TL;DR: What to Check When Your Furnace Won’t Turn On
Before calling for furnace repair:
- Make sure your thermostat is set to Heat and set above the current room temperature.
- Replace the thermostat batteries if it uses them.
- Check the furnace power switch and circuit breaker.
- Look at your furnace filter and replace it if it is badly clogged.
- Make sure the furnace access panel is completely closed.
- Look for a blinking furnace error light if you can see one without removing anything.
- On a high-efficiency furnace, check that the outdoor intake and exhaust pipes are not blocked by snow, ice, or debris.
If all of those look normal and your furnace still won’t turn on, there is probably something going on inside the system that needs to be diagnosed.
You turn up the thermostat, wait for the familiar sound of the furnace starting, and… nothing.
When your furnace won’t turn on, it does not automatically mean you are looking at a major repair or a new furnace. Sometimes the problem is as simple as a thermostat setting, a tripped breaker, a dirty filter, or a furnace door that is not completely closed.
Other times, the furnace is trying to protect itself because something in the ignition, airflow, electrical, or venting system is not working correctly.
The important thing is knowing what you can safely check yourself and when it is time to stop troubleshooting.
At Accutrol Complete Home Services, we prefer to start by figuring out what is actually wrong before jumping to conclusions about the repair. Here is the same basic process we recommend when a furnace will not start.
First, What Is Your Furnace Actually Doing?
“Furnace won’t turn on” can mean several different things.
Before you touch anything, stand near the furnace while someone turns the thermostat up. Pay attention to what happens.
The difference between total silence, clicking, humming, and a furnace that starts and immediately stops can give you a much better idea of what is happening.
The Furnace Is Completely Silent
If absolutely nothing happens when the thermostat calls for heat, start with the basics.
The furnace may not be receiving the signal to run, or it may not have electrical power.
A completely silent furnace can be caused by things like the thermostat, furnace switch, circuit breaker, access-panel safety switch, wiring, or the furnace controls.
Do not assume that because you have a gas furnace it does not need electricity. Modern gas furnaces rely on electrical power for the thermostat, control board, blower motor, igniter, and safety controls.
The Furnace Clicks or Hums but Never Starts
Clicking or humming tells us something different.
The furnace may be receiving the call for heat and beginning its startup sequence, but something is preventing it from completing that sequence.
Possible causes include the igniter, inducer motor, pressure switch, capacitor, gas supply, or another electrical component.
At this point, we generally do not recommend trying to take the furnace apart yourself. The sounds are useful information for the technician, though, so make a note of exactly what you hear.
The Furnace Starts and Then Shuts Off
If your furnace turns on but only runs briefly, you may not have a true no-start problem at all.
The furnace could be shutting itself down because of an airflow issue, overheating, flame-sensing issue, or another safety control.
If yours repeatedly starts and stops, there are several common causes behind a furnace that keeps turning on and off.
7 Things to Check Before Calling for Furnace Repair
1. Check the Thermostat
This sounds obvious, but it should always be your first check.
Make sure the thermostat is:
Set to Heat, not Cool or Off.
Set several degrees above the current indoor temperature.
Not stuck in an Away, Eco, or unusual programmed schedule.
If your thermostat uses replaceable batteries, put in a fresh set.
Then give the furnace a few minutes. Some systems intentionally have a short delay between the thermostat calling for heat and the furnace starting.
If the thermostat screen is completely blank, that is also useful information. The issue may involve thermostat power, batteries, a tripped breaker, wiring, or a furnace safety switch.
2. Make Sure the Furnace Power Switch Is On
Most furnaces have a nearby power switch that looks surprisingly similar to a normal light switch.
It can get flipped off accidentally while someone is working around the furnace, moving boxes, changing a filter, or doing other work in the utility room.
Make sure it is in the ON position.
Then wait a few minutes to see whether the furnace responds.
3. Check the Furnace Circuit Breaker
Find your electrical panel and look for the breaker supplying the furnace.
A tripped breaker does not always look completely off. Sometimes it sits between the ON and OFF positions.
If it has tripped, you can normally move it completely to OFF and then reset it to ON once.
The word once matters.
If the breaker immediately trips again, do not keep resetting it. A breaker that repeatedly trips can indicate an electrical problem that needs to be diagnosed.
Repeatedly forcing it back on is not fixing the underlying problem.
4. Check the Air Filter
A dirty furnace filter can cause more trouble than most homeowners realize.
When the filter becomes severely clogged, the furnace struggles to move enough air across the heat exchanger. That can cause the furnace to overheat and trigger safety controls that shut the system down.
Pull the filter out and take a look.
If it is packed with dust and you cannot easily see light through it, replace it with the correct size and type.
Filter quality and replacement frequency matter too. We break that down more thoroughly in Everything You Need to Know About Furnace Filters.
5. Make Sure the Furnace Door Is Completely Closed
This is one of the easiest things to miss.
Many furnaces have a safety switch behind the blower compartment door. When the door is removed, the switch cuts power to the furnace.
That is intentional. It prevents the furnace from operating while the compartment is open.
If you recently changed the filter, had maintenance performed, or moved the furnace panel for some reason, make sure the door is seated correctly.
A furnace with a slightly misaligned access panel can sometimes look completely dead.
Do not bypass or tape down the safety switch. Just make sure the panel is installed correctly.
6. Look for a Blinking Error Light
Many modern furnaces have a small diagnostic light on the control board.
You may be able to see it through a small window in the furnace door without opening anything.
If the light is blinking, count the flashes or take a quick video with your phone.
For example, you might see three flashes, a pause, and then three flashes again.
Those codes vary by furnace manufacturer, but they can give the technician an important clue about why the furnace will not start.
This is one reason we usually recommend checking the light before repeatedly turning power off and back on. Resetting the furnace may clear the fault code you were trying to identify.
7. Check the Intake and Exhaust Pipes on a High-Efficiency Furnace
If you have a high-efficiency furnace, you may have PVC pipes running from the furnace to the outside of the house.
One brings combustion air in and another vents exhaust gases out.
In Colorado, snow and ice can occasionally become a problem around those pipes.
You can safely walk outside and visually check that the openings are not obviously blocked by snow, ice, leaves, or other debris.
High-efficiency furnaces also produce condensation while operating. If the condensate cannot drain properly, a safety switch may prevent the furnace from running.
That is generally where we would stop the homeowner troubleshooting. Do not start disconnecting pressure switches, drain lines, or venting components unless you know exactly what you are doing.
What If the Thermostat Says “Heat” but the Furnace Still Won’t Turn On?
This confuses a lot of homeowners.
Your thermostat can display Heat On while the furnace remains completely quiet.
That does not necessarily mean the thermostat is working perfectly.
It mainly means the thermostat believes it is requesting heat.
The furnace still has to receive that signal and successfully complete its own startup sequence.
If the thermostat says heat but the furnace is not running, the problem could still involve thermostat wiring, furnace power, an ignition failure, a safety lockout, the pressure switch, the control board, or another failed component.
Once you have confirmed the basic thermostat settings and power checks, diagnosing those parts usually requires testing at the furnace.
Common Furnace Problems That Usually Need a Technician
Failed Furnace Igniter
Modern gas furnaces commonly use an electronic igniter rather than an old standing pilot light.
If the igniter fails, the furnace may begin its startup sequence but never light the burners.
You may hear the furnace trying to start without ever getting heat.
Dirty or Failing Flame Sensor
Once the burners ignite, the furnace needs to confirm that a flame is actually present.
If the flame sensor cannot properly detect the burner flame, the furnace may shut the gas off shortly after ignition.
If your furnace lights and then shuts back down a few seconds later, this can be one possible cause.
Pressure Switch or Venting Problem
Before many furnaces ignite, the system checks that combustion gases can vent properly.
A pressure-switch fault can be caused by the switch itself, the inducer system, venting, condensate, or another airflow problem.
Because this is tied to the furnace’s combustion safety system, this is not something we recommend bypassing.
Blower Motor or Electrical Component Failure
Humming without normal operation can point toward a motor, capacitor, electrical, or control issue.
The exact failure cannot reliably be diagnosed just from the sound, but telling your technician what you heard can help narrow things down.
Furnace Control Board Problem
The control board is essentially the furnace’s brain.
It manages the sequence between the thermostat, inducer, ignition system, gas valve, blower, and various safety switches.
If that board or one of its inputs fails, the furnace may behave inconsistently or fail to start altogether.
Stop Troubleshooting If You Notice These Warning Signs
There are times when the goal is no longer “get the furnace running.”
The goal becomes making sure everyone in the house is safe.
If you smell gas, hear a carbon monoxide alarm, see smoke or scorching, or notice a strong electrical burning smell, stop troubleshooting.
If you suspect a gas leak or carbon monoxide problem, get everyone out of the home and contact the appropriate emergency service or utility company from a safe location.
A slight dusty smell during the first furnace run of the season can be normal, but other smells can mean something very different. Our guide on why a furnace smells like burning explains the difference.
Carbon monoxide concerns can also be related to combustion or heat-exchanger problems. There are several signs of a cracked furnace heat exchanger homeowners should know.
When Is It Time to Call for Furnace Repair?
If you have checked the thermostat, furnace switch, breaker, filter, and access panel and the furnace still won’t turn on, you have probably reached the point where guessing is not going to help.
We would also call for service if:
The breaker keeps tripping.
The furnace repeatedly tries to ignite but fails.
It starts and shuts down again.
You hear unusual clicking, humming, banging, or grinding.
You can see a furnace fault code.
You are repeatedly resetting the furnace to make it run.
The important thing is figuring out why the furnace stopped.
At Accutrol Complete Home Services, furnace service starts with a $75 diagnosis. We service all furnace brands, and completed furnace repairs include a 1-year parts and labor warranty.
Our approach is pretty simple. Diagnose the problem first, explain what we found, and then help the homeowner understand the options.
If the repair makes sense, repair it.
If the furnace is older and the repair does not make financial sense, that is when it is worth discussing whether you should repair or replace your furnace.
Replacement does not automatically mean you need to make a decision on the spot. Accutrol provides free furnace replacement estimates, and understanding furnace replacement cost in Denver can help you compare your options.
Before the Technician Arrives, Make Note of What the Furnace Is Doing
A little information can make the diagnostic process much easier.
Pay attention to what happens when you increase the thermostat temperature.
Is the furnace totally silent?
Does it click?
Does a motor start?
Do the burners ignite?
Does it shut down after a few seconds?
Are there blinking lights?
You do not need to know what any of it means. That is our job.
Just being able to explain exactly what the furnace did can be much more useful than saying, “It stopped working.”
If you can easily see the furnace brand, model number, or fault code, taking a photo can also help.
How to Reduce the Chances of Another No-Heat Problem
You cannot prevent every failed igniter, motor, sensor, or control board.
You can, however, avoid some unnecessary furnace problems.
Keep up with filter changes, avoid blocking supply and return vents, pay attention when the furnace begins making unusual sounds, and do not ignore repeated startup or cycling problems.
According to the U.S. Department of Energy, keeping your heating system properly maintained and replacing dirty filters can improve efficiency and help prevent unnecessary strain on the system. U.S. Department of Energy furnace maintenance guidance
Routine service also gives you a chance to catch worn or dirty components before the furnace quits on one of the coldest nights of the year.
Furnace Won’t Turn On? Get the Problem Diagnosed
If you have checked the thermostat, power switch, breaker, filter, and furnace door and you still have no heat, there is probably something happening inside the system that needs to be tested.
A furnace has several built-in safety controls for a reason. Once the simple checks are out of the way, we would rather diagnose the actual problem than have a homeowner keep resetting the system and hoping it starts.
Accutrol Complete Home Services provides furnace repair throughout Denver and surrounding areas. We service all furnace brands, start with a $75 furnace diagnosis, and back completed repairs with a 1-year parts and labor warranty.
The goal is simple: find out what is wrong, explain what it takes to fix it, and let you decide what makes sense.
If you need furnace repair, contact us today.
FAQs: Furnace Won’t Turn On
What is the first thing I should check if my furnace won’t turn on?
Start with the thermostat. Make sure it is set to Heat, set above the current room temperature, and has working batteries if required. After that, check the furnace switch, circuit breaker, filter, and access panel.
Can a dirty air filter stop my furnace from turning on?
Yes. A severely restricted filter can reduce airflow enough for the furnace to overheat and trigger a safety shutdown. Replacing a badly clogged filter is one of the simplest things you can check yourself.
Why does my thermostat say Heat but my furnace isn’t running?
The thermostat may be calling for heat while the furnace is unable to complete its startup sequence. The problem could involve furnace power, thermostat wiring, ignition, venting, a safety switch, or another component.
Why does my furnace click but not turn on?
Clicking often means at least part of the startup sequence is happening. Depending on exactly where that sequence stops, possible causes include the igniter, inducer, pressure switch, gas supply, or electrical controls.
Why does my furnace start and then shut off?
If it only runs briefly, the furnace may be shutting down because of an airflow, flame-sensing, overheating, or safety-control issue. Repeated starting and stopping should be diagnosed rather than continually reset.
Should I keep resetting my furnace if it won’t start?
No. One reset may sometimes clear a temporary lockout, but repeatedly resetting a furnace can hide useful fault information without fixing the underlying problem. If the furnace keeps locking out, have the cause diagnosed.
Does Accutrol repair all furnace brands?
Yes. We at Accutrol Complete Home Services service all furnace brands.
How much does Accutrol charge to diagnose a furnace that won’t turn on?
Our furnace diagnostic fee is $75. Once the problem is diagnosed, the repair options can be explained before moving forward.
Does Accutrol offer same-day furnace repair?
We do not offer 24-hour emergency service. Depending on the schedule, same-day service may sometimes be available, but it is not guaranteed.
What warranty does Accutrol provide on furnace repairs?
Completed furnace repairs are backed by a 1-year parts and labor warranty.
