Honeywell ST9400C

The Honeywell ST9400C is a 7-day electronic programmer that allows you to set different schedules for both your central heating system and hot water tank. It has a total of 3 on and off periods during a day and comes with a variety of control options and features, all of which combined make it an excellent comprehensive heating management system for your home.


 Honeywell ST9400C Thermostat Overview- 7day-2-channel


Features and Specifications For The ST9400C

The Honeywell ST9400C comes with a large display, which is great for the programming process. It shows you the time, date and day of the week. The left side of the screen shows you the heating controls, while the hot water controls are placed on the right.

When you pull the flap up, you’ll see all the control buttons. The main control you need to be acquainted with is the slider at the bottom right of the device. It’s got different modes that will let you set the date, time and heating programs.

You can set a schedule for Monday and decide when you want the programmer to heat your hot water tank and copy that schedule onto Tuesday, Wednesday and so on. You can make amendments for each day and repeat the process for your central heater separately. The device will operate at a range of 0 to 40 °C and is easily adjustable for the winter/summer season. The standout feature of the Honeywell ST9400C is that it’s really simple to install and use.

All instructions are given in clear English, so you don’t need to worry about “learning” how to use it. Perhaps, the best part is the button-press option to copy all your heat settings onto the next day. Hence, those who don’t want to spend too much time on the nitty-gritty can opt for a ready-made schedule that’s pre-fed into the programmer. Additionally, the device is not very energy-intensive and uses only about 10 watts of power. That should help save at least something on the electricity bills.


Honeywell ST9400C Problems and Limitations

Although the programmer comes with a 1-year warranty, its screen sometimes glitches to show you an “internal fault” sign. This is where you get a blank display and the controls become unresponsive. This could mean two things: either the internal capacitors have failed, or the small lithium battery inside has gone flat. Another Honeywell thermostat that can suffer potential problems with the screen is the CMT927.


How to Solve the Problem?

One way to sort out the problem is to swap the programmer with the exact same replacement model. The programmer’s back plate is already attached to the wall, so all you need to do is undo the two screws at the bottom of the faulty unit, take it out and clip in the replacement unit the same way. The whole thing will cost you about £80 at most (brand new unit, or considerably cheaper used), and you’ll have a fully functioning unit up and running in minutes.


The Honeywell ST9400C is a comprehensive programming device for all your household heating needs. While it is simple to use and easy on the budget, this one may trouble you with a blank display issue at some point, but it’s nothing a replacement model can’t fix. And its worth noting that the problem does not affect all units. Far from it. Generally a very good, dependable programmer.

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4 Thoughts to “Honeywell ST9400C”

  1. […] One that springs to mind which, for most systems will require only basic rewiring, is the Honeywell ST9400C. There’s no shortage of 9400C’s available to buy online, both brand new and used. Most […]

  2. Screw Loose

    Experiencing this issue at the moment, hence me ending up at this page. I know I have to buy a new one, but don’t really feel like buying the same model after it breaking like it has. Got some good years out of it, though, it must be said.

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