--- title: "Mr Heater Big Maxx Fan Keeps Running" canonical: "https://gadgetguides.org/mr-heater-big-maxx-fan-keeps-running/" author: "Arian" published: "2026-09-10T19:10:20+00:00" modified: "2026-09-10T19:10:20+00:00" language: "en-US" site: "Gadget Guides" description: "Your Mr Heater Big Maxx fan keeps running after the burner fires down, and you're probably wondering if the unit is about to die. It's not. The fan limit…" categories: "Help Guide" attribution: "Gadget Guides (https://gadgetguides.org/)" --- # Mr Heater Big Maxx Fan Keeps Running Your Mr Heater Big Maxx fan keeps running after the burner fires down, and you're probably wondering if the unit is about to die. It's not. The fan limit switch or a stuck relay is the usual suspect, and both are cheap, DIY-friendly repairs. Most homeowners catch this early, before it causes any real damage. These heaters run on a straightforward control circuit. A 24 VAC thermostat calls for heat, the gas valve opens, and a fan limit switch tells the blower when to start and stop. As of 2026, Mr. Heater still wires the Big Maxx the same way across most models, so the diagnostic steps hold up. Let's trace the problem from symptom to fix. ![Mr Heater Big Maxx Fan Keeps Running](https://gadgetguides.org/wp-content/uploads/2026/09/mr-heater-big-maxx-fan-keeps-running-mtvwjne2.webp) Image source: Bing (Web (fair-use with source credit)) ## The Problem: What's Actually Happening and Why It Matters To stop the fan, you need to know what makes it run. The burner heats up the heat exchanger, the air around it warms, and a probe on the fan limit switch senses that temperature. When it crosses the setpoint, the switch closes and the blower comes on. When the burner shuts off and things cool down, the switch opens, killing power to the motor. If that switch sticks closed, it doesn't matter that the heat exchanger is cold. The circuit stays complete, and the fan keeps spinning. In our research, this is the most common reason behind a Mr Heater Big Maxx fan keeps running complaint. Another culprit is a relay welded shut on the control board, or a thermostat that keeps sending 24 VAC to the gas valve and fan circuit. Both keep the blower alive. Figure out which one you're dealing with before you buy any parts. ## Quick Diagnosis: Check This First Before You Do Anything Else **Quick answer:** If the fan runs nonstop after the burner shuts off, test the fan limit switch first. That's the part that fails most often. A stuck relay is second, and a faulty thermostat is third. Before you buy a replacement, do a 10-minute safety shutdown. Turn the gas valve to OFF and kill power at the breaker. Let the unit cool to room temperature. Then open the front access panel and look at the wiring. Three quick checks point you in the right direction: - With the heater cold, the fan limit switch should show no continuity on the fan side. If it does, it's stuck closed. - Set your thermostat to the lowest setting. There should be no 24 VAC across the R and W terminals. If voltage stays, the thermostat is the problem. - If both checks pass, the relay or control board is likely holding the fan on. ## The Decision Tree: Trace Your Exact Symptom to the Fix Your next step depends on the exact behavior. Work through these branches in order. ### Branch 1: Fan Runs After Burner Shuts Off If the blower keeps spinning for a few minutes or indefinitely after the heat cycles off, that's the classic stuck fan limit switch. The burner is done, the heat exchanger cools, but the fan contact stays closed. Test the switch with a multimeter and replace it if it reads shorted at room temperature. This fixes the issue in most Big Maxx models. ### Branch 2: Fan Runs All the Time, Even When Cold If you walk in on a cold morning and the fan is already running, that's not heat-related. A relay with welded contacts or a wiring short is keeping power on the motor. Check the relay next, then inspect the wiring harness for rubbed or pinched wires. The fan limit switch can still be at fault, but less often when the unit never warmed up. ### Branch 3: Fan Never Runs at All This branch is the opposite problem, but it's worth a mention. If the fan won't start even with the burner hot, the fan limit switch could be stuck open, the high limit could be tripped, or the motor capacitor is dead. That's a separate diagnosis, but the same fan limit switch test applies. ## How to Test the Fan Limit Switch (The Most Likely Culprit) ![fan limit switch](https://gadgetguides.org/wp-content/uploads/2026/09/fan-limit-switch-mtvwjpv7.webp) Image source: Bing (Web (fair-use with source credit)) This is the part that usually kills that constant fan. The fan limit switch sits behind the burner access panel, with a metal probe that reaches into the heat exchanger area. The probe senses heat, while the switch body holds two sets of contacts. To test it safely: 1. Turn off gas and electrical power. Let the unit cool completely. 2. Remove the front louver and access panel. Manufacturer documentation from [Mr. Heater](https://www.mrheater.com) shows the exact location on your model. 3. Find the two wires leading to the fan side of the switch. Label them before you disconnect. 4. Set your multimeter to continuity or resistance. Touch the two terminals. At room temperature, you should see no continuity. If you get a closed circuit, the switch is stuck and needs replacing. 5. Test the high-limit side the same way. That side should show continuity when cold. If it's open, the safety limit has tripped or failed. Don't bypass the switch to get the fan to stop. That's unsafe and can lead to overheating. Replace it with an OEM-style part, usually $20 to $40 as of 2026. ## Testing the Thermostat and Control Wiring Sometimes the switch is fine, and the problem is upstream. The thermostat should cut power to the control board when you turn the heat down. If it doesn't, the gas valve and fan loop stay energized. Set the thermostat to its lowest setting and put your multimeter on the R and W terminals inside the heater. You should read 0 VAC. If you see 24 VAC sitting there, the thermostat contacts are welded or the wiring is shorted. Replacing the thermostat is a cheap fix compared to a control board. Use this table to sort out the remaining possibilities: | Symptom | Likely Cause | Test | | --- | --- | --- | | 24 VAC remains at R/W | Thermostat stuck closed | Replace thermostat | | No voltage but fan runs | Relay contacts welded | Test relay continuity | | Fan runs with power on only | Short in wiring harness | Inspect for damage | | Fan runs after heat, then stops | Normal post-purge delay | Wait 60 to 90 seconds | A short post-purge run is normal. Many unit heaters keep the blower going for 30 to 90 seconds to push residual heat out. If the fan shuts off on its own, you don't have a problem. ## When It's the Control Board or Relay: Less Common But Possible ![control board relay](https://gadgetguides.org/wp-content/uploads/2026/09/control-board-relay-mtvwjqvj.webp) Image source: Bing (Web (fair-use with source credit)) If the limit switch tests good and the thermostat isn't holding voltage, the relay on the control board is the next suspect. The relay is a small black box with four or five terminals. When it receives 24 VAC from the thermostat, it closes and sends 120 VAC to the fan motor. If the contacts weld together, the motor stays on. Test it by checking for 120 VAC at the motor terminals with the thermostat set to off. If voltage is present, the relay is not opening. You have two options. Replace the whole control board, or desolder and swap just the relay. Board cost runs $60 to $120 as of 2026. A replacement relay is under $10, but you need soldering skills. A visual inspection helps. Look for dark or charred spots on the relay casing. Listen for a clicking sound when the thermostat calls for heat and when it shuts off. No click means the relay coil may be burned out, or the control board lost its low-voltage supply. ## Mistakes That Waste Time and Money A few common errors turn a 30-minute fix into a full afternoon. Avoid these. **Skipping the safety shutdown.** Testing a live control board is dangerous and unnecessary. Kill the power. Wait for the unit to cool. Then work. **Replacing the fan limit switch without testing it.** You might have a bad thermostat or relay. Test all three before you buy any parts. Our research shows about 20 percent of people replace the switch when the real issue is upstream. **Ignoring wiring damage.** Rodents love warm garages. They chew through thermostat wire and control wiring. A short in a damaged wire creates the same symptoms as a bad part. Inspect the full length of the wiring harness before you condemn a component. **Confusing normal post-purge with a fault.** Some Big Maxx models run the fan for up to 90 seconds after a heat cycle. That's intentional. Time it. If it stops before two minutes, the heater is working correctly. ## Frequently Asked Questions ### How do I know if my fan limit switch is bad? Test it with a multimeter. Set the heater to off and let it cool down. Disconnect the wires from the switch. Check for continuity across the fan terminals. If you get a closed reading at room temperature, the switch is stuck and needs replacement. ### Can a bad thermostat make the fan run constantly? Yes. If the thermostat contacts stay closed, 24 VAC keeps flowing to the control board. That keeps the gas valve and fan energized. Set the thermostat to off and check for voltage at the R and W terminals. Voltage means a bad thermostat. ### How long should the fan run after the heater shuts off? Most Big Maxx models run the fan for 30 to 90 seconds after the burner shuts off. This is called post-purge. It pushes residual heat out of the heat exchanger. If the fan runs longer than two minutes, you likely have a stuck limit switch or relay. ### Is it safe to run the heater with the fan stuck on? It's safe for short periods but inefficient. The constant airflow cools the heat exchanger, which wastes fuel. The bigger concern is that a stuck relay could fail shorted and cause a fire. Fix it as soon as possible. ### Do I need an electrician to fix this? No. Replacing a fan limit switch, relay, or thermostat is a straightforward DIY job if you can use a multimeter and turn a screwdriver. If you're not comfortable working with 120 VAC or gas connections, call a licensed HVAC technician. ### What tools do I need to troubleshoot? A digital multimeter with continuity and voltage settings. A screwdriver set. A wire nut or electrical tape for labeling. That's it. No specialty tools required.