5 Common Reasons Why Your Gas Furnace Won't Start

5 Common Reasons Why Your Gas Furnace Won't Start
Roger Bakies
Residential Sales Professional

I have been in the residential/light commercial HVAC business for 30 years. I grew up in a sheet metal fabrication shop and have installed, serviced, sold and helped people choose new systems to best fit their needs and lifestyle. I look forward to helping you pick the best fit for your home!

About This Article

Will your furnace start right away after sitting idle for months? If it doesn’t, what might be wrong? We explore common reasons why it may not start up immediately.

Why won’t my gas furnace start?

  • Check your thermostat
  • Does your furnace have electricity?
  • Is gas getting to your furnace?
  • Check the pilot light
  • Check your furnace’s filter

Autumn is a great time to have a maintenance technician come out to check on your furnace. If you skip this visit, you’ll find out soon enough if everything is working when you turn on your furnace for the first time.

If your air conditioning or heat pump quits on you during the summer, the worst that can happen is that you’ll be uncomfortable for a day or two.

But if your furnace dies in the winter, you’re going to be cold and worry about your pipes bursting.

Although you can always call an HVAC service technician, you might have to wait a few days or more for that service. The phone call for service that you put off suddenly becomes an emergency repair, and you’ll accept any help from any HVAC contractor who can get to your house the quickest. There’s no time to vet who will do the best job.

At Fire & Ice, we get hundreds of service calls for furnaces during the winter months. And we hear from plenty of homeowners who discover that their furnace won’t start after sitting idle for months.

Let’s go through some common issues that can affect your furnace’s ability to start.

1. Check Your Thermostat

First, make certain your thermostat is calling for heat. You may have to turn it up for it to send the message. If it’s 72 degrees inside and you have it set at 69, it’s not going to start. It could also be that the thermostat is set to “fan only,” or you simply had it set to “cool.”

It’s entirely possible that a malfunctioning thermostat could be the issue. If it isn’t giving the furnace the correct signal, check to see if it’s giving you error codes.

Smart thermostats can help you diagnose where the problem is. You can even let them tell your HVAC dealer what’s gone wrong.

2. Does the Furnace Have Power?

This may sound really obvious, but make sure your furnace has electricity. It’s possible that the breaker has tripped. If you reset the breaker to give the furnace power, and it trips again, it’s time to call an electrician or an HVAC contractor.

Once you’ve checked the circuit box, make sure the power cord is connected to the furnace, and then check to make sure that the furnace itself is on. Maybe the power switch on the inside was turned off during a maintenance call.

Gas furnaces need electricity, too, so you should go through the steps listed above to make sure the electricity is getting to where it needs to go.

3. Is Gas Getting to the Furnace?

Is your gas line intact? One way to check is to check to see if you still have hot water. If you have gas to your hot water heater or your stove but not to your furnace, that line could be damaged.

If you have no gas at all, make sure the gas company didn’t come and take your gas meter. We’ve seen this happen, when the bill payment was late and there was a lock on the meter. It’s also possible that your neighbors don’t have gas either, meaning that a major break in the line needs to be addressed.

If it’s a modern-style furnace, you should listen for the draft motor to start up. If the draft motor starts up and the furnace doesn’t go any further, then you’ve either got a bad igniter, or if it’s a direct-spark ignition, maybe the spark module is bad. Unless you’re really handy, an HVAC contractor is required to fix these issues.

Or the pressure switch may have failed. When you’re starting your furnace for the first time of the season, that’s not uncommon. It’s possible the inducer motor might have some debris in it and will fail. Again, call an HVAC contractor to repair this.

Three people standing next to a furnace

4. Check the Pilot Light

If your gas furnace is old enough to have a pilot light, is the pilot lit and blue?

We’ve seen cases where the pilot light is lit, but it’s a lazy, yellow flame. That means that the pilot is dirty and it’s not producing enough heat. It needs to be cleaned, but it will still function.

If it’s the type of furnace that has a port with a light in it, is the light on? If it’s blinking, it’s telling you that something is wrong.

5. Take a Look at the Furnace Filter

Make sure your furnace filter is clean. A clogged filter creates too much static pressure and restricts airflow. If there’s not enough airflow, your furnace might not start.

HVAC contractoir replacing a furnace filter

Turning Your Furnace On for the First Time

When your furnace comes on for the first time, you will likely get a whiff of something burning. It’s likely dust that settled on the heat exchanger. This is normal, and you don’t need to worry about that smell. If the smell continues after a few minutes, there’s something else wrong, and it’s likely a major problem.

If you have a heat pump, there’s a reversing valve that activates. It’s the part of the system that switches the unit from producing cold air to hot, and vice versa.

Once the heat pump comes on and you’re satisfied with its heating, you should set your thermostat to make a call for emergency heat. That will activate the heat strips inside your furnace. Out of all of the components in an electric furnace, heat strips are the least likely to fail because they have no moving parts.

Again, you might get a slight burning smell from dust being singed. This is also normal. If the burning smell persists, it’s time to call in an expert.

My Furnace Works, But It’s Still Having Problems

Okay. Your furnace is humming away, which means that most of the parts and pieces are doing what they’re supposed to do. You’re ready to forget about any HVAC problems until summer rolls around and you need to get your air conditioner working.

But something’s still wrong. You’re not getting the warmth you had last winter. Maybe your system is shutting off before the optimum temperature is reached. Maybe the air coming out of the vents feels cooler than normal.

There are a few things that might be wrong.

1. A gas furnace needs to exhaust waste materials (including carbon monoxide) either up through the chimney flue, or outside via a PVC pipe. Sometimes you could have a blockage in the flue. We’ve seen birds get stuck in the pipes.

2. Is the filter clean? If it’s really dirty, your furnace might run, but if it restricts too much air because of all the dust and debris it’s caught, your furnace won’t heat adequately. It can trap only so much dust and bacteria. Once it’s saturated, dust will find its way around it. It can wind up in the furnace’s motor, which creates an inefficient unit that has to work harder. That will affect your energy bills.

You can change your filter yourself if you have a replacement, or an HVAC contractor will happily do it for you.

Buy Your Filter

3. The blower motor might be out of commission. If it was running fine during the summer, when it was blowing conditioned air through the ducts, your blower motor is more than likely OK. But sometimes you can get flames, you have heat, but the blower isn’t running. The furnace will smell hot. That means that the blower motor has failed.

4. Most modern-style furnaces have a flame sensor, and if that’s dirty, the flames will turn on, but they won’t maintain that heat.

5. Are the registers open? Closed registers are a temporary fix for a larger problem. You can close down registers in an effort to divert air from one ductwork run to another, but if you’re doing this, you’re creating problematic static pressure. And if you’re trying to cope with cold spots in your home, it means that your furnace isn’t performing at optimal levels.

Graphic of furnace

Furnace Maintenance and Repair Through Fire & Ice

We’ve all been there. One day it’s 90 with high humidity, and you’re comfy while your AC does its job.

Before you know it, the temperature dips to 50 degrees at night, and you set your thermostat for heat, knowing that it hasn’t been turned on for months. Fingers crossed.

A regular maintenance visit - once in the spring for your AC, once in the fall for your furnace - goes a long way to ensure that everything will be working when you need it.

We are ready for your call, whether it’s a routine once-over or a repair. And if you’re interested in furnaces and would like to learn more, we offer these suggestions:

Furnace Troubleshooting: Common Heating Problems & Repairs

A Guide to High-Efficiency Furnaces

HVAC Buyer’s Guide

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