Mr Heater Big Maxx Fan Keeps Running [5 Easy Solutions]

Mr Heater Big Maxx fan keeps running due to the blocked flue, clogged pressure switch tube, broken pressure switch, bad fan limit switch, and a fan limit switch that’s set to manual override.

Keep reading to know how you can resolve these issues and get your heater running normally ASAP.

Mr Heater Big Maxx Fan Keeps Running [5 Easy Solutions]

Here, we’ll discuss the solutions that you can try to fix your heater blower fan from running constantly. After diagnosing which of the following reasons is behind your blower fan malfunction, go for the relevant solution. 

1. Blocked Flue

Your heater produces gases and vapor when it runs. These combustion fumes travel outside your house through the flue. When you get a blocked flue, these fumes have nowhere to go but your house.

That can be dangerous as carbon monoxide can build up in your house. It is always better to have a carbon monoxide detector at home if you are using any heating appliances. That way, you stay safe from the risks of poisoning.

When your unit’s flue is obstructed, the unit will be shut off at the pressure switch. This may be what is causing your Mr Heater Big Maxx’s fan to keep running continuously.


You can tell if there is a blockage in your flue if you look for the following signs:

  • The smell of smoke when the heater is turned on
  • Debris falling from the flue liner
  • White remnants on the flue liner’s walls

If you can spot these signs, your flue liner is most likely blocked and needs cleaning immediately. Cleaning the flue passageway and the flue box is part of your heater maintenance. You should check them before every heating season and perform a thorough cleaning.

Follow these simple steps to clean your heater’s flue passageway and flue box:

Step 1: At first, you’ll need to turn off the gas and electrical supply to your heater.

Step 2: Spot the combustion air blower wiring. Disconnect it.

Step 3: Remove the flue box by unscrewing it with a screwdriver. You may need to remove the blower assembly from the flue box next to clean the flue box properly with a wire brush. 

Step 4: Remove and clean the turbulators. Remove burners and clean them with a wire brush. Then reassemble the unit.

2. Clogged Pressure Switch Tube

The combustion air pressure switch first detects that the blower is running and only then allows power to the ignition controller. So a clogged pressure switch tube or condensation tube function will hamper and result in no heat coming out of the heater while the fan keeps running non-stop.


Your heater will give an error code reading that indicated that the pressure switch is closed. That means you have condensation build-up in your pressure switch tube. Here’s how you get it out:

Step 1: Turn off the power supply to the unit. Remove your heater cover so you can access the pressure switch tube. You’ll notice some water in there.

Step 2: Disconnect the tube from the fitting Get a vacuum cleaner. Simply put the hose over the line before turning your vacuum on. 

Step 3: Make sure you’re disconnecting your vacuum lines from your pressure switch. The pressure switch can’t handle as much vacuum as this is going to put forth through it. Mark the vacuum lines before disconnecting. If you put them back on in the reverse order, your heater won’t work and start. 

Step 4: Now, turn the vacuum on and water will start coming out quickly. 

Step 5: After that, a good best practice is to take your hose off here and look at the outlet hole to make sure nothing is blocking it.

Step 6: Reconnect your lines here to your pressure switch, making sure you don’t reverse those. You can now click your furnace back on. 

3. Broken Pressure Switch

A broken pressure switch will also prevent the heater from igniting and keep the fan running.


Replace the pressure switch.

4. Bad Fan Limit Switch

The fan limit switch is what directs your furnace’s fan to turn on and off. This is what stops your heater from blowing cool air into your house when the furnace turns off automatically after the desired temperature is reached. When this switch is broken, your blower fan will be unable to turn off automatically as it normally does.


You’ll need to replace the fan limit switch. Hiring a licensed professional is a good idea if you don’t have any experience with electrical appliances at all. 

5. Fan Limit Switch Set To Manual Override

When the fan limit switch is set to manual override instead of automatic, the blower fan will keep operating.


Locate your fan limit switch by consulting the manual. Then check to see if the little button is pressed. If it is, that means your blower fan is set to manual override.

In this state, your fan will keep running non-stop. To fix this, simply pull out the button and your fan limit switch will be reset to automatic. In automatic mode, your blower fan switches on and off as per the thermostat’s call for heat.


How do I reset my Mr heater?

Ans: Shake your heater lightly from side to side. Press the heater power button twice. This will reset your heater.

How much does it cost to replace a limit switch on a furnace?

Ans: The part itself will cost around $5-20. If you hire a licensed electrician, the replacement can cost up to $150 to $300. This includes diagnosis as well.

What causes a limit switch to go bad?

Ans: Improper airflow resulting in repeated overheating of your heat exchange can damage your limit switch. You should keep your heater filter clean and make sure your air inlet and outlet aren’t obstructed to maintain good airflow.

What does a roll-out switch do?

Ans: This is a safety device. When flames inside a gas furnace somehow creep back into your heater unit, the roll-out switch protects the heater from overheating, exhaust build-up, etc. by switching off the unit.

Where is the reset button on my furnace?

In most models, the reset button is going to be found in the blower compartment. Check the sides of the blower motor for the reset button. 


With the simple procedures we’ve discussed above, hopefully, you can solve the issue behind why your Mr. Heater Big Maxx fan keeps running.

If you have more queries, we’d love to hear you. Leave us a comment below. You’ll hear from us soon. 

About William

William is the founder of He has real life practical skills in fixing smoker & heating appliance issues. He loves to share his knowledge & helps others in fixing their appliances & saving their money. William firmly believes that anyone can repair his or her unit with the correct guidance & knowledge. See more at about us.

6 thoughts on “Mr Heater Big Maxx Fan Keeps Running [5 Easy Solutions]”

    • Hi Randy

      The common reasons that can be behind why your Heater fan keeps running are blocked flue, clogged pressure switch tube, broken pressure switch, bad fan limit switch, and a fan limit switch that’s set to manual override. Fix all these issues. You can read our guide again.


  1. Is there is any solutions for a pressure switch to not plug up? We have had a pressure switch plug up 4 times in the last month and a half. Wondering what the next steps would be. We have replaced the pressure switch already also.

    • Hi Emily

      Generally, a pressure switch may not work due to one or more reasons like a blocked intake air vent or a combustion air vent, failure of the draft inducer motor, clogged condensate drainage, electrical failure of the pressure switch, or leak around assemblies. First, try to find out the main culprit behind the issue and fix it.

      If you are not comfortable with the whole process, take help from a technician.



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