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Question DetailsAsked on 9/8/2016

why does my furnance mother board continue to blow fuse

I lost power a few days ago while I was work. When I got home I was able to reset the breaker to turn everything back on except for the A/C. After troubleshooting I found that the fuse on the mother board was blown. I changed the fuse and as soon as I pressed the door button in order for it to start, the fuse blew. I repeated this twice just to trouble shoot from the thermostat.

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4 Answers



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Answered 3 years ago by Member Services


Sounds like the power outage may have included a power surge or lightning hit or such which caused a short in your board or in some circuit in the furnace leading to the board. The boards are low-voltage, so a hit with high voltage can arc between circuit board paths, leaving a metallic (therefore conducting) path that results in a short. Or if a good overvoltage event, could have arced at a connection or through insulation causing a short.

What you need is a "cold trace" job on the unit by a Heating and A/C contractor - meaning each circuit and path is traced "cold" or without power, to check for shorts (or open circuits also, though that would not blow the fuse) and fix the source, then put in new fuse, and check if the board still works right.

Some techs know how to do this - others will not, and of course how much the board costs makes a difference too - many will just replace the board, though they should check all circuits coming into the board (sensors and safety switches and such) for shorts FIRST so they do not just blow the new board because of a short in something that plugs into the board.

Answered 3 years ago by LCD


The fuse on the mainboard of the furnace/blower is what powers the thermostat. Before replacing furnaces, I'd try removing the thermostat it's connected to. Most of them can be removed from the wall without having to disconnect every wire, but if that's not the case here, the one you want is red (that's the power from furnace).

THEN try replacing the fuse. If it doesn't blow this time, it's the thermostat (check for wires touching. If it STILL blows, doublecheck for wires touching at the installation, and maybe disconnect them all, spread them wide apart, and then try again. If it's definitely not the wires, it's probably time to hire a pro, or replace the furnace board.

Answered 3 years ago by Richter12


Good input by Richter12 - though different brands of furnaces/boilers have fuses or push-to-reset circuit breakers in different places, and sometimes more than one on a given unit - can be on the thermostat leads only, on the main output leads from the low-voltage transformer, on eductor fan and damper solenoid power feed (for condensing units), on emergency shutdown sensors, on gas control valve leads, or protecting the entire mother circuit board - that I have seen. I have seen as many as 4 on a single furnace.

On the thermostat - sometimes you can see mounting screws if you pop off the display cover (on digitals) or the outer housing cover (on non-digital) - many others have slotted holes in the back like many fire alarms, so you lift the unit vertically to disengage the thermostat from the wall. Others have a mounting plate screwed to the wall, then the thermostat unit slides up out of the mounting plate in slots. I have also seen one type of round thermostat which used slotted holes on the back but twisted about 30 degrees to remove it. Regardless of mounting method, be careful not to pull it far away from the wall till you see how much spare leadwire there is - the mounting hardware for the wires is flimsy in some cases and you can damage the thermostat if you pull on the wires.

One other thought on the thermostat - check the batteries - if you had an overvoltage event you could have burst batteries shorting out the thermostat.

I would not keep testing by changing fuses and trying to restart it - the overloads could be a fire hazard, and certainly if the fault is in the circuit board repeatedly over-powering it cannot be good for it. "Cold circuit testing" is the solution for that type of problem.

Answered 3 years ago by LCD

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