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How To Remove, Inspect, Clean And Replace Pulse Furnace Spark Plugs

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Different kinds of furnaces from places like Wightman Mechanical are in-use for heating homes and other buildings, but the pulse furnace is a somewhat-unique technology. If you have a pulse furnace, you need to understand the importance of  spark plugs as well as how you should clean or replace them. Below is what you should know:

What is a pulse furnace?

During World War Two, the German military used the V-1 flying bomb to terrorize the city of London; this revolutionary weapon was powered by a pulsejet engine. Today, this same pulse technology is being used to power home heating furnaces with great efficiency. Pulse furnaces operate using a fairly-simple process; below is each step in the furnace heating cycle:

1. Fuel and air are combined and then atomized in the combustion chamber.

2. A spark plug ignites the vapor which rapidly burns and generates hot gases.

3. The burned hot gases pass out of the combustion chamber into an exhaust pipe where the heat energy is captured by the furnace.

4. As the combustion gases leave the exhaust pipe, they open a flapper valve that also controls the introduction of more fuel and air into the chamber.

5. Some of the hot gases generated by the previous combustion cycle are reflected back into the chamber; this causes the new injection of fuel/air to ignite, and the process repeats itself indefinitely until the furnace fuel/air supply is withheld.

This process only requires the spark plug for initial ignition of the furnace, but the spark plug electrodes are exposed to a constant barrage of high temperatures and fuels.  That's why the spark plug needs periodic cleaning and replacement to maintain optimal operation of the furnace; below is how to remove and clean or replace these components:

Tools and materials needed to remove and clean the plug

  • Handheld propane torch
  • Shop towel
  • Bright flashlight or lamp
  • Clamping pliers
  • Deep socket set
  • Screwdrivers

Removing and cleaning the spark plug

1. Disconnect the power source to your furnace – always turn off the power at the breaker panel before beginning work; simply turning down the thermostat isn't sufficient to protect you from potential electrical shock or burns.

2. Remove the access panel to the furnace – use a screwdriver to remove the side access panel to your furnace so you can view the spark plug installation. The spark plug will be screwed into the side of the combustion chamber, and an ignition wire will be attached to the end of the plug. Consult your furnace manual or parts diagram if you have difficulty locating the plug.

3. Remove the spark plug – once you locate the plug, pull the ignition wire from the end of the plug. Next, match the appropriate-sized deep socket that fits your plug, and remove the plug from its mount.

4. Examine the plug for defects – wipe down the plug with a shop towel to remove soot and other deposits. Hold the spark plug under a bright flashlight or lamp, and carefully examine it for any visual defects such as cracks in the ceramic insulator or other breaks or obvious flaws. In addition, inspect the side of the insulator for dark, vertical lines; these are indicative of body-side electrical shorts, and you can be sure the internal electrode is broken or not otherwise conducting current. Throw away any spark plugs that show signs of body-side shorting.

5. Clean the spark plug – once you have verified the overall integrity of the spark plug, clamp the tip of it inside a pair of clamping pliers. Next, apply a blue flame from a portable propane torch to the electrode end of the plug to burn away carbon deposits and other residues. Keep the electrode moving through the flame, and be careful not to hold the end of the plug in place for too long. Continuously check the electrodes for cleanliness, and set the plug aside to cool once it is free of foreign matter.

6. Reinstall the spark plug – once you are finished cleaning the plug, reinsert it into the threads on the combustion chamber wall. Tighten it with your deep socket set, but don't apply too much force or you may strip the threads in the housing.