Rheem model RGAA-125A .Air conditioner works. When I put on heat fan doesn't blow

Filter is always on the return side of the unit .
Look at your unit and trace out the ductwork.
If your unit is up flow meaning it blows air upward then that is your supply vent ,and the return should be at the side of the furnace.
Normally there is a slot on the return vent where you can put your filter ,also there can be filter grill too .

Thermostat is wired wrong on terminals or at inside unit. It could be heat pump

Possible bad motor and/or bad capacitor and/or bad blower relay.

Hi,

Here is a tip that will help you try to figure out why your furnace is not working.

Basic Furnace or Heating System Troubleshooting

heatman101

If I recall correctly, model number Rheem furnace you listed has electronic ignition to light the pilot, when the home's thermostat calls for heat. Is this what you're having a problem with?

The indoor fan will continue to run. When heat pump goes into defrost the outdoor fan stops, the reversing valve switches and the unit is basically working in the cooling mode. In order for you not to feel a blast of cold air in the winter the electric strip heat also comes on so the indoor fan must run while the strip heaters are on.

heat pumps don't work as effient at that temp thats why they have 2nd stage supplemtial heat

pls check your settings. if not you have a defective heater.

On a furnace the fan is controlled by a computer and a sensor ot a oldstyle fan thermostat. Basically it waits till the furnace heats up a little and then turns it on and after it shuts off it will run till the thermostat cools back down may take a few mins. In cool mode the thermostat controls the fan so it shuts down immediatly. This is why the thermostat asks you if its electric or gas heat. Should have a switch somewhere for this.