The start capacitor in a furnace offers an electrical power up boost when the unit first starts running. The capacitor helps counteract any potential disturbances in the electrical supply and ensures the system starts to run properly. If your furnace has suddenly stopped powering up correctly, the start capacitor is a good place to start your troubleshooting.

You will need to remove the start capacitor then test it using a multimeter to determine whether the part was the problem. The process isn't complicated if you have multimeter experience, but you should always call in a heating and air conditioning services company for help if you have any qualms or questions.

Things You Need:

• Socket
• Screwdriver
• Socket
• Multimeter
• New capacitor, if needed

Step 1: Prep and Drain the Capacitor

Switch off the main power supply to your furnace using the shut-off at the outlet or the breaker. Locate the top access panel on the furnace and twist the knobs to loosen that panel. Pull down on the panel to remove and set off to the side.

Locate the mounting fasteners for the lower access panel, which screw through the lower part of the frame surrounding the opened upper panel. Use a socket to remove the fasteners, set the fasteners aside for safe keeping, and then pull up on the lower panel to remove and set aside.

Look inside the lower access panel and you should see the start capacitor. Make sure you are looking at the start capacitor and not the run capacitor, which is also in the same area but requires a different procedure for draining. Use your owner's manual as a guide if unsure.

Unhook the wires from the terminals of the capacitor. Set your multimeter to AC readings, hook one probe to each of the terminal ends, and wait until the AC reading drops to zero to show that the stored current is drained out.

Step 2: Test the Start Capacitor

Use a screwdriver or socket to unhook the mounting screws holding the capacitor in place. Place the capacitor on a flat work surface to continue the testing process.

Turn your multimeter to Ohms readings. Hook one probe each up to the two terminals of the capacitor and check the reading. The reading should fall within the Ohms range printed on the capacitor. If the numbers don't match, you need to replace the capacitor. If the numbers do match, call an HVAC tech to diagnose your system for the real problem.

Step 3: Install the New Start Capacitor

Install the new capacitor by lining up the mounting screw holes on both the capacitor and the wall of the unit. Insert the fasteners into the holes then use your screwdriver or socket to tighten the fasteners firmly in place. Hook the wires back up to the capacitor terminals.

Slide the lower access panel back in place and secure with its fasteners. Slide the upper panel in place and twist the knobs to secure. Restore power to the unit then make sure the problem went away. For more information, see a website such as http://www.getzschman.com.