Quick Answer: Why Won’t My Furnace Turn On?
The most common reasons a furnace won’t turn on are a thermostat setting, a tripped breaker or power switch, a clogged air filter, no gas or propane reaching the furnace, a dirty flame sensor or failed igniter, a blocked vent or condensate drain, or a failed part such as the control board. In homes with a dual fuel system, the furnace may also be off on purpose because the heat pump is handling the heating.
Safety first: if you smell gas (a rotten-egg odor) or your carbon monoxide alarm goes off, don’t troubleshoot anything. Leave the house right away and call your gas company or 911 from outside.
The first cold night of the season has a way of finding problems. You turn up the thermostat, wait for the familiar click and whoosh of the furnace, and nothing happens. The house stays cold.
A furnace that won’t turn on is one of the most common heating problems every fall, including here in Sussex County, DE and on Maryland’s Eastern Shore. After sitting unused all summer, furnaces are more likely to have a dirty sensor, a closed gas valve, or a setting nobody remembers changing. Some of the causes below you can check yourself in a few minutes. Others need a licensed technician, especially anything involving gas.
1. The Thermostat Isn’t Calling for Heat
This sounds too simple, but it’s where every technician starts. Check that the thermostat is set to Heat, not Cool or Off, and that the temperature is set a few degrees above the current room temperature. If the screen is blank or dim, replace the batteries.
Have a dual fuel system? Many homes in our area pair a heat pump with a gas or propane furnace. In those homes, the furnace normally stays off until the outdoor temperature drops below the switchover point, often around 35°F. On a chilly but mild day, it’s normal for the heat pump to do the work while the furnace sits quietly. If the house is warm, nothing is wrong. If the house is cold and neither system is running, keep reading.
Can you fix it yourself? Usually, yes.
2. No Power to the Furnace
A furnace needs electricity even if it burns gas. Check two things:
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The breaker. Look in your electrical panel for a tripped breaker labeled furnace or HVAC, and reset it once. Power blips during fall and winter storms are a common cause.
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The furnace switch. Most furnaces have an on/off switch near the unit that looks like a light switch. It’s easy to flip off by accident when storing things nearby.
If the breaker trips again right after you reset it, stop and call a technician. That points to an electrical problem.
Can you fix it yourself? Yes, unless the breaker keeps tripping.
3. A Clogged Air Filter
A filter packed with dust restricts airflow, and the furnace can overheat. When that happens, a safety switch called the high-limit switch shuts the burners off to protect the furnace. The furnace may start, run briefly, then shut down, or it may not start at all.
Turn off the furnace, pull the filter, and hold it up to the light. If you can’t see light through it, replace it, matching the size printed on the frame and the airflow arrow. Most filters need changing every 1 to 3 months during heating season.
Can you fix it yourself? Yes.
4. No Gas or Propane Reaching the Furnace
If the furnace tries to start but never lights, it may not be getting fuel.
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Natural gas homes. Check that the gas valve near the furnace is open (the handle runs parallel to the pipe). If other gas appliances, like a range or water heater, also aren’t working, call your gas utility.
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Propane homes. Many homes in Sussex County, DE and on Maryland’s Eastern Shore are outside the natural gas lines and heat with propane. Check the gauge on your tank. If the tank ran empty, call your propane supplier. When a tank runs completely dry, the supplier usually has to do a safety check before the system can be relit, so don’t try to relight it yourself.
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Second homes. If you closed up a beach house in Rehoboth, Bethany, or Ocean City for the off-season, the gas valve may have been turned off. Opening it is simple, but if a pilot light then needs relighting and you’re not comfortable doing it, call a technician.
Can you fix it yourself? Only the simple checks. Leave anything beyond opening a valve to a professional.
5. A Dirty Flame Sensor or Failed Igniter
Modern furnaces light the burners with an electronic igniter, then use a small metal rod called a flame sensor to confirm there’s a flame. If the sensor is coated with residue, the furnace can’t “see” the flame and shuts the gas off as a safety measure. The usual sign is a furnace that lights for a few seconds, then shuts off and tries again.
Igniters also wear out. A cracked igniter won’t glow, so the burners never light.
Flame sensor and igniter work involves the gas side of the furnace and fragile parts, so this is best left to a technician. It’s also one of the things checked during a fall tune-up.
Can you fix it yourself? We don’t recommend it.
6. A Blocked Vent or Clogged Condensate Drain
High-efficiency furnaces vent through white plastic pipes that run outside, usually through a side wall or the roof. If leaves, a bird’s nest, or drifted snow blocks those pipes, a safety switch stops the furnace from running.
These furnaces also produce water, which drains away through a small tube. If that drain clogs, the furnace can shut itself down. Look for water around the base of the furnace.
You can safely check that the outside vent pipes are clear of debris. A clogged drain or a tripped pressure switch is a job for a technician.
Can you fix it yourself? You can clear debris from the outside pipe openings. Everything else needs a pro.
7. A Failed Part
If you’ve checked everything above and the furnace still won’t start, the cause is likely a failed component, such as the control board, the inducer motor, a pressure switch, the gas valve, or the blower motor. Many furnaces have a small light on the control board that blinks a code. Write down the pattern before you call, since it helps the technician diagnose the problem faster.
Can you fix it yourself? No.
DIY Check vs. When to Call a Professional
| Cause | DIY Check | Needs a Pro |
|---|---|---|
| Thermostat setting or batteries | ✅ Yes | If wiring is the problem |
| Breaker or furnace switch | ✅ Yes | If the breaker trips again |
| Clogged air filter | ✅ Yes | — |
| No gas or propane | Check the valve or tank gauge | ✅ Relighting, empty propane tank |
| Flame sensor or igniter | — | ✅ Always |
| Blocked vent or condensate drain | Clear the outside pipe opening | ✅ Drain, pressure switch |
| Failed part | — | ✅ Always |
When to Call for Emergency Furnace Repair
Call right away, day or night, if:
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You smell gas or your carbon monoxide alarm sounds (leave the house first)
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The house is getting cold and someone at home is very young, elderly, or unwell
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Temperatures are near freezing and pipes could freeze
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The breaker trips again as soon as you reset it
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You hear banging, grinding, or squealing from the furnace
Should You Repair or Replace?
Most gas furnaces last about 15 to 20 years. If yours is in that range and needs repairs every winter, replacement may cost less over time. If you’re considering a new system, a dual fuel setup, which pairs a heat pump with a furnace, is a popular choice in our mild coastal climate.
Preventing a No-Heat Night
Most of the causes above, including dirty flame sensors, clogged drains, and worn igniters, are exactly what a fall tune-up is designed to catch. A technician checks the burners, the ignition system, the safety switches, and the venting before heating season starts, so problems get fixed on your schedule instead of on the coldest night of the year.
Frequently Asked Questions
How do I reset my furnace? Turn the furnace switch off, or switch off its breaker, wait about 30 seconds, and turn it back on. Some furnaces also have a reset button. If the furnace locks out again after a reset, don’t keep resetting it. Call a technician, because repeated resets can hide a safety problem.
Why does my thermostat click but the furnace doesn’t turn on? The click means the thermostat is calling for heat, so the problem is usually at the furnace: no power, no fuel, a safety switch that has tripped, or a failed igniter or control board.
Why does my furnace turn on and then shut off after a few seconds? This is often a dirty flame sensor, a clogged filter causing the furnace to overheat, or a blocked vent pipe. A technician can test which one it is.
Is it normal for my furnace not to run when I have a dual fuel system? Yes. In a dual fuel system, the heat pump handles heating until the outdoor temperature drops below the switchover point, often around 35°F. Then the furnace takes over. If the house is warm, the system is working as designed.
How much does furnace repair cost? It depends on the cause. Replacing a filter costs very little, while a new control board or inducer motor costs more. A technician can give you an exact price once the problem is diagnosed.
Get Your Heat Back On
A furnace that won’t start is stressful, especially when the temperature is dropping. Start with the simple checks: thermostat, power, filter, and fuel. If none of those solve it, it’s time for a professional.
PROTECH HVAC, LLC is a locally owned, family-operated HVAC company based in Selbyville, DE, licensed in Delaware (#HM-0011213) and Maryland (#08331698). We offer furnace repair on all makes and models and 24/7 emergency service across Sussex County, DE and Maryland’s Eastern Shore, including Rehoboth Beach, Millsboro, Berlin, and Salisbury.
Call (443) 664-3900 any time, day or night, or request service online.