Gas heating, the compressor does not run.

Electric resistance heating, the compressor does not run.

Heat pump/reverse cycle heating, the compressor runs*.

What type is your system?

*An auxiliary/back-up heat source is also normally involved (gas or electric resistance) to function 1) as a booster for the compressor if needed under some weather conditions, 2) as the sole heat source in some freezing/below freezing weather conditions, 3) as the sole heat source in instances of compressor failure. Condition 3 is controlled by the homeowner via manually setting the mode switch on the thermostat to Emergency Heat. Condition 1 is controlled automatically by the thermostat in response to how far the room temperature may fall below the setpoint. Condition 2 is also controlled by thermostat in response to outdoor temperature conditions if the particular thermostat has the capability to do so, otherwise the homeowner can manually engage the Emergency Heat setting as per Condition 3.

Assuming your "central air" is in fact a heatpump, the thermostat may be set to "emergency heat" or else there's an issue with the system that will likely require a service call.

We have a RHEEM RP1430FJ1NA SEER heat pump. I verified it is running (under cooling mode). A couple of years after we had it installed we had a RXBH-series electric heat kit added to cope for when temperatures get below 32. (it was 20 this morning in Milwaukee). All I am trying to understand is the RP1430FJ1NA meant to come on in sub 32 conditions or does the air handler and electric heat kit operate without the compressor? (The air handler was definitely producing warm air by itself.)

See my explanation of Condition 2. Your thermostat may have an option/function that locks-out the compressor when the outdoor temp is at or below a specific temperature.

Got it . thanks

"A couple of years after we had it installed we had a RXBH-series electric heat kit added to cope for when temperatures get below 32."

Check out dadoes suggestion first but when they added the heat strips, they may have wired it up incorrectly.

A couple of years after we had it installed we had a RXBH-series electric heat kit added to cope for when temperatures get below 32. (it was 20 this morning in Milwaukee).

The winter design temperature for Milwaukee is about -2F degrees. It seems strange to install a heat pump in Milwaukee with no electric heating elements. Were you able to keep the house at a comfortable temperature in the winter prior to the installation of the electric heat strips? The Rheem RP1430FJ1NA heat pump is 2.5 tons. According to the chart I found, the heat output at 17F and 47F degrees is 16,600 and 27,000 BTUs. At 32F the output will be about in the middle of these values. Is this heat pump for a section of the house? Are there other sources of heat for the rest of the house?

We have radiant heat for most of the house but there is only one small radiator on the third floor (attic) where my wife's office is... so we opted for the heat pump when we chose an AC to supplement the heating up there. Sometimes the hear pump is handy in the fall before the boiler has kicked in. That is why we didnt initially add the heating element but then later we thought it could be handy for those real cold days despite it being expensive to run. If it is too inefficient for the exterior heat pump to operate when it is too cold then I'm fine just having the heating element/air handler operate for brief periods to warm up the attic. I just couldn't recall if the heat pump was meant to come on or not under those conditions.

Your explanation makes more sense. The other function of the heat strip is to warm the air during a defrost cycle. In the defrost mode the heat pump reverses itself and temporarily operates in cooling mode. The heat strip warms the cold air during this process.

The thermostat connected to an outside temperature sensor determines when to switch from the heat pump to heat strips heating only. The installer should have set this up when the heat pump was installed.