My blower motor isn't shutting off after my heat shuts off, causing it to blow cold air into my house. I suspect that the fan limit switch is bad, but the furnace has at least four limit switches. Are these all tied to the blower motor? I'm not sure which ones I should check for the fan limit switch. I'm confident it isn't the thermostat as I swapped it out with another one and had the same issue. It continues to run even after I put the thermostat in the off position.
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What make and model is your furnace? Can you post its wiring diagram, for that matter?– ThreePhaseEelCommented Jan 8, 2021 at 3:35
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1Make is ICP model is N8MSL.– PhoenixCommented Jan 8, 2021 at 3:47
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1Make is ICP model is N8MSL. Not sure how to upload pictures since I can't find the manual online. Looks like it comes from the board to flame roll out switch 1 to limit switch 1 to limit switch 2 to draft sage guard switch to flame roll out 2 back to the board. Im assuming one of the two limit switches in this circuit is what I need to check?– PhoenixCommented Jan 8, 2021 at 3:57
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Please edit your post to include the make/model - comments can disappear at any time. Also, while you're editing, you can drag/drop pics (clear, focused ones that you've taken with your phone/camera) right into the web browser & the site will upload & host them for you.– FreeManCommented Jan 8, 2021 at 12:29
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Have you verified that the fan “on” switch or setting on the thermostat is in the off position?– Ed BealCommented Jan 8, 2021 at 16:21
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1 Answer
Your control board's likely shot
On a newer furnace like yours, the fan limit/thermostat function is performed by the control board and its onboard fan relay. This means that a continuously running blower is generally the fault of the fan relay getting welded shut, or occasionally something else failing inside the control board's logic. Either way, replacing the control board should cure the issue.
answered Jan 8, 2021 at 4:01
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Try tapping the relay smartly several times with a screwdriver handle. Do this while the blower should already be off. The relay might unstick. Then get the relay or board fixed. Commented Jan 8, 2021 at 6:05
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1It could be the thermostat, I had a friend tell me about a customer that had changed control board, contactor, capacitor and the blower motor before calling. He told him of all the $ he spent and could not figure it out. He went to the thermostat and told the owner he could fix it guaranteed for 500$ he wrote up the quote and gave him the bill for 100$ then walked the owner over to the thermostat flipped the switch and the fan shut off. He could have really stuck it to this guy but it was the simplest of fixes. Control board failures usually have error codes in the form of flashing lights.– Ed BealCommented Jan 8, 2021 at 16:28