Fill one bucket with warm water and 1/2 cup of dishwashing detergent. Fill the other bucket with clean, warm water.
Turn off the power to your furnace. Either unplug your furnace or flip the electric breaker at the breaker box. For safety reasons, the power must be turned off.
Access the furnace blower. Remove the front access cover of your furnace. These covers may simply sit in place, screw into place or slide into place. Remove it by unscrewing it, lifting it up or sliding it out of position. At the bottom of the furnace, you will see the blower fan. It may be located on the right or left side. This fan is the furnace blower; you do not need to remove it to clean it. You just need to be able to get to it with your toothbrush to scrub it.
Clean the fan blades. Dip one toothbrush into soapy water, and scrub one fan blade at a time. Take your time and scrub all of the dirt, grime and gunk off each blade before moving on to the next.
Use the second toothbrush to apply the clean water. Dip your toothbrush into the clean water and scrub the fan blades. This will remove the soap that was left behind from the washing process.
Wipe the fan blades with the cleaning rag. The blower apparatus (fan blades) must be dry.
Reassemble the furnace. Screw or slide the front access cover into place. Wait approximately one hour before turning the power back on.