How to Troubleshoot and Replace an Attic Exhaust Fan

attic fan on roof in front of leaves
  • 4 hours
  • Intermediate
  • 150
What You'll Need
Replacement attic exhaust fan motor
Wire cutters
Wire strippers
Penetrating spray
Wire caps
What You'll Need
Replacement attic exhaust fan motor
Wire cutters
Wire strippers
Penetrating spray
Wire caps

An attic exhaust fan's main purpose is to remove hot air from your attic. When it fails, the attic becomes hotter, and the overall house temperature rises, causing more work for the AC. In cold regions, warm attic air can cause snow to melt on the roof, creating ice dams and causing damaging water leaks. Ventilation fans tend to fail for one of three reasons:

a) An electrical failure due to a breaker/fuse, a short circuit, or a bad connection.

b) A motor failure that could be mechanical as well as electrical in nature.

c) A faulty thermostat.

Step 1 - Perform a Physical Check of the Fan

1.1 - Open the electric panel and switch off the circuit breaker inside.

1.2 - Give the fan a spin to see if it rotates freely. If it does, take it a step further if it is belt driven and verify if the belt is broken, and check the belt's tension.

1.3 - Replace the belt if it's damaged or adjust its tension if it's loose.

1.4 - If it drags or is jammed up, that might be caused by a foreign object in its path or a motor bearing gone bad. If nothing physical is restricting its movement, the motor will likely need to be repaired or replaced.

large fan

Step 2 - Check the Voltage at the Fan

How the Circuit Works

In a properly wired circuit, the power cable should come from the electric panel at the source, then feed into the thermostat's electric box where the Hotwire (black) is connected to the thermostat's white wire. The Neutral (white) wire should be connected to the white wire from a second cable feeding into the motor.

The thermostat's black wire should be connected to the motor's black wire, as it provides the voltage. Acting like a normal switch, it can cut off the hot-line from the motor when the attic temperature drops below its set temperature, thus opening the thermostat's contacts.

Safety Note - The following steps will expose you to live circuits and components. Don't attempt them unless you're certified or well experienced with such electrical work. If you're not totally comfortable working on a live circuit, you must call an electrician to complete the troubleshooting and repair the problem. If you decide to proceed, put on protective insulated gloves and safety glasses.

2.1 - Check the dedicated circuit breaker at the electrical panel to see if it’s tripped. Switch it to OFF then back to ON for extra measure. If the breaker trips or if the lever stays loose instead of being tightly held, proceed to Step 3 (3.1 to 3.6).

2.2 - With everything good at the circuit breaker panel and the breaker switch back OFF, put on your protective gloves and remove the cover from the electrical access on the motor.

2.3 - Locate both terminals on the motor where the cable from the thermostat is connected, then turn the circuit breaker to the circuit back ON.

2.4 - Rotate the thermostat's dial to its lowest (coldest) setting. If it's a mechanical device, you should hear the click when it closes.

2.4 - Adjust your multimeter on a high scale AC Volts setting, and take a voltage reading by touching the probes across the two terminals. If you don't get a reading, fully rotate the dial the other way to make sure that it doesn't operate in reverse.

2.5 - If you get 120 volts reading for either one of the two measurements and the fan doesn't operate the problem lies within the motor. It could come from an open winding or loose wire.

2.6 - If you don’t get any reading, it could be from a tripped breaker (Step 3) or a bad thermostat (Step 4).

Step 3 - Check the Circuit Breaker

3.1 - Before taking everything apart, check the electrical panel to see if the fuse or the breaker is tripped. If the fuse is burnt, replace it with another of the same rating.

If it’s wired to a circuit breaker, make sure it is switched ON by sliding the lever all the way to the OFF position and back all the way to the ON position, making sure it securely hooks and stays in position. If it springs back somewhat or trips again, you either have a short circuit somewhere in that circuit or else a defective circuit breaker.

3.2 - To quickly check if the breaker is faulty, carefully remove the front cover from the electric panel. Removing the cover will expose you to multiple live circuits and 200 plus Amps, so unless you’re qualified or extremely comfortable working inside electrical panels, get a professional electrician to complete this task.

3.3 - If you decide to go ahead with it, make sure first to put on a pair of insulated gloves, then loosen the terminal screw of that particular breaker, and disconnect the circuit’s wire from it.

If the breaker still doesn’t stay on or its lever is loose, replace it with one of the same ratings. If it stays on, use your multimeter to get a voltage reading across the breaker’s terminal screw and ground. If you don’t get a reading, the breaker again is defective and needs replacement.

3.4 - Pull out the breaker from its slot and insert the replacement breaker in its place. Switch the new breaker OFF.

3.5 - Loosen up the terminal screw and insert the circuit’s wire into position, securing it in place. Switch the breaker back ON—it shouldn’t trip.

3.6 - If the new circuit breaker tripped, you probably have a short in one of the appliances plugged into one of the outlets or within the wiring in that particular circuit. Remove and disconnect every appliance that is plugged into that circuit to isolate the circuit from the defective appliance.

3.7 - Reset the breaker and if it doesn’t trip, check to see if the fan is working.

3.8 - If the breaker keeps tripping, you probably have a short-circuit within the circuit itself. You can either call an electrician to fix it or starting at the far end, disconnect every outlet, one at a time until you get to the one tripping the breaker. If you can’t localize the short circuit inside that electrical box, you could have a problem inside the wall and should get an electrician to fix it.

Step 4 - Check the Thermostat

4.1 - Following up from step 2.6 where you couldn’t get any voltage reading at the motor if the circuit breaker is ON and in working order, as determined in Step 2, the next step would be to remove the access cover to the thermostat.

4.2 - With the thermostat set at the lowest (or coldest) setting, take a voltage reading between the ground and each of the two terminals or wires from the thermostat:

a) A voltage reading present at the white thermostat wire indicates that the voltage gets to the thermostat. No voltage indicates that there is no voltage coming from the electric panel, that the circuit is opened along that line, and may require a certified electrician to troubleshoot.

b) Voltage present at the black or colored thermostat wire indicates that the voltage gets through the thermostat and should also appear at the motor unless there is a break in the wire caused by a loose connection or connector, or a broken wire. The breaker must be switched off to troubleshoot for the physical break but should be easy enough to find using continuity testing.

Step 5 - Remove the Fan

large fan in vent system

Having reached this step into troubleshooting and fixing the attic fan should lead you to consider the possibility of replacing either the fan motor or probably the whole fan.

Once you’ve decided that the fan motor has to be repaired or replaced, you have to remove it from the wall or from its casing to be able to bring it to an electrical repair shop for evaluation or motor substitution.

Their technicians will likely have plenty of experience to expertly guide you through the appropriate solution, especially if the fan is an older type and hard to find with the proper bolt pattern—fixing it can be nothing compared to finding the right substitute which can sometimes constitute the true challenge.

5.1 - Before you begin, make sure that you cut the power to the attic fan. Locate the correct circuit breaker inside the main panel and switch it to the off position. Remove the motor’s access cover and with the terminals exposed, use a test light or a multimeter to take a reading between the neutral white wire and each of the colored wires inside its enclosure to ensure that there is absolutely no voltage present.

5.2 - Disconnect the three wires coming from the power source feeding into the fan’s terminal box. Loosen the cable clamp and pull out the wires from the box.

Step 6 - Remove the Fan

6.1 - Look around the fan where it meets with the wall and find how the fan is attached to the structure, if it’s with screws or bolts, where they’re located, how many are used to hold the fan in place, and what tools you’ll need to remove the fan.

6.2 - You should also check how the fan motor and impeller assembly are attached to the fan’s main housing. As the motor is often attached to the wire guard, removing the guard/motor/impeller assembly instead could offer an easier solution to get the motor out.

6.3 - Using the proper screwdrivers and wrenches, remove the three or four screws/bolts holding the complete unit in place, or the ones holding the guard to the housing, depending on how you decided to get to the motor. Removing one of the top fastener last will make it easier to hold the part in place until it’s time to take it out.

Step 7 - Remove the Motor from the Unit Assembly

hands holding attic fan in metal vent

All fan assemblies are not standardized with the same bolt patterns, bolt sizes, motor and blade sizes, weight, and design. So the description given here is very general and should be approached with an open mind looking for divergences in mechanical concepts.

7.1 - You can now take the unit to your workbench to take it apart and deal with the motor itself. Place the motor on the workbench with the impeller up and spray a fair amount of penetrating oil or a good quality nut loosener onto the set screws and the motor shaft where the blades are attached.

Being exposed to the exterior moisture of the attic, the impeller’s sleeve could’ve built up a coating of rust, so you’ll have to take your time and soak the juncture several times with the solution to give it a chance to get inside and between the metal parts and break down the rust.

7.2 - Don’t try to force the set screws out all at once. Instead, work them slightly back and forth at first, getting them to move a little and gradually gain more movement as the rusty bond breaks down. Even with the set crews loosened up, the impeller sleeve could also be rust bound to the motor shaft and you might need a puller to get the impeller off.

7.3 - With the impeller removed, you can get the proper sized wrench and remove the wire guard from the motor. With this done, you can now take the motor to the repair shop to see if it’s salvageable or if you need to purchase a new one.

Step 8 - Replace or Repair the Motor

If your fan motor is equipped with a start capacitor or run capacitor, the capacitor will often be the likely cause for the motor not working. You should therefore take the motor to an electrical repair shop to get it checked before discarding it.

A replacement capacitor might cost you around $15. A repair shop will also have more expertise in checking the viability of your motor and selecting the right motor as a replacement as they deal with that sort of problem on a daily basis.

An electrical hardware supply store can also help if you simply decide to replace the motor or even get a brand new ventilation fan. But in order to properly match up the motor, you should take the old one with you when you go to the store, as they might have to refer to the bolt pattern to select the proper substitute if an exact replacement is not available.

Step 9 - Re-Install the Attic Fan

attic fan on roof

The following steps are simply the execution of steps 5 to 8 in reversed order to get the fan back in place and working.

9.1 - If you got your motor repaired or if you purchased a new motor, the next step is to mount it back up to the guard or to the housing.

9.2 - Proceed to install the blades to the motor shaft and secure it in place by tightening the set screws. A light coating of grease on the shaft would considerably slow down the corrosion.

9.3 - Reassemble the motor/fan/guard assembly to the housing.

9.4 - Re-install the completed assembly on the wall and secure it in place.

9.5 - You can now open the motor’s terminal box, add the cable clamp to it and pass the wires through it then secure the cable in place.

9.6 - Connect the wires to their proper color-coded wire terminals. Close the terminal box back up. You can now switch the breaker back on and test your fan.

If you’d like to further expand your knowledge with more specific details or different options to consider, you can follow the links and read about attic ventilation and roof insulation or different attic fan options you’d like to consider.