Few things feel worse than turning up the thermostat on a cold evening and hearing nothing from the furnace. Sometimes the cause is simple: a thermostat set to the wrong mode, a switch that was turned off, or a filter that has been ignored for too long. Other times, a furnace that will not start points to something a technician needs to handle, such as an ignition problem, a failed control, or a safety device that has shut the system down for a reason.
This guide covers the checks a homeowner can reasonably make before calling for furnace repair, and where to stop. It cannot diagnose your particular system.
One rule applies to every step below. Do not open gas components, do not bypass or jumper a safety switch, and do not attempt electrical repairs. Those jobs belong to a trained technician, and doing them yourself is how a no-heat problem becomes a safety problem.
Start With the Thermostat
The thermostat tells the furnace when to run, so look there first. Confirm that it is set to heat rather than cool or off, and that the target temperature is a few degrees above the current room temperature. A thermostat set to 68 in a house that already reads 69 will not ask for heat, no matter how cold it feels to you.
If the display is blank or dim, the batteries may be worn out. Many thermostats use batteries even when they are wired to the furnace. Check the schedule or program settings too, since an overnight setback, an away mode left on after a trip, or a setting changed by accident can keep a furnace from starting when you expect it to. If the thermostat still seems unresponsive after fresh batteries, the thermostat itself could be the part that needs attention.
Check Whether the Furnace Has Power
A gas furnace still depends on electricity for its control board, ignition system, and blower. Start with the furnace’s service switch, which many homes have on or near the unit and which usually looks like an ordinary light switch. Make sure it is on. These switches can be flipped by accident, especially after cleaning or a filter change.
Next, look at the breaker panel, as long as the area is dry and you can reach it safely. If the furnace breaker has tripped, switch it fully off and then back on one time. If the furnace starts and runs normally, you may have had a one-time overload. If the breaker trips again, leave it off and call a professional. Resetting a breaker over and over can be unsafe, and a breaker that keeps tripping is telling you something is wrong.
Look at the Air Filter
A badly clogged filter restricts airflow, and restricted airflow can make a furnace run hotter than it should. Many furnaces have a limit control that shuts the burners off when that happens, so a severely restricted filter may be one reason a system will not run. It is a possible cause, not a guaranteed one, and a clean filter does not rule out a bigger problem.
If you cannot remember the last change, take a look. ENERGY STAR advises checking the filter monthly during heavy heating or cooling seasons and replacing it at least every three months. Turn the furnace off at its switch before opening the filter compartment. For help choosing a schedule, see our guide on how often to change a furnace filter.
Make Sure Vents and Returns Aren’t Blocked
A furnace pushes warm air out through supply registers and pulls room air back in through return vents. When either side is blocked, the system has to work harder and may overheat and shut down sooner than it otherwise would. Look for furniture against registers, rugs over floor vents, and return grilles hidden behind boxes. Blocked airflow more often explains weak or uneven heat than a furnace that will not start at all, so treat this as a quick check rather than the likely culprit.
Check the Furnace Door or Access Panel
Many furnaces include a safety switch that cuts power when the blower compartment door or access panel is not properly in place. If a panel was removed recently, during a filter change for example, and it is not seated correctly, the furnace may not start. Trane notes that an open blower door can cut power to the furnace and its gas valve.
Turn off the furnace’s power switch, reseat the door following the manufacturer’s instructions until it sits flush and latches, then turn the power back on. Do not press the switch down with tape, wedge it, or defeat it in any way. It exists to keep you away from moving parts and live components. Never open the burner or gas compartment.
What If the Furnace Tries to Start but Shuts Off?
Some furnaces are not completely silent when something is wrong. You may hear a click or a fan spin up before everything stops and no warm air arrives. A gas furnace goes through a startup sequence before it makes heat, and if one step does not complete, the controls can halt the process. Possible causes include an ignition problem, a dirty flame sensor, a venting issue, or a safety control responding to a fault. None of those is a do-it-yourself repair, and a reset button on a safety control is not something to keep pressing.
If your furnace starts but then runs for only a few minutes, shuts off, and repeats, that is a different pattern. Our guide to furnace short cycling covers it. This article is about a furnace that does not start, or does not finish starting.
What If You Smell Something Unusual?
Many odors are minor. Our guide to furnace smells and when to call a technician explains which smells deserve attention.
The smell to treat as urgent is gas. Gas utilities add an odorant to natural gas, and many people describe it as a rotten-egg smell. The Pennsylvania Public Utility Commission advises anyone who smells gas indoors to leave right away without turning lights on or off, making calls from inside, or creating a spark. Once you are a safe distance away, call 911 and your gas utility, and stay out until they say it is safe. A furnace that will not start is never a reason to investigate a gas smell yourself or try to relight anything.
When the Problem Needs an HVAC Technician
If the basic checks do not get the furnace running, or the problem keeps returning, it is time to stop troubleshooting. These situations call for a professional:
- Ignition or control problems, such as a furnace that clicks or hums but never produces heat.
- A breaker that trips again after one reset, which points to an electrical problem that needs diagnosis.
- Blower problems, such as a motor that hums without moving air or no airflow from the vents.
- Safety switch problems, since a switch that keeps shutting the furnace down is reporting a real issue.
- Recurring shutdowns. A furnace that works for a day and quits again has not been fixed.
- No heat after the basic checks, when the thermostat, power, filter, vents, and panel all look fine.
Our HVAC service and repair work covers diagnosing problems like these, and our heating services page lists the systems we work on, including gas furnaces. If your furnace gave warning signs before it stopped, such as new noises or uneven heat, our guide to the signs your furnace may need repair explains what those can mean. The Consumer Product Safety Commission also recommends a yearly professional inspection of your heating system.
When No Heat Is an Emergency
Most no-heat problems are urgent but not dangerous. A few situations should not wait for a repair appointment:
- A gas smell. Leave the home and call 911 and your gas utility from a safe distance.
- A carbon monoxide alarm. The CPSC says to get outside to fresh air, call 911 or the fire department, and stay out until responders say it is safe. Headache, dizziness, nausea, or flu-like symptoms without a fever can be signs of exposure.
- Visible smoke or flames. Get everyone out and call 911, and do not go back in.
- An electrical burning smell. Turn the system off at the thermostat and call a licensed technician. If it comes with smoke, sparks, or a hot electrical panel, treat it as a fire and call 911.
- Unsafe indoor temperatures. The National Institute on Aging says homes at 60 to 65 degrees Fahrenheit can put older adults at risk of hypothermia and recommends setting the heat to at least 68. If you cannot heat the home, consider staying with a friend or relative, and call 911 for signs of hypothermia.
Never use an oven, grill, or gas stove to heat the house, and be cautious with space heaters, which can create fire and carbon monoxide hazards of their own.
Furnace Won’t Start? QualiTech Can Diagnose the Problem
If you have worked through the checks above and the furnace still will not run, the next step is a professional diagnosis. QualiTech Heating & Cooling works with homeowners in Philadelphia, Bucks, and Montgomery counties on furnaces, gas boilers, heat pumps, and other heating equipment, and we list 24/7 emergency service for breakdowns that cannot wait. Tell us what the furnace is doing and what you already checked.
To schedule heating system diagnosis and repair, contact QualiTech online or call (215) 499-7155.