Mr Heater Big Maxx Fan Keeps Running [Reasons & Easy Fixes]

Mr Heater Big Maxx fan keeps running for a few minutes after the thermostat is turned off as part of the cool-down process. This helps to distribute any remaining heat throughout the home and ensures that the system shuts down safely. It also helps to prevent the buildup of heat within the heater itself.

But if it runs continuously, it is an indication of potential issues relating to:

  • Incorrect thermostat setting
  • Bad thermostat
  • Malfunctioning pressure switch 
  • Faulty control board 
  • Or the defective fan itself 

Start by checking the fan setting in the thermostat and making sure it is in auto mode. Also, inspect the thermostat, pressure switch, and control board for proper operation. Replace the faulty parts. If the issue persists, consult a qualified HVAC technician.

mr heater big maxx fan keeps running

Possible Reasons & Solutions When Mr Heater Big Maxx Fan Keeps on Running [Quick Overview]

Before digging deeper, let’s have a short glimpse at our quick table below, which highlights all the probable reasons for this issue with quick fixes:

Probable Reasons Recommended Fix 
Incorrect thermostat setting Check and ensure the thermostat is set to “Auto” fan mode.
Malfunctioning thermostat Inspect the wiring connection of the thermostat. Fix the loosened one. If required, replace the bad thermostat.
Malfunctioning pressure switch Clean the blockages from the heater vents. Inspect the pressure tube for any clogs and clean it.
Bad pressure switch Replace the bad pressure switch.
Faulty control board or fan motorReplace the faulty control board or the complete fan assembly 

How To Fix Mr Heater Big Maxx Fan Keeps Running [Easy Solutions]

Continuous operation of the fan when the thermostat is off could lead to increased energy consumption and potential wear on the fan motor. Thus, addressing the issue promptly is crucial.

Here are some basic troubleshooting and easy solutions you can apply to your Big Maxx fan cycling properly again.

1. Delayed System Shutdown

If your Mr. Heater Big Maxx fan won’t shut off even after the thermostat is off, it simply would be a system turn-off time delay from the system.

The fact is simply like this: After you set the temperature, the thermostat calls for heat, and the burners start to heat the heat exchanger. When the heat exchanger warms enough, the fan of the heater turns on and blows air over the hot heat exchanger. This conditioned air then blows throughout your room via ductwork.

The heater blows hot air until the thermostat says to stop. Then burners turn off and the fan continues to run (usually 60–90 seconds or so) to push the remaining heat from the heat exchanger as well as to get down the heat-soaked burner temperature to a safe level.

How to Fix:

If your heater fan keeps running due to a delayed system shutdown, there is nothing to take action in this instance. Just wait for some time to cool off the system.

If there is still the same case, be sure something is wrong within the system that needs your attention. Keep reading to get details.

Also read: How to troubleshoot the common issues with Mr heater Big Maxx.

2. Incorrect Thermostat Setting

Incorrect temperature settings are one of the big factors to blame if your heater fan keeps running with the thermostat off. The most common mistake that users make is setting the thermostat fan setting to MANUAL mode.

Actually, Mr. Heater’s fan switch has three positions: AUTO, OFF, and MANUAL. In AUTO, the blower turns on and off as with the heater cycling. But in the manual setting, the fan will run continuously even when the heating element is not in operation.

thermostat fan of mr. heater

[Note: Since the fan motor runs nonstop, it consumes electricity even when the heating system isn’t actively working. This ultimately causes a higher energy bill as well.]

Another big mistake could be setting the thermostat too high. If the thermostat is set to a temperature higher than the current room temperature, the heating system will keep running to reach the set temperature, keeping the fan on continuously.

How to Fix:

Start by inspecting the thermostat temperature of your heater. Make sure it is set lower than your current room temperature. When the room is heated, wait 3–4 minutes, and the fan should stop running.

If so, check whether the thermostat is in the ON position. Switch it to AUTO. In auto-fan mode, the fan will circulate air until the room temperature reaches the set temperature in the thermostat.

inspecting the thermostat temperature of mr. heater

3. Faulty Wiring Connection To Thermostat

If it is all okay with your thermostat setting and still the heater fan won’t stop, a faulty wiring connection to the thermostat is another notable factor to blame. The thermostat is mainly wired back to the heater, which manages all the operations, including opening and closing the blower motor.

If there are loose connections or damaged wires, the thermostat may not send the correct signals to the heating system, resulting in continuous operation.

At times, the thermostat malfunctions due to a high-voltage wiring connection that prevents it from signaling with the HAVAC system.

How to Fix:

Start by removing the thermostat cover and inspecting the internal wiring connections individually. Look for any loose, damaged, or disconnected high-voltage wires. Tighten the loose connection or replace the worn-out one.

inspecting the internal wiring connection individually of mr. heater

Also, ensure all the wiring connections are firmly attached with their respective mounting screw. 

Look for corrosion on the wire terminals or burn marks indicating high resistance connections. Clean terminals with sandpaper or replace wires if corroded.

If required, take a multimeter and check the continuity between the wire terminals on the thermostat side and the heater side. No continuity indicates a break in the wiring. 

If there are still issues with the high-voltage thermostat wiring connection, I’d recommend to contact with a professional since messing with wires can be dangerous as well.

4. Bad Thermostat

If you’re still struggling with your thermostat, the best possibility is that it is defective and causing your heater fan to run continuously. If so, your thermostat needs a quick replacement.

But wait! Thermostat miscalibration is another factor you can check out.

Actually, thermostats need to be calibrated accurately to ensure they provide correct temperature readings. If the thermostat is miscalibrated, it may inaccurately perceive the room temperature, leading to the heating system and fan running continuously.

The thermostat loses its calibration due to dirt buildup or if it gets bumped. Don’t worry! You can easily recalibrate it.

How to Fix:

Before starting, remove the thermostat cover. Now remove any dust or dirt you find inside the thermostat using a soft cloth or Q-tip.

Then, check and ensure that the batteries on your thermostat aren’t dead. If so, replace them first. Now, reattach the cover back in place and check whether your thermostat is showing your current workshop temperature. Use a separate thermometer to confirm the fact.

check whether your thermostat is showing your current workshop temperature

If not, re-calibrate your thermostat following the instructions in the below video. You can check your manual as well.

In the worst cases, you may need to replace your bad thermostat.

5. Malfunctioning Pressure Switch

The pressure switch is an essential safety device in the gas heater that ensures proper ventilation and combustion within the system. This mainly works through sensing the negative pressure created by the draft inducer motor.

As the heater turns on, the pressure switch opens. When the heater turns on properly, it creates negative pressure, allowing the exhaust to leave the unit properly. When the switch senses this negative pressure, it closes it up and allows the unit to fire up and continue the heating process.

pressure switch of mr. heater

But in case the pressure switch malfunctions, it may fail to detect the proper pressure, or it may get stuck in the open or closed position.

If the pressure switch is stuck in the closed position, the control board may take this as a signal that there is adequate pressure, allowing the fan to continue running. If that is so, you will find three flashes on the controller.

To address the issue, here are a few factors to check out as effective solutions:

i) Blocked Flue

If the pressure switch is failing to close, the best possibility is that your venting is blocked up. The pressure switch of a heater is mainly connected to the flue pipe.

If the pipe is blocked by debris or soot, it prevents the free flow of combustion gases.

blocked flue of mr. heater

The exhaust gases recirculate back into the intake vent and limit the airflow to close the switch. As a result, the system will remain running, including the fan.

How to Fix:

Disconnect the power to your heater and turn off the gas supply. This will allow safe access to the venting. Now start cleaning as follows:

  • Remove the vent pipe from the exhaust outlet. Often, it is attached by a twist lock or screws.
  • Use a wire brush to scrub the interior of the vent pipe thoroughly to remove any scale, rust, grease, or debris buildup. Brush from both ends.
  • Vacuum out any loose debris through the vent pipe. You can also blow it out with compressed air. Wear eye protection.
  • When cleaning is done, reattach the venting to the heater exhaust outlet, ensuring a snug connection.

ii) Clogged Pressure Switch Tube

If the pressure switch on your heater isn’t functioning as it should, I bet the switch hose that goes to the fan assembly is clogged with debris. A blocked or disconnected switch hose is another big culprit, causing the switch to stick open and the fan to keep running.

How to Fix:

Inspect the end of the pressure switch tubing from the EXHAUST FAN hose barb. Remove it and check for any obstructions. Try to remove the clogs with a screwdriver or something similar.

Push that through the length of the hose barb, plus at least another 1/2 inch, into the exhaust fan housing. This will clear out anything stopping the vacuum from engaging the pressure switch.

pressure switch tube of mr. heater

[But be careful not to damage the switch body. If required, you can blow compressed air through it to remove loose debris.]

When it is free of debris, reattach the connection.

iii) Bad Pressure Switch

If cleaning the exhaust vent and unclogging the switch hose doesn’t resolve your heater fan issue, you might be dealing with a bad pressure switch at all. Luckily, the issue has an easy fix. All you have to do is test the switch for continuity and replace it if required.

If you are technically inclined, I recommend you call an HVAC technician to complete the task for you.

6. A Bad Circuit Board Or Shorted Wire

The circuit board is the essential part of the heater and mainly refers to the brain of the heater, controlling the burner, ignition, blower, and other essential functions.

The wires attached to the board allow the fan motor to run and stop as per the signal.  In case the control board malfunctions, it fails to send the right signal to the fan, leading to it running continuously.

Though the control boards are durable enough, several complexities make them vulnerable to failure. Like:

  • Shorted wiring connection: This is a common factor due to the control board malfunction. The best chances are that any of the wiring attached to the board is loosened or fried out.  Loose or damaged connections prevent it from controlling other critical parts of the system.
  • Power surges: If there are large fluctuations in the electrical power supplied to the heater, it can damage the sensitive electronics on the control board. Power surges can happen during storms or from issues with the home’s electrical wiring.
  • Moisture damage: If moisture gets into the control board housing, it can corrode connections and short out components. High humidity, condensation, and leaky ductwork near the heater can allow moisture in.
  • Bad capacitors: Your Mr. heater control board has multiple capacitors that regulate power to the blower motor and fan. If the capacitor is worn out or damaged, it can fail to properly regulate power to the motor, causing the blower motor to receive uncontrolled power. This makes it run even when it should be off.
control board of mr. heater
  • Dust and debris: The buildup of dust and debris on the control board and its components can cause overheating issues and electrical shorts, leading to failure.

How to Fix:

It’s quite common to have heater control board issues. If that is so, you can confirm the fact by checking the continuity of the board. Take a voltage meter and test it electrically. If the voltage between the wires shows a reading of 120 volts AC, then the power works fine. If not, be sure your board needs repair or replacement.

For instance, we will recommend hiring a professional to diagnose and repair the board.

7. Defective Blower Motor Itself

Finally, a defective blower itself can cause it to run continuously. Here are potential reasons why a fan can become defective and lead to this problem:

  • Unbalanced fan: If the fan blades become bent or worn unevenly, it creates an imbalance. This added strain can cause the motor to run erratically, including spinning nonstop.
  • Bent fan shaft: The shaft connecting the fan to the motor can get bent due to wear or impact. Much like an unbalanced fan, a bent shaft puts extra load on the motor, which can lead to continuous operation.
  • Failed motor: In severe cases, the entire blower fan motor fails and malfunctions. This can manifest as the fan spinning uncontrollably when power is applied.

How to Fix:

It’s important to have a professional inspect and repair the heater blower motor and the overall fan assembly to ensure safe and efficient operation. They will assess the fan and suggest to you whether it needs repair or replacement.


Is it normal for the fan to run for a period of time after the heater has shut off?

Yes, it is normal for the fan to continue running for a brief period after the heater has shut off. This is often part of the cool-down process to dissipate any remaining heat and ensure safe operation.

Are there any adjustments or settings that can affect the fan operation on the Big Maxx heater?

Yes, there are several adjustments and settings that can affect the fan operation on the Big Maxx heater. These may include the thermostat settings, fan speed adjustments, and any settings related to the fan control or limit switches.

Why does My Big Maxx fan run but there is no heat?

If the fan runs but no heat comes out, there may be a problem with the heat exchanger, gas valve, igniter, or flame sensor not allowing the burner to light. There could also be a blockage preventing airflow.

What should I do if resetting the power doesn’t stop the fan?

If a simple power reset does not stop the fan, there is likely an electrical or mechanical issue. Check the error codes on the control board if equipped. Test components related to the fan for faults. The fan relay, limit switch, or motor may need replacement.

How difficult is it to replace a faulty Big Maxx fan motor?

Replacing the actual fan motor will require disassembling the unit to access the motor, disconnecting the wiring, removing mounting hardware, and installing the new motor. This should be done by a qualified technician.

In Closing:

Your Mr. Heater Big Maxx fan keeps running even when the unit is off, which is an indication of the wrong thermostat setting or a faulty thermostat or control board. After turning off the power, if the fan doesn’t stop within 150 seconds, check the fan setting in the thermostat to ensure it is in auto mode.

I hope this will fix the issue. If not, you’ll need to replace the faulty parts, like a bad thermostat or control board.

Proper maintenance is key to preventing unusual issues with the unit. Like periodically cleaning the housing to prevent blockages, inspecting the power system going through it, or examining the wiring condition for damage that could shorten and malfunction!

If you still need any further help, you can contact our HVAC experts. Just leave a comment to get the best answer to your queries.

About David

David is a man who loves doing research and finding out the best solution to any type of heating appliance issues like heaters and fireplaces. Whenever he finds the perfect solutions to any issues of heating appliances after doing in-depth research, he loves to share them with his readers. In fact, his aim is to help his readers to fix the issues with their heating appliances easily.More about us .

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