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Well...they are kinda the same. The condenser is where the refrigerant is condensed from a gas to a liquid. When in AC mode this happens outside at the AC unit. (You ever put your hand over a running AC unit in the summer? Feels warm!). When in Heat Pump mode the process is reversed. Instead of condensing the gas to liquid outside (which creates heat) the condensing happens inside and helps warm the building up (with the help of the fan of course). So essential the condenser is where the gas is condensed into a liquid, which creates heat. A heat pump can reverse the direction of the flow and cause this condensing to happen either outside or inside.
This is a very watered down version of what happens but I hope this is sufficient for what you are asking.
The "R" terminal on the back of the stat should be 24 volt supply to the stat. For a heat pump, when the stat calls for heat, the "W" terminal goes hot (24 V) and should actuate the reversing valve and start the outside unit for heat. when the stat calls for cooling, The "Y" terminal goes hot and starts the outside unit without the reversing valve. The heat strip is used to temper the air while the outside coil is defrosting when in heating. You need an OHM meter to test for a short in the heat strip. A lot of installers are wiring it to heat all the time when in the heating mode if the heat output is too low on really cold days. Try jumping "R" to "Y" and the outside unit should start. Disconnect the heat strip from power while testing. This is really brief, its a big subject. Good luck.
Unless you just installed or had the thermostat installed recently, I would not point at the thermostat as the problem. With what you have described I would suggest have your units serviced as it sounds like a loss of charge, or failed defrost control board. The "Aux" comes on when the unit is using secondary heating (electric heat strips). That occurs when the demand temperature (what you set it to) in Heat mode is more than the current room temperature, usually ~2-4 degrees higher. Also happens in defrost mode. Defrost is a heat pump cycle that melts any build up of ice on the condenser unit (outside unit). That process is simply the unit running in cool mode which will heat up the outside unit to melt any ice. Defrost cycles vary with manufacture but usually you can tell if is in defrost by looking at the outside unit, during defrost the unit will be warm or hot sometimes there is steam and the compressor will be running but not the fan. Defrost only happens on heat pumps in "Heat" mode. Rheem has a 10 yr compressor and parts warranty, so call the installer or Rheem authorized service company.
Rather lengthy but here it is.
The outside unit(heat pump) has a reversing valve, a defrost board, a crankcase heater, a metering device(txv or piston) and an accumulator. This is what makes it a heat pump instead of a straight air condensing unit.
When in the heat mode the outside unit becomes the evaporator and the inside unit becomes the condensing unit.Before somebody corrects me i am just stating what is different in a heat pump.
When the system goes into defrost several things happen. First the system has to call for a defrost cycle. This happens on a time and temperature sequence. When the defrost board times out for a defrost check(the defrost board has a jumper pin to determine how long before the system will check for a temperature(thermal switch) is open or closed. This switch is in the normally open position. It will close at 35 degrees(depends on the switch). This switch is located on the copper line in the condensing unit. If this switch is closed then the system will go into defrost. Now for the good part. This will shut down the outside fan, switch the reversing valve, and the outside unit will essentially go into cool mode. Because the fan is off the compressor will heat up very rapidly and the refrigerant passing thru the coils will be hot as well. This will cause the ice buildup on the outside unit to melt(defrost). This will continue until a temperature is met or it times out. Then the system will return to the heat cycle. One other thing, depending on how your system is wired the inside unit will turn on the auxillary heat while the defrost cycle is in process. That way you will still have heat while the defrost cycle is in process.
It is not uncommon for steam to come out of your outside unit while this is going on. It can also be quiet noisey sometimes.
Hope this explains what you wanted to know. If not just reply to this with any other question you have about the cycle.