Why isn't my frigidaire heat/air not blowing warm air at all! Model LRA18HMU2

SOURCE: reverse unit. Heater only blows air, not warm air

If you have a reverse cycle unit (heat pump) your compressor and your fan should be running. You need to determine if your compressor is running.

SOURCE: Frigidaire air conditioner - no cool air coming out.

if you used it alot, i would check it for freon.
checkthe air filter, make sure it's not dirty.
check the front panel, make sure there's no "ice/frost" in the unit.

SOURCE: Central heat does not blow hot air, just warm air weakly

Sounds like some of the elements are not working. You being in Florida they may not have put very many elements in. It was 61 degrees last week. I know because I'm a Dolphin Fan. You will need a current sensing meter to check the elements.

Testimonial: "Perfect... The problem was with the Defrost control unit... apparently the outdoor unit was icing up every night, defrost control element was bad!"

SOURCE: heating unit blows warm air then turns to cold air

it sounds like unit could be kicking out the low pressure switch or if it is a electric furnace it could be the sequincers

SOURCE: I have an Intertherm electric

http://www.nordyne.com/Literature/707716c.pdf
http://www.nordyne.com/Literature/359a.pdf
The diagrams above may not be exactly what you are needing. But they will be close. Usually the diagram for the furnace is on the inside of the blower fan door. Two things come to mind. It seems like your limit switch(s) are cutting out, then in, then out, etc and may have failed totally. These are in the system to prevent the temperature in the furnace from getting too high. Why they are cutting in and out may be due to the lack of sufficient air passing through the furnace. This can be caused by, 1. the fan speed is too slow, or 2. the air filter is needing to be changed. I am not sure how proficient you are with electrical troubleshooting. I would (turn off all power to furnace) find the limit switches (usually at the top of the heating elements) and check to see (with an ohm meter) if the switches are allowing the meter's current to flow through them. Your meter should read near zero, if not, then I would replace the switch with an exact one. If you are proficient with electrical troubleshooting, I would place the furnace in the "call for heat" position, with the power on, (be careful, this stuff can hurt) and I would see if I have voltage at the heating elements, and I would check with my voltmeter across the two leads on each limit switch if there is voltage present. If there is voltage present, then the switch is probably ****.
Depending on the furnace you have, you might have a number of switches that control the heating elements, these units are usually time delay and if they fail, you should replace them with the same delay time. I hope this helps.