Furnace service switch rusting


  #1  
Old 05-21-21, 12:51 PM
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Exclamation Furnace service switch rusting

Hi. The power switch, on/off power switch mounted to the exterior side of my furnace (not the breaker), is badly rusted. It's inside the house, in the utility room in my basement. Natural gas, 93% or there abouts, efficiency unit. It vents out the side of the house via PVC. The only other appliance is that area is a natural gas water heater vented thru the roof. There's no visible water or condensation inside or on the exterior of the furnace. The room is dry. There is a condensate pump that drains from the furnace to the home sewer waste pipe ( PVC from a bathroom on the floor above) via a flexible plastic tube which is not blocked. The pump drains. All were professionally installed several years ago and have worked flawlessly.

Back to the switch - the AC wiring is fed from the breaker box, located in another area of the basement. At the furnace the wiring comes down from the ceiling above (open basement ceiling, all work exposed) via a PVC tube to a metal, new-work electrical box housing the switch. The box is screwed directly to the furnace side. The switch is on/off just like your basic light switch but heavier duty rating. The box interior, the metal cover for the box and the switch itself all have some fairly severe rusting. But it functions fine. Heat and AC all work. The switch works. But why is it rusting?? It's been getting worse over time, as you might guess. No water/moisture apparent in the wiring or tubing. I want to get this figured out. I plan to finish reno'ing the place and hopefully sell end of summer or early fall this year. Thank you in advance!
 

Last edited by IDMostThings; 05-21-21 at 12:52 PM. Reason: clarification
  #2  
Old 05-21-21, 01:05 PM
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Welcome to the forums.

Don't know what to say..... it takes water/moisture to get rust.

Possible problems:
1) A/C evaporator coil drip pan is overflowing on very humid days.
2) The furnace is the condensing type that uses a condensation drain.
Possibly a clog in that drain system.
 
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Old 05-21-21, 01:15 PM
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If the switch is attached to the plenum in summer there may be condensation getting it wet. It could also be that the box had a thin layer of plating that just didn't last.
 
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Old 05-21-21, 01:29 PM
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The box is mounted BELOW the plenum and is on the side of the main furnace cabinet - directly next to where the exhaust is vented via the plastic pipe to outside. I guess humidity is being transmitted to the switch box via the screws/holes that mount it to the furnace. Would a removing the old box and installing a new one (and switch) then caulking any holes correct it (using the high heat variety caulk such as for a stove hood vent)?
 
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Old 05-22-21, 11:34 AM
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I can see replacing it. I don't see the need to seal it.
 
 

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