Everything about the Honeywell ST7100 is rather basic, and for some people quite outdated. But that’s not always a bad thing. This is an extremely reliable thermostat that’s capable of running a whole host of different home heating and water systems / set ups. It’s a 7 Day 2 Channel Programmer that’s very easy to use, making still a popular choice even now. A good, easy to perform swap, or direct replacement for this unit is, of course, the exact same model. This poses the least hassle if you’re someone who wants to be up and running quickly and with minimal fuss.
Unfortunately, the problem is the sourcing of the unit itself. Due to their age, the Honeywell ST7100 is pretty hard to find for sale. Even on websites such as eBay, where vintage second hand thermostats are put up for sale regularly, ST7100’s are rarely found. In fact the most recent used example I could find was from last year and that sold for over £50. So as you can imagine, if you need a replacement in a hurry, it’s not a good idea to wait for the same model to come up at auction or on a heating supply website that deals in used condition spares.
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The best thing to do is buy a newer thermostat that’s fairly reliable as an easy to wire compatible replacement. 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 of the time a new backplate + minimal wiring (Earth not required for the ST9400C) and the replacement unit should work just fine. Another consideration worth swapping the 7100 with is the cheaper Honeywell ST9400A (24 hour 2 channel programmer).
However, if you want to enhance your heating experience and make things more “Smart”, controlling heating settings remotely (via the App on Smartphone or PC), you can always replace the ST7100 with a Smart thermostat, such as a Nest or Hive device. Of course, depending on your specific set-up, this can be a bit more complicated to get up and running.
There are a lot of helpful guides online that show exactly how to do this. Some offer a complete walkthrough, showing wiring diagrams, potential issues you may encounter, and basically giving the laymen a complete installation tutorial step by step from start to finish. Here’s a helpful page regarding the wiring setup if replacing the ST7100 with a NEST. And another source with lots of information about swapping the ST7100 with a Hive. Much more available on the Internet.