Set your wall thermostat to the lowest temperature setting. Locate the breaker inside your main breaker panel that supplies electricity to the furnace motors and place it in the "Off" position.
Check for leaking gas from your Rheem furnace prior to turning it on. Smell around the furnace and along the floor. If you smell gas, leave your residence immediately and call your gas provider. Do not attempt to start your furnace.
Grab the handle at the bottom of your Rheem furnace and pull the control door from the burner compartment. The burner compartment houses the gas valve, direct spark igniter, burners and blower motor.
Turn the on/off switch on the gas valve to the "Off" position. Wait 5 minutes to allow gas to clear from the burner compartment. If you smell gas after 5 minutes, leave your residence immediately and call your gas provider.
Remove the door from the blower compartment located above the burner compartment to access the furnace filters. The filter is located on the bottom or side of the compartment in an up-flow furnace. Pull the retaining rod from the up-flow furnace filter and pull the filter from the blower compartment. Locate the down-flow furnace filter at the top of the blower compartment. Push up and in on the filter to release it from the retaining bracket, pulling it from the furnace.
Vacuum the dust and dirt from the furnace filter. Clean the filter with mild dish-washing soap and water, rinsing the filter until the water runs clear. Allow the cleaned filter to air dry, and replace it into the furnace. Replace the door on the blower compartment.
Turn the on/off switch on the gas valve to the "On" position. Replace the control door to close the burner compartment.
Place the breaker in the "On" position to supply electricity to your furnace. Set your wall thermostat to your desired temperature setting. The direct spark igniter lights the burner once the ambient room temperature trips the wall thermostat, signaling the thermostat to turn on the furnace.