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How To

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Furnace Short Circuit

The central heating and air in this house stopped working and needed to be diagnosed. There's an obvious problem with a short circuit at the sub-panel power feed but whenever a short circuit develops it's important to understand what caused the short in the first place.

We did a lot of disassembly and testing to find the possible contributors to the short but we were only mildly successful in finding the source of the problem.

We ended up replacing the main power cabling to the unit – check out the project to see just what it took to bring the unit back online.

Note 1 - The main circuit breaker for this furnace was switched to off before work began. Voltage at the subpanel within the furnace closet was checked with a digital multimeter to validate that no voltage was present prior to disassembly.

∗Note 2- Every single wire presented within this project was removed, inspected, and reinstalled in an attempt to locate the short circuit.

furnace burned wire

Here's the source of the problem. This picture represents an electrical sub-panel that is installed within the same closet where the furnace resides.

The wire on the left (A) should have a bright red protective casing.

furnace short circuit

As a comparison, here's the same circuit breaker with the new wiring installed and the old wire shown in the foreground.

The casing on the original red wire at left has burned off due to high heat and about 8" of the casing has been turned to ash.

film icon

The video outlines the problem and the steps that were taken to repair the central heating and air conditioner.

furnace power cable

The power cables are routed from the sub-panel and enter the left side of the furnace. These wires tie into another set of circuit breakers that are contained within the furnace. The casing on these wires has bubbled up but we found no evidence that a short exists here.

The red wires have the most potential for overheating and melting but again there's no evidence of that occurring.

The wire-distance from the sub-panel to this location is about 3 feet.

furnace circuit breakers

The wires enter circuit breakers within the furnace. As shown, these breakers were manually switched to the off or "open" position as the testing progressed.

circuit breaker exit wires

The wires exit the circuit breakers and connect to mechanical relays within the furnace.

furnace relays

The relays are mechanical in nature and cycle on and off thousands of times throughout their service life. This constant switching makes them highly suspect as a root cause of the problem.

…but we found no evidence of a short circuit with these relays.

furnace power cable

It's a little anticlimactic but after checking everything we could think of to locate the short we replaced the power cable from the sub-panel to the furnace. We're guessing the wiring shorted on the subpanel casing. The casing entry hole is sharp and it may have cut the casing on the burned red wire.

There are three wires within the cable casing: red, black and green. The red and black wires are for power and connect to circuit breakers. The green wire is a ground wire. The ground wire grounds the furnace cabinet to the subpanel casing. If either of these components were to somehow become electrified due to a major malfunction the electricity will be wicked away to ground and reduce the likelihood of injury if they were touched by someone.

furnace cable connection

Cable installation is easy. The straight end of the cable is threaded and attaches to the subpanel with a nut – just thread it on and tighten it up.

furnace power cable curved end

The curved end of the cable attaches in the same manner to the furnace casing.  A plastic nut acts as a keeper and tightens up from the inside of the furnace.

closed furnace

Connect the wires to the circuit breakers in the subpanel and furnace using the same orientation as the old cable.

Tighten everything up and set all of the breakers to the on position including the main panel servicing the house.