EM HEAT.... does it stand for ECONOMY HEAT or EMERGENCY HEAT? They both conflict. Someone's misinformed.

Emergency heat is when furnace coils only are working.

Emergency heating via heat elements will only run till temps get to a certain point then the elements turn off and the heart pump takes over and that is the economy setting. Emergency heat is used to quickly raise temps then heat pump takes over. The elements are very expensive to power ergo the switchover.

Has this always been this way? When the emergency heat is on the outside unit shouldn't run. When in normal heat, no emergency heat, the outside unit should be running unless the temperature is below the balance point ( usually around 35 degrees). If this has always been this way and the outside temperature is above 40 degrees you might have your 1st stage and 2nd stage wires switched.

does the t-stat have a button for emergency heat?

Yes, you may turn to em heat only. May this confirmation help you.

I think you probably have a heat pump. Aux heat is electrical resistance heat coils that produce heat when the heat pump can not produce enough to keep the temperature up (caused by low outside temperature or a problem with the refrigerant system). This resistance heat can be switched on by the emerg. heat button. It also can come on automaticlly (sp) and will be indicated by the aux being shown on the thermostat. Sometime if you set the heat up several degrees by thermostat the aux will come on to help the system catch up to the desired temperature setting. The emerg. heat is a way to turn on the AUX manually. If you do not have a heat pump system then it is possible that you have a heat pump thermostat installed on a non heat pump system. Emergency heat and Aux are both the secondary.

Hope this makes sense1

Bob

Economy Mode = EM
Auxelary Mode = AUX
in em mode heat pump does most of the work in aux either an electric heating element is used or gas depending on if it is gas or all electric

Sorry, neither of these answers are completely correct.
You have a heat pump (or the wrong thermostat). Let's assume you have a heat pump.

In air conditioning mode, it works like every air conditioner you have ever had, but...

In heat mode, it reverses its operation. Have you ever felt the air coming out of the outdoor unit of your A/C unit? It's hot, isn't it. And the air coming out of the indoor unit (out of the registers) is cold. Now for a heat pump to produce heat it simply runs the air conditioner in reverse and the heat comes out in the house and the cold is released outside. Neat, huh!

Here's the problem with heat pumps...when it is really cold outside the heat pump can't produce enough heat to heat your home. So it has an additional heat source called "Auxiliary Heat". This heat comes on automatically when the house doesn't get warm enough. The source of this heat is based on the region of the country you are in. North/Northeast generally have oil heat, other regions have gas, and still others have to use electricity to heat. In Texas, we usually use electricity as the supplementary heat on heat pumps. VERY EXPENSIVE!

Now the "Emergency Heat"...this is exactly as stated in Solution #2. This is manually turned on by YOU at the thermostat when your heat pump fails. This turns on the auxilliary heaters and turns off the heat pump (remember, the reverse air conditioner). Again, this can be quite expensive to run if your heat source uses electricity! Gas and oil may be cheaper. The emergency heat is only designed (normally) to keep the house livable (not comfortable) until the Heating Tech can get out to you and fix your heat pump.

Something else you should know. It is normal for a heat pump's outdoor coil to frost up during heating mode. It will detect this and go into DEFROST mode and melt the frost off the coil. While it is doing this, it will turn on the auxilliary heater to keep the air blowing in the house at a reasonable "warm" temperature, but it will not be as hot as normal. In fact, heat pumps produce a lower temperature heat than traditional heaters. So the air may feel cooler during heating than you are use to anyway. This is normal and is not a sign of a problem.

So what do you do: Set your thermostat to the temperature you want and set the controls for HEAT/COOL and FAN-AUTO/ON and leave the EMERG HEAT off unless your heat pump breaks.

As always, keep your filters clean and your outdoor unit's coils clean and free of debris.

Hope this explains your question for you!

the switch is on your thermostat - look for something that says "system" or "Em heat, heat, off, cool" switch it to "cool" and enjoy. If that doesn't work, call a HVAC service tech. You could have bigger problems than just the system switch. Good luck

The T874R thermostat should be a heat pump thermostat. EM means emergency heat which you obviously are not using. That means that you just need to determine if your heat strip is on while you air conditioner is working. If so you may need to replace your thermostat or you heating relay or sequencer.