Programmer Horstman to Honeywell ST9400C

11 Dec 2015
Reaction score
United Kingdom
I'm replacing Programmer Horstman H27XL with a Honeywell ST9400C but having problems with the wiring as the Horstman had 9 connectors and the Honeywell has only 7.

I've attached a photo of the Horstman wiring and a photo of how I've wired in the Honeywell.

I don't think it's heating the water

The central heating works

What have I done wrong?

I have an interconnecting loop over



  • Horstman.jpg
    240.5 KB · Views: 120
  • Honeywell (1).jpg
    Honeywell (1).jpg
    193.2 KB · Views: 119
Last edited:
Sponsored Links
Thanks Chris,

I did as you said and now I've hot water but no central heating
  • Thanks
Reactions: CBW
Is the heating still on and thermostat still calling for heat?
Sponsored Links
The heating is still on and calling for heat.

The black wire is still in 4
Mmm. It should be - especially if nothing has changed regarding the black wire.

I can't help but think something else has 'turned off' the heating.
  • Like
Reactions: CBW
Have you disturbed the black wire or has come loose? Photo of changed wiring please
The black wire is securely in place.

I've just put the wiring back as I had it originally in the photo, and as before, the central heating is working but not the hot water
  • Thanks
Reactions: CBW
can you post a photo of the new wiring?
With the wiring as per the attached photo, the hot water heating doesn't work.

The central heating only runs if the hot water is selected to 'OFF'


  • 20231219_205946.jpg
    198 KB · Views: 62
  • Thanks
Reactions: CBW
Do you know what type of heating system you have?
Despite the modern wiring, might you have a Gravity system?
  • Thanks
Reactions: CBW

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links