Top 10 Reasons Why Your Furnace Is Overheating: With DIY Solution

So your Furnace is overheating and I know it can be quite frustrating. There are many factors that can cause furnace overheating.

As an HVAC technician, I know what’re the most occurring problems that can lead to furnace overheating.

So in this article, I have listed the top 10 major reasons why your furnace is overheating and how you can solve them.  

Top 10 Reasons For Furnace Overheating


Reason No #1: Dirty Furnace Filter

dirty furnace filter

The most common reason why a furnace overheats is a dirty furnace filter. I understand that these things are very easy to forget about you don’t think about your furnace every day.

But if you don’t replace your furnace filter for a while it’ll start to restrict the airflow and that will cause your furnace to overheat. And just so you know these filters should be replaced on a monthly basis or at most every two months.

You don’t want to leave them in there longer because then your furnace will start to overheat. Some of the filters claim that it lasts up to three months and really when they say three months your house would have to be super clean no dust.

And maybe between the transition period, you know between summer and winter where you’re not really using your furnace or your ac.

In that case, yes your filter would last longer because your blower motor is not coming on as often. This is a personal recommendation for those who are having overheating issues.

You should not use premium grade filters they don’t actually filter that much. But they do cause a lot more of an air restriction on the furnace and that causes a lot of furnaces to overheat.

You should go for the medium-grade furnace filter they don’t restrict airflow that much.

Reason No #2: Dirty Evaporator Coil

Reason number two is a dirty evaporator coil. If you have an up-flow furnace the  A-coil will be sitting on top of the furnace. The reason it’s called an A-coil is that it’s shaped like a letter A.

So how does this A-coil overheats the furnace? You know whatever gets past the furnace filter will inevitably end up plugging your A-coil.

So as the air is going through your furnace (through the heat exchanger) that air is supposed to absorb the heat and bring it to the rest of your house.

But when that coil starts to get plugged up not enough air is able to get past it so that heat starts to get trapped inside the furnace.

And as it gets hotter and hotter that trips your high limit switch and turns off the burners. Cleaning an A-coil is not really a DIY thing you have to hire a ductwork company to do it. 

Reason No #3: Bad Blower Motor

Reason number three for furnace overheating is a bad blower motor. So if for some reason your blower motor is not turning on.

Let’s say the blower motor is burnt out then your furnace, of course, will overheat because the flames are on but there’s no airflow going through it so the furnace overheats.

A lot of times if a blower motor is burnt out you’ll smell some kind of a burnt electrical smell throughout the house and especially around the furnace.

And then if you just take off your furnace door and reach in there and just touch the blower motor a lot of times it’ll be really hot.

The capacitor can be another reason why the blower motor might not be starting. Sometimes capacitors become really weak or dead and the capacitor is pretty much like a car battery for a car.

It’s like a battery for the fan motor so if the capacitor is fully dead then that motor will not run.

But from what I see in the field usually if that capacitor is fully dead that means the blower motor is already damaged so even if you replace just the capacitor chances are like a month down the road you’re going to be replacing that motor anyway.

So if I find dead capacitors on the job I just replace the motor and the capacitor right away. You can replace the capacitor by yourself but replacing a blower motor is pretty hard. You can try to do it but I would suggest calling a technician.

Must Check Our Full Tutorial Guide On How to check and Test A Bad Blower Motor

Reason No #4: Weak High Limit 

Reason number four is a weak high limit. So the high limit is basically a temperature sensor switch that trips when it detects overheating in the furnace.

The high limit switch effectively shuts down the furnace operation until the unit cools.

If your furnace overheats a lot for some reason let’s say you’re bad at replacing your filter for example that limit will get weak and it’ll open ahead of time so for example, my limit is supposed to open up at 190 degrees if the furnace ever gets to 190 degrees inside the furnace will trip that limit and turn off the burners.

But if my limit is weak then it might be opening at 170 degrees instead so even though my furnace is not actually overheating that high limit opens up and turns off the burners. If your high limit switch is weak then just replace it with a new one.

Reason No #5: Bypass Humidifier

Sometimes humidifiers can be the reason for overheating. Not all the furnaces will have a humidifier but if you have one and it’s causing overheating.

You can just turn it off and see if it’s really causing the problem. You’ll need to call a technician for the inspection.

Reason No #6: High Gas Pressure

Reason number six is high gas pressure. So this will usually be on newer furnaces. So for some reason when the furnace was installed maybe the gas pressure was left a little too high it wasn’t adjusted.

And of course, you should only be touching your gas valve if you actually know what you’re doing. You need to have some kind of an HVAC background otherwise you can make things worse. You’ll need to call a technician about this problem.

Reason No #7: Bad Control Board

Reason number seven is a bad control board. So if your board for some reason is not sending power to the blower motor. Then of course that blower motor will not turn on and your furnace will overheat in no time.

If you have a multimeter you can check if your control board is sending 120 volts to your blower motor. If your control board is bad then you’ll have to replace it with a new one.

Check Our Guide On Bypassing the thermostat and control board

Reason No #8: Too many Return Vents Closed

return grills

Reason number eight is blocked or closed return vents. Those should always be open once in a while. I see people put like a couch in front of it or a box in front of it and that’ll block a return especially if it’s a big one.

If your furnace does not have enough return air coming in it’ll overheat really fast. A simple solution for this problem is to just keep your return vents open.

Reason No #9: Bad Heat Exchanger

Reason number nine is a bad heat exchanger. Either the primary heat exchanger or if you have a high-efficiency furnace the secondary heat exchanger can be bad as well.

I have had times where the furnace was overheating and when I checked my carbon monoxide emissions they were like 5 000 parts per million which are super high.

And that also means that there’s incomplete combustion so that the heat exchanger was getting a lot hotter than it should be. You’ll have to call an HVAC technician about this problem.

Check Our Step by step guide on troubleshooting your furnace if it’s not igniting

Reason No #10: Weak Inducer Motor

Reason number 10 is a weak inducer motor and these can be kind of tricky to track down as well just like the blower motor.

This is not a common problem but it can be the reason why you’re furnace is overheating. This is not a DIY thing either you have to call a technician for this. 

Also, Check Our Tutorial/Guide On Most reported Furnace Repair Problems with DIY Solution

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