I must say that these CT30 and CT80 thermostats and your existing Enersaver thermostat look nice. The big back lighted displays would be much easier to read than the small non-illuminated LCD display on my old Honeywell Chronotherm III. Yet I would be reluctant to install one of them on my heat pump system for several reasons.

First off, I see in the instruction manuals for the CT30 and CT80 that they don’t have provisions for using the so called “system monitor” or “fault” signal that many heat pumps offer. In my case the adaptive defrost controller (ADC) will flash a red LED on my indoor thermostat to alert me that the heat pump coils may not be defrosting properly. In addition, that same signal wire will send a “reset” command to the ADC just by toggling the “mode” switch on the indoor thermostat between normal / auto and the emergency heat modes.

Second, the fact that the Enersaver (not sure about the CT30/80 units) has these adjustable “deadband” and “lag” parameters suggests that the thermostat does not have a very sophisticated “recovery algorithm”. A good recovery algorithm goes a long way in minimizing the use of auxiliary heat.

Like you, I too was struggling with my system using expensive auxiliary heat all to often. Even the best indoor thermostat recovery algorithm can’t cope with an unusually cold weather event. Yet I was also seeing my auxiliary heat come on after manually bumping the indoor set-point up just 1 degree on a mild winter day.Hmmm…..

Then I discovered that my outdoor thermostat, which was supposed to block the activation of auxiliary heat when the outside temperature was above the thermal balance point of the system, had issues. These issues included, very poor calibration, way too much hysteresis and lousy repeatability. Once I got that resolved, the energy savings came pouring in.

Hi Daniel,

First, thank you for all the effort you but into this site. I only found it the other day and have not had time to absorb all the good stuff you have written. Well done!

Yes, my Honeywell Chronotherm III (model T8611G) is a very heat pump centric thermostat. It was installed by my mechanical contractor in 1999 as part of a new Trane heat pump system I purchased from him when my 1979 vintage Armstrong heat pump died. Interesting enough, the TG8611G was not the most current model of Chronotherm III thermostats in 1999. The printing date on my TG8611G owners manual is 1987. While some of the other contractors I got bids from were promoting the newer versions, my contractor refused to sell me anything but the old TG8611G. He said the recovery algorithm in the (then) newer model was getting too much negative feedback from customers, so he was sticking with the old tried-n-true TG8611G.

What you have done by manually disabling your indoor thermostat from activating aux heat is sound, simple and cost effective. If you would like to automate things, adding an outdoor thermostat (ODT) can be pretty easy. The trick is in finding a decent ODT and then making sure it’s located properly (i.e. in the shade and not so close the heat pump that it makes false readings). As you see fit, send me an email and I can try and help.

While I’m pretty happy with the system I have now, I too have a growing list of thing I will want when it comes time to replace my existing system. Maybe we can compare notes

Thank again,

Steve