I just replaced an older Honey well T-stat on my gas furnace/AC unit and it worked great for a few days, then it would intermittently not shut off the gas/furnace once it reached heating setpoint. My older t-stat did the same thing. I don't think it's the furnace because once setpoint is reached, the "Heat On" indicator should turn off on the display; it doesn't. Do you know what causes this?

I have not looked at that particular model, but some electronic t-stats have anticipator settings and differentials you can change. See if it has these settings and write back. Anticipators shut off the heat before the temp at the t-stat is actually raised to keep the temp from going over the set point. Another thing you need to look at is the location. Is the t-stat 3 feet away from an exterior wall or heat source such as an electric lamp,or source of cold air? Also, can a draft get to the t-stat? It sounds a lot like the latter. Where the t-stat is mounted is very important as well as what is behind the t-stat. The old one was probably okay. I have seen drafts come from the wall where the t-stat is mounted and sealing behind it with foam solved the problem. Tell me what you find and I will look into it further. DrTom4444

Thanks for the quick reply. Just so you know, I have worked on complex industrial and commercial HVAC systems for many years. There are no anticipator settings but there are settings to, 1) protect a compressor from starting too soon after shut down and 2) to limit the number of cycles/hour depending on if the heat source is electric, gas or oil. I have it set on the gas setting which is what I use. This t-stat is about the most simplistic device available, other than a mercury bulb style unit. I've lived in this house for 12 years and have not had any problems with the mounting location which is on an interior wall, the main hallway just below the return air grill in the ceiling. I suspect faulty electronics so am curious if this model has a history of this issue. It worked great for about a week or two before it started doing this. You can observe the display when the temperature drops below set point and the call for heat signal starts the furnace. I can tell that it is sensing a heat rise on the display as the temperature rises, but as it reaches set point, the call for heat should turn off and it doesn't always do that; the "Heat On" indicator stays on when it should turn off and you should also hear a relay click open internally, it does not. It is an intermittent failure. At that point I have to lower the set point 1 or 2 degrees for it to open the call-for-heat signal. At that point the gas shuts off and the fan continues to run until the air cools down and stops too (no issues there). If I leave the set point at the new lower level the problem goes away, i.e. it will control just fine at the lower set point.