What is the difference between Auxiliary heat and Emergency heat ?

I am often asked that question!!  Why? Because some thermostats come with separate Aux and E terminals and most newer thermostats (like the TH8321WF1001) come with a single AUX/E terminal. So what do you do if you have two separate wires in AUX and in E on your old thermostat and you are switching to a TH8321?

Well, let's start with some concepts. First, this is ONLY applicable if your system is a heat pump. Heat pumps use the same compressor to heat or cool, by reversing the flow of refrigerant (simplifying). If you have separate Air conditioning and heating systems, or just a heating system, stop reading! However, if you do have a heat pump, you probably know that if the heat coming from the "compressed refrigerant" is not enough on a very, very cold winter day like -20F (Florida residents stop reading here, please), these heat pump systems generally have a backup, since they lose capacity and efficiency in very, very, very cold weather. A 'separate', if you will, system that generally has a much greater heating capacity but is much less efficient than the compressor heat at milder temperatures ( that is, costs more $ to run) comes to the rescue . These backup systems can be oil, gas or electric (rare in very cold zones). The ones with gas or oil backup systems are called "dual fuel" heatpumps, because the basic heat pump (the compressor that supplies cold or heat, remember?) is electric and, if the backup is not electric- there are two different 'fuels'. So we are going to call "compressor heat" when it comes from the compressor and "backup heat" when it comes from the backup system, OK??

Now, the compressor heat and the back up heat are often "1 stage". That is they are either ON or OFF. But more sophisticated systems can be 2 stage for the compressor heat and/or 2 stage for the backup heat or even 3 stage for one or the other or both (very rare). Which means that, when they are needed, the first stage is turned on , and if it doesn't cut the mustard, the 2nd stage is kicked in and so forth. Remember all compressor heat stages would be 'kicked in' before any backup heat stages are 'kicked in', if needed!

I don't know if I've lost you, hopefully you are still tuned in. But if you are reading this because you are making the thermostat switch and have 2 wires and one terminal, in order to continue to follow this write up you have to know what kind of HVAC system you have. That is, how many compressor stages and how many backup heat stages do you have?? .

If you have a heat pump with  1 compressor stage and 1 backup heat stage (most common), then auxiliary heat is the same as emergency heat and, if you have two separate wires reaching your thermostat , they are probably joined at the unit so you can go ahead and join them and connect to AUX/E terminal . Now that terminal will send the signal to the backup heat when the "compressor generated heat'  is not enough, and the thermostat determines it needs to kick it in (will show AUX HEAT) OR when you press the emergency heat button (will show EMER HEAT). The TH8321 lets you set some parameters in case you are willing to suffer a little ( Back up heat Temp droop, backup heat timer) before the thermostat decides to turn on the more expensive backup heat, but if you are desperate, you can hit emergency heat and turn it on at your whim. Note: If it's a dual fuel system (oil/gas backup, remember?) you CANNOT have the 'compressor generated heat'  and the oil/gas back up heat running at the same time. So when the AUX heat turns on because the compressor heat is not cutting the mustard based on what you programmed ( backup heat lockout, compressor lockout, etc.) or you, wimp, hit the emergency heat button, the compressor heat will be cut off and only the expensive backup heat will run. If you have electric backup heat (rare but does exist in milder weather zones), when the aux heat is turned on because the compressor heat is not cutting the mustard or because you pressed the emergency heat button, the compressor heat will continue to work, together with the backup heat! But the compressor heat may be so inefficient on a  very cold night that you may want to turn it off and the TH8321 allows you to set that as well (Compressor Lockout Temperature)

So, in short, if you have  a single stage heatpump with backup heat: it's OK to join AUX and E and make sure you program the thermostat to tell it when to add (if electric backup) or switch to (if oil, gas) the backup heat stage.

If you have more than one compressor stage or more than one backup heat stage, the TH8321WF1001 can generally accommodate it, but in that case a more careful study is needed to know what to do with the separate AUX and E wires. For two compressor stages and a single backup stage, the old AUX wire might go to W2 and the E wire to AUX/E. In this case you do not want to manually switch to the more expensive  EMER heat until the thermostat kicks in the Aux 2nd stage, unless you know that the compressor has failed!!

Northerners please contribute your thoughts!!!!