Bryant furnace under water. Doesn't run now

Question

I have a Bryant gas furnace. We got heavy rain and power out and had more than 10 inches of water in the basement before I could get a pump going.

The unit is unhappy. No response to heat call. LED starts flashing the instant 120V is supplied.

• The LED flashes constantly (a bit faster than 1Hz). I don't see anything on the label which matches that description. (I'm discounting that the "polarity reversed" error is relevant).
• I tried removing the panel with the labels to see if there were any water behind it. There are three obvious screws. But when those were removed, the panel felt very solid. Nothing budged.
• Fan control on thermostat still runs the fan.

Is there anything I should be able to do here? I can imagine that the filters were partially underwater and need replacement, and at least some of the controls and equipment are well above the high-water mark. But I don't know what is down in the bottom and if this unit can be salvaged.

Answer 1

Given that the trouble indication shows polarity reversal you might as well get a volt meter and find out. Yes polarity reversal is a real thing. Some furnace controls test for it because this condition can interfere with their flame proving circuitry.

The test is simple enough. Measure voltage from the black/line supply wire to the chassis of the furnace. It should be around 120 volts. Measure from the white/neutral to the chassis; it should be near 0 volts. For a little extra reassurance you could measure to the ground pin of a nearby outlet (or a 3-wire extension cord) rather than measuring to the furnace chassis.

Obviously the flood didn't sneak in and swap the wires while you were busy pumping water. I'm actually thinking that the furnace connection to the equipment grounding conductor, aka earth ground, may have failed. Perhaps the furnace control is unable to tell the difference between loss of ground and polarity reversal.

By the way, that lower panel is sealing the blower compartment. You'd do well to work a little harder at getting that open so you can do a visual check inside. If the water level rose enough to even partially submerge the control board, it's definitely within the realm of possibility that electrolysis or stray currents have damaged solder joints, traces, or components on the circuit board.

Answer 2

In this case, patience....

I had pulled the filter doors and threw away the filters to give air a bit more access to the back of the unit and let stuff dry. (I was never able to figure out how to pull the lower panel with the labels in the photo).

It's getting close to 48 hours after I got the water drained and the unit now powers up properly.

Looking inside the filter bays I only see a fan, and the wiring for the fan comes in from above, so I think it's likely that there's no circuitry or wiring at all at the bottom of the unit that would have been actually under water.

Given that, I'm assuming the faults were either a humidity sensor stopping things or humidity on some equipment causing a problem. Now that it's dried out, seems to be working okay.