Furnace Won’t Turn ON – Causes And Solutions

So, your furnace won’t turn ON. You’ve tried a few times now, and there’s still no heat. You’ve even tried the old trick where you switch off the unit and wait a few minutes before turning it back ON.

But still, nothing happens. What is wrong? And, is it something you can fix yourself right away, or do you need to call your HVAC technician?

Well, it depends. The furnace can refuse to turn ON for several reasons. Some of the issues are minor problems that you can fix yourself while others are too technical or unsafe for DIY.

Below are the five most likely reasons why your furnace won’t turn ON and what you can do in each case. We’ll assume that you have electricity in your home, evident from the working lighting fixtures.

If you have a power blackout, then you may have to wait until power returns to use the furnace.

6 Reasons Why Your Furnace Won’t Turn ON

1: Thermostat Issues

The first place you should check if you’re experiencing the issue for the first time, an indication that the rest of the furnace is likely in good shape, is the thermostat. If all else is working well, then you may have input the wrong thermostat settings. Or perhaps the thermostat isn’t getting power.

This is especially common for wall thermostats. Here’s how to diagnose thermostat issues;

  • Check for power challenges

Check the thermostat display. Does the display show any readings, correct or not? If it’s completely blank, then the thermostat is not getting power. If the thermostat isn’t getting power, the furnace cannot run as the furnace takes instructions from the furnace.

Solution: Try replacing the batteries. If the thermostat works when you replace the batteries, you’ve fixed the issue. However, if it doesn’t or dealing with a hardwired thermostat, it could be a deeper problem. Consult your HVAC technician.

  • Diagnosing thermostat settings

If the thermostat has readings on the screen, check the readings closely. Thermostats are responsible for maintaining ambient temperature in your home. Therefore, incorrect settings can spell doom and may even cause the furnace to remain OFF when you need heating.

You want to check for two things specifically – the operating mode and Time, Date, and timer.

Solutions: First, make sure the furnace is set to “Heat” or “Heating” mode, depending on the furnace model. If it’s on “Cool,” “Hold,” “Vacation,” or other modes, change it. More importantly, double-check that the set temperature is higher than the current temperature.

Secondly, make sure the time, date, and timer settings are correct. If any of these three are off, the furnace may not produce the desired heating levels or fail to work altogether.

2: The Furnace is Not Getting Power

Many people wonder whether the furnace uses electricity and, if so, why? The reason is that the furnace, like all other home appliances, needs electricity to run internal controls.

For instance, the blower motors within your furnace depend on electricity to pull cold air into the furnace and blow warm air into your home.

Built-in thermostats also need electricity. The flames or heat from the furnace cannot power these parts. For the above reasons, the furnace may refuse to turn ON if your furnace has electricity/power issues or if it’s not getting power in the first place. So, why is the furnace not getting power when the rest of the home has electricity? It can be one of three reasons.

  • Tripped breaker: All furnaces have a circuit breaker that trips (disconnects) power in case of power surges. Make sure the breaker isn’t tripped.
  • ON/OFF switches: The furnace has an electrical switch, typically located on the wall next to the furnace. Make sure it’s switched ON.
  • Fuses: Finally, furnaces also have electrical fuses positioned strategically throughout the electrical circuitry to protect the furnace and ensure user safety. The furnace may not turn ON if one of the fuses is blown.

Solution: If the circuit breaker is tripped, flip it back to an ON position. The same applies to the switches. If you have trouble flipping your breaker or other switches back ON, an electrician or HVAC professional. 

As for the fuses, you need to replace the blown ones. It’s best to let a professional electrician handle the replacement. However, it’s also doable if you’re confident in your DIY skills.

  1. Switch OFF the furnace at the breaker.
  2. Open the furnace access door and locate the circuit board.
  3. Locate the main fuse. It’s usually a cylindrical item with a bright color. Most furnace fuses are removable
  4. Take it out and inspect it for damage using a flashlight. Damaged fuses are burned and have a burned smell.
  5. Purchase a replacement if the fuse is blown. Make sure to find the exact make and model.
  6. Insert the replacement, making sure it fits snugly.
  7. Now, switch the breaker ON and turn on the furnace to see if it works. If it still doesn’t start, call an HVAC specialist.

3: Ignition System Issues

Your furnace may also refuse to turn ON If it has a faulty ignition system. Unfortunately, you’ll need to be highly specific here as different furnaces use different ignition systems. Nevertheless, the following are the most common issues;

  • Pilot light issues 

Older furnaces use a standing pilot system to light the burners. The standing pilot burns continuously, as long as you have gas. When you turn on your furnace, the main valve opens, and gas from the burners spontaneously ignites the burners.

Unfortunately, standing pilot systems suffer many drawbacks. For one, the pilot flame can go off if there’s a strong wind or if the pilot orifice is blocked. The pilot light can also go off if the pilot light flame sensor (where applicable) is defective.

Solution: Make sure the pilot system is clean and well maintained. Additionally, ensure the flame is healthy and burning normally. If it goes off anyway, remove the furnace access door to access the pilot assembly. Then locate the “Reset” switch, turn it off for 5-15 minutes, and then back ON. If the pilot light still doesn’t work, call your HVAC technician.

  • Spark ignition issues 

If your furnace doesn’t have a standing pilot light, you most likely have what’s known as a spark-ignition furnace. Spark ignition furnaces (also known as intermittent pilot systems) only light the pilot when the thermostat calls for heating. When you turn on your furnace, an electrical spark is created that ignites the pilot. Then the pilot lights the burners.

If the pilot light doesn’t light when you turn on the furnace, you have a problem. Often, you can fix the problem with a simple cleaning.

Solution: Switch off power to the furnace by turning it off at the breaker and turning off the gas supply. Then remove the furnace access panel to locate the pilot system. The spark ignitor is a ceramic component with wire connections at the end.

Use compressed air to dust off the ignitors, put back the panel, turn on electricity and gas, and test the furnace. If it still doesn’t work. It’s time to call the pros.

  • Hot surface ignition system issues

Finally, hot surface ignition is the most recent advancement in furnace ignition technology. It doesn’t use a pilot flame at all. Instead, it comprises a conductive material that glows red when you turn ON the furnace and ultimately lights the burners.

If your hot surface ignition system doesn’t glow red when you turn on the furnace, there’s a problem. Maybe the voltage is too low or too high. Or maybe the ceramic igniter is damaged from overuse or contamination.  

Solution: The easiest solution here is to replace the ceramic ignitor. Alternatively, call an HVAC pro to take a look at the system.

4: It’s in “Lockout”

Furnace lockout is quite common and may also keep you from enjoying a warm home. So, what exactly is a furnace lockout, and what causes it? Furnace lockout refers to when your furnace enters a nonresponsive state.

During this period, the furnace becomes nonresponsive to most input commands. It may even refuse to start altogether. Essentially, the furnace controllers shut off fuel and electricity supply to prevent unsafe operation.

A furnace will remain in “lockout” until you manually reset it. A furnace typically enters lockout mode due to problems in three areas, i.e., the ignition system, flame sensor, and limit switches.

  • Ignition system issues: A defective igniter that causes the user to attempt to ignite the furnace severally may result in a lockout. As we’ve seen, ignition system issues vary significantly depending on the type of furnace.
  • Flame sensor issues: The flame sensor is a small wire within the burner system that detects the presence of a flame on the burner and keeps the furnace ON or turns it OFF, depending on the findings. If it can’t detect or “see” a flame, it will cut fuel supply to prevent gas poisoning and fire risk. If the flame sensor repeatedly tries to find a flame in vain, it can put the furnace in lockout mode until you reset the unit.
  • Limit switch issues: The ordinary furnace has several limit switches, including the flame rollout switch, temperature limit switch, and pressure inducer switch, which “limit” furnace operation under specified circumstances. For instance, the flame rollout switch discontinues furnace operation if the flame floats out into undesignated zones. These limit switches can also send the furnace into “lockout” if the underlying issue isn’t resolved after a pre-determined number of attempts.

Solution 

The only solution to a locked-out furnace is to reset it manually. Fortunately, it’s very easy to reset a furnace.

  1. Turn off the furnace
  2. Wait 30 minutes for it to cool down
  3. Locate the “Reset” button. It’s located near the motor housing
  4. Press the button

You’ve just reset the furnace. If the furnace doesn’t restart, try resetting it again twice. If it still won’t turn ON, it’s time to call the pros.

5: Blower or Pressure Inducer Issues

Finally, your furnace may also refuse to turn ON if the blower system or furnace inducer motor has an issue. The blower system is the fan system that blows warm air into the house. It also creates the air movement necessary to draw cool air into the furnace.

Meanwhile, the furnace inducer motor helps remove exhaust gases out of the furnace and outside the house. The furnace may refuse to come ON if any of these two parts are compromised. So, what could go wrong?

  • Dirt/blockage 

The most common issue whenever one of the furnace fans becomes defective is dirt or blockage. For instance, perhaps the fan has accumulated debris and can’t turn as a result. Or maybe a piece of furniture is blocking the airflow.

Solution: The solution here is regular maintenance. Regularly dust the motor and ensure at least three feet clearance around the furnace for unimpeded airflow.

  • Electrical issues

Another possibility is an electrical malfunction. Is the fan getting electric power? If not, then you need to fix it right away. Check for loose wires, disconnections, and control board issues.

Solution: If you find any loose wires or connections, tighten them or replace the wiring. Also, try unplugging the fan and plugging it back. Does that solve the problem? If not, check the fuse box.

Maybe the unit encountered an electrical surge, and the fuse blew. If so, you need to replace the furnace. If it still doesn’t work, contact an HVAC pro.

  • Is it a bad capacitor?

Furnace fans use capacitors to store starting power as the electrical supply to the fan is rarely enough to start the motor. Therefore, if the capacitor fails, the motor cannot start. You can tell that the blower capacitor is bad if the fan hums but doesn’t run.

Solution: The only solution to a bad blower capacitor is to replace the capacitor. Consider consulting your HVAC technician for advice.

Summary

There you go – five common reasons why a furnace may refuse to turn ON. Other issues to check include whether the unit is getting gas and if the float switch is up. If your DIY skills don’t seem to be getting you anywhere, consult a professional HVAC technician.

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