand is on. The st9400 must be configured as a mini programmer to achieve this.

[schoolprint]

Hi, we have the ST9400C next to the boiler and in our house the CM927 thermostat.

The water heating seems to work fine … the heating this year we are having a slight issue.  The heating comes on at the right time and the radiators all warm up - but when the thermostat reaches the correct temperature the radiators do not switch off.

I have checked the pairing between the CM927 and the little box (HC60NG Relay Box) on the wall beneath the ST9400C and they seem to be paired OK.  The flame does go off of the main thermostat when the heat reaches the desired temperature and also the green light on the small unit goes off.  But the radiators seem to just stay on … where as before the radiators used to switch off at the desired heat.

Any advice would be greatly appreciated.

[meerujaselexstephan]

I have a same problem now, I didn’t hear any click sound, hot water and heating doesn’t work. Must need to change the capacitor?

[prjroberts]

Any estimates on how much to repair if you called out a plumber who says this is the problem and needs replacing? £264

[elainebowen]

My Honeywell timer switch is playing up, I think I'll replace whole unit....like for like...isolate electrical circuit and fit same..unit, probably easier for me. I'll take photo to merchants...think I may be able to fit okay. If not sure get professional in...have a look at it first.

[rebeccaford]

Hi I can't find a capacitor with the exact details that my one has out of this Honeywell model but can someone please send me a link of ones I can buy