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Furnace Not Working After Duct Cleaning? 3 Common Causes

Furnaces provide heat in buildings by heating fuel and air, passing the air through a heat exchanger, which then propels warm air through air ducts into rooms using vents.

Your furnace isn’t working after duct cleaning because you left a component in the furnace turned off or accidentally detached when cleaning the duct. There are a lot of components in a heating system, and even seasoned experts sometimes forget to turn one back on or reattach after repairs.

This article will help provide a detailed guide of where to look when your furnace is not working, especially when the ducts are clean and the furnace worked before the cleanup. I’ll also discuss steps geared towards making sure your furnace is well maintained to prevent frequent malfunctions.

Why Your Furnace Won’t Work

Furnaces are an example of a forced-air heating system. They are powered by natural gas and electricity and cold, filtered air, which is heated and propelled through ducts and released into rooms via strategically placed vents throughout a building.

Distinguishing different types of malfunctions will assist you in diagnosing your problem better and fixing it faster. These are the types of malfunctions:

1. Furnace Won’t Heat Up

After cleaning the air duct, the next logical step is usually cleaning dust off of the furnace. The furnace has many mobile parts, including the power switch, gas shut off, and draft monitor. These can be switched off accidentally, especially when cleaning.

Check the Furnace Switch

Without power, a furnace won’t heat your house. So the first step in diagnosing your furnace is to check whether the furnace switch is on. You can find it near the furnace, and it can be shut off very easily. If the switch is off, turn it back on. If this still doesn’t turn on, check the main power supply. If the HVAC breaker box is off, turn it on.

Check the Gas Valve

If the gas is off and the fan on the furnace is running, your inducer fan will come on, trigger the hot surface igniter to start, which will then open the gas valve. The gas valve will realize there is no gas and close itself.

This process will recur three to five times depending on the type of furnace you have, and then it will activate lockdown mode. In lockdown mode, the furnace remains idle for three hours, and then it will try this process again.

To fix this, turn the gas valve, then flip the power switch off and on again. This will reset the furnace.

Check Your Thermostat

Most people overlook thermostats when troubleshooting. Thermostats can either be electric or use batteries. If you are using a battery-powered thermostat, check whether the batteries are dead and replace them.

Is your thermostat set to heat? Ensure you set it a few degrees higher than your room’s temperature. If nothing happens, unclip the thermostat cover and dust off any debris, then configure the wires correctly.

Check the Panel Lid

In newer devices, the panel door has an interlock sensor lever; a safety feature shuts down your heating system if the door is not placed properly. If everything is okay inside and your furnace still won’t work, check whether you have replaced the door properly.

2. The Furnace Burner Comes On and Then Goes Off After a Few Seconds

This problem is also known as short cycling.

To fix this problem, check the flame sensor. The flame sensor is a bent metal that detects that the gas valve is emitting a flame when the furnace is on. This thin metal is important in preventing gas leaks in your building, which is why everything shuts off immediately it malfunctions.

A dirty flame sensor will also trigger the lockdown feature in newer furnace models. The most common cause of this malfunction is carbon build-up to its ends.

To fix a flame sensor, you will need:

These are the steps required to fix the sensor:

  1. Flip off the power switch and the gas valve. This is an important safety precaution when repairing any electric device to prevent electrocution and gas leaks.
  1. Remove the flame sensor. To get to the sensor, you will have to detach the top door. It might be screwed on or easy to detach. If screwed on, use the hex driver to unscrew the lid. Ensure that you put the screws in a safe place to prevent runaways.

You will find the sensor in the burner assembly. Remove its screws using the hex driver. If you’re not sure where to find the sensor, I recommend watching this helpful video about furnace components.

  1. Scrub off the carbon using the scouring pad or wire tool until clean
  2. Clean the hex tool with a paper towel and screw it back on.
  3. Replace the furnace door and test whether the problem is fixed.

3. The Heater Won’t Reach the Set Temperature

It’s a cold day, and you have set your thermostat to heat the house; the thermostat is on, but you still feel cold.

Check the Air Filter

A dirty air filter will suck in dust and dirt fragments from the air, clogging it up and straining the heat exchanger. The clog up limits air supply to the heat exchanger, therefore, causing overheating. When the overheating becomes chronic, it results in short-cycling in furnaces, and eventually, you may have to replace faulty components.

Check the Fan Limit Switch

The fan limit switch ensures that the furnace produces the temperature you specified on the thermostat before distributing heat through the building. The furnace is meant to deactivate itself after reaching the preset temperature and not overheat.

According to wmhendersonic, most limit switches are preset to 200°F (93°C) to turn off the burner, 130°F (54°C) for the blower, and 100°F (38°C) to trigger the blower to turn on again. The limit switch will trigger the blower assembly, which then goes off.

If faulty, the fan limit will keep fluctuating its readings, keeping turning the furnaces on and off prematurely. To fix a limit switch, clean off any dirt that has accumulated on it.

Know Your Error Indicator Codes

When your furnace has a problem, it will mostly flash an LED warning. These warnings are coded in different colors ranging from red, yellow, amber, and green. Taking note of the error’s color and the number of blinks can help you troubleshoot a problem faster.

The codes are usually located on the inside of your furnace, at the back of the panel door.

Know When To Call a Professional

If your DIY repair and care doesn’t yield results, then you have done your best, and there is no shame in calling a certified repair technician.

Final Thoughts

Furnaces are a staple, especially in countries that experience extreme weather. It is important to ensure your entire heating system undergoes a regular checkup. Make sure that the air ducts are clean, replace air filters regularly so that you don’t overwork your heat exchanger, and remember to replace your thermostat’s battery.

When you decide to clean up your furnace, take care not to trip or rip off any wires and switches. As a precaution, after every cleanup, confirm all the switches are on, and the panel door is locked properly.