Honeywell ST6400C Programmer

The Honeywell ST6400C Programmer is an old boy these days. So much so, that like the ST7100, Honeywell no longer make this model. It can still be found for sale online in used condition, and some of them are in much better condition than others.

If you happen to buy one in good condition, it will obviously be in a clean cosmetic condition and should still have the plastic front flap present. If it does, there's a good chance that the flap should still contain the written instructions, which can be seen when pulling the flap down. If it doesn't, not to worry, this model is rather foolproof when it comes to programming. for more information on this, either read on, or watch the very informative video below.

This programmer is seven-day capable, and if you don't know what that means, it basically lets you program in different heat settings for every day of the week. Note: The ST6400C also features different settings for central heating and hot water control. So it can still be very effective even today, in 2020.

Video Instructions: How to use the Honeywell ST6400C

This programmer is extremely easy to operate. The user gets use of up to three on/off periods each day, but due to the design, this model does not save the time settings if the worst happens and you suffer a power loss of some sort. However, it's not hard to re-set the time, and here is how it is done.

Open the plastic protective cover on the front of the ST6400C and proceed to change the vertical slider on the far right to the top position (set day / time). After this,  use the square buttons which read plus + minus and above those two, the day button to program the date and time to what you need.

When set, move the vertical slider back down to the bottom position. The two horizontal sliders on the left: One controls the heating system and the other controls the hot water supply. They both have the following options available to the user: Off, auto (will go on and off at the time schedule you have set) and continuous (always on).

This model also has an override option (in certain modes) and a copy feature. With the copy function the user can easily copy heat / water timer settings from one day to another. Example: Tuesday, to another day, such as Friday.

There's an instant reset button also present that can help solve some of the common "problems" with this model. One of those being if the screen is totally blank (no display). To reset the unit push something like the end of a pen or match stick on to the reset button, hold for a second or two, and that will reset the programmer.

If you ended up here looking for what to replace a Honeywell ST6400C with, one model would be the ST7100. But like the ST6400C, this model is now somewhat obsolete. So, as a very easy replacement, I would recommend the Danfoss Randall FP715 Si. Looking for the manual (PDF) for the ST6400C? You can read / download that here.