By: Peter Kirtschej, Service Coordinator, ABR Wholesalers, Inc.
March 14, 2014
In our last newsletter, we talked about flame rectification. One of the reasons we have flame sense failures is due to faulty wiring or faulty connections. The following steps will help you when checking the wiring and connections on a furnace.
1. Disconnect the wires from the control board where the line voltage connections come in.
2. Set the multi meter to the omega symbol (this symbol looks like an upside down horse shoe). This setting will check resistance and conductivity of a connection between points A and B.
3. Take one lead from the multi meter, plug it in to the ground wire, and plug the other lead into cabinet ground. This will let you know whether the furnace has a good ground connection inside the furnace cabinet. A good reading would be roughly 0 to 2 ohms.
4. Next, check the ground wire to neutral wire connection. A good reading is 0 to 2 ohms. In theory, this would mean that the connection between the panel box and the furnace is good.
5. If the resistance reading was high, this would indicate that there is a faulty connection between the furnace and the panel box. At this point, start tracing the wire back to the panel box to make sure there were no junctions that may be loose or corroded.
6. Make sure that the furnace is on its own circuit. Per code, a furnace needs to be on its own circuit, but sometimes home owners tie in lights and receptacles to the furnace circuit. When this is done, every connection in a wire causes more resistance and more heat inside that wire. This means that from point A to point B we now have more resistance, in turn giving us a worse connection.
7. If the wire looks good feeding from the panel box, then check the connections inside the panel box. Make sure that the ground and neutral lugs are tight inside, and if you are qualified, comfortable, and have the proper safety gear, I would also check the main lugs to make sure they are tight.
When talking about a good connection and checking for good connections, remember that if you are checking a wire that is stranded and has, for example, 15 small wires wrapped together inside, it is possible to have a good connection from point A to point B. Indicating a good ohm reading; however, does not guarantee that there is enough gauge wire for the control board of a furnace to register flame sense. For example, when checking for resistance between points A and B on a 5 foot long wire as thin as a human hair that had no breaks, the meter might show a good connection and might have good conductivity as well. However, this would not be a thick enough wire for the control board of a furnace to rectify flame. When the wire isn't thick enough, the stranded flame sensor wire should be replaced with a new stranded wire or a piece of 12/2 wire. Simply cut a 3 foot section of 12/2 from your roll of wire and pull one of the insulated wires out of the casing and replace the flame sensor wire with that wire. Pulling the pin out of the connector should allow the 12/2 to fit. If the pin connection is good, then just cut the flame sensor wire 3 or 4 inches away from the plug and wire nut the 12/2 to that pig tail.
Checking ground and neutral connections is important when diagnosing the cause of flame sense failures. For any questions, or to purchase UEI multimeter DL379, please contact your local ABR branch - or visit us on our website: www.abrwholesalers.com