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When your heater stops working on a cold day, the cause is often something small that you can find and fix in a few minutes. Other causes need a licensed technician, and a few are dangerous enough that you should leave the house right away. This guide walks through safe checks for furnaces, heat pumps, boilers, and portable heaters, and it explains when it is time to call a professional.
First, Stay Safe
Heating equipment causes real harm when it fails or is misused. The National Fire Protection Association estimates that U.S. fire departments responded to about 40,997 heating-equipment fires a year from 2018 to 2022, with about 470 civilian deaths a year. Peer-reviewed research also finds that more than 200 people die each year in the U.S. from carbon monoxide poisoning in homes, and about two-thirds of those deaths are linked to heating equipment. The U.S. Consumer Product Safety Commission reports that portable heaters were involved in an average of 1,600 fires a year from 2020 to 2022, causing about 70 deaths and 150 injuries a year.
Before you try any fix, keep these rules in mind:
- If you smell gas, see soot around the heater, or hear your carbon monoxide alarm, go outside right away. Call the gas company or 911 from outside, and do not turn any switches on or off on your way out.
- Never leave a portable heater running while you are asleep or away. Keep it at least 3 feet from curtains, bedding, furniture, and anything else that can burn.
- Turn off power to the heater at the breaker before you remove any furnace panel.
Identify Your Heater Type
The right fix depends on what kind of heater you have, so start by finding out which one is in your home.
- A gas furnace burns natural gas or propane and blows warm air through your ducts.
- An electric furnace heats the air with electric heating elements and also uses ducts.
- A heat pump moves heat from the outdoor air into your home. It uses an outdoor unit that looks like an air conditioner.
- A boiler heats water and sends it to radiators, baseboard units, or in-floor tubing.
- A portable space heater plugs into an outlet or runs on fuel and heats one room.
If your problem is a lack of hot water at the tap and not a lack of heat in the house, see our hot water heater not working guide instead.
Quick Checks That Fix Many Heaters
These three checks apply to nearly every central heating system, and they solve a large share of no-heat calls.
- Check the thermostat. Set it to Heat and the fan to Auto, then raise the temperature 3 to 5 degrees above the room temperature. Turn off any schedule or hold that may be overriding your setting. Replace the batteries if the screen is dim or blank, and confirm the date and time are correct. If you have a smart thermostat, make sure it is connected to your WiFi.
- Check the power. Look at the furnace switch, which often resembles a light switch on or near the unit. Make sure the furnace door or panel is closed fully, because a safety switch cuts power when it is loose. Then check your breaker panel. If the heater’s breaker has tripped, reset it one time only. If it trips again, leave it off and call a professional.
- Check the airflow. Replace the air filter if it is dirty, since a clogged filter can make a heater shut itself off. Open all supply registers and return vents, and move any furniture or rugs that block them.
Troubleshooting by System Type
If the quick checks did not help, work through the section that matches your heater.
Gas Furnace: Won’t Start or Blows Cold Air
Gas furnaces are the most common central heater in the U.S. Confirm that the gas supply valve to the furnace is open and that other gas appliances, such as your stove, are working. If you recently had a gas interruption, relight a pilot light only if you do not smell gas and you follow the manufacturer’s instructions exactly.
If you can see the burner through the small sight glass, watch what happens when the thermostat calls for heat. A normal cycle begins with the inducer fan starting, then the ignitor glowing or sparking, then the burners lighting, and finally the blower starting. Where the cycle stops tells you a lot, and it is useful information to give a technician.
Some causes are within reach of a homeowner: a clogged filter, a blocked intake or exhaust pipe (the white PVC pipes that run outside),a thermostat setting, a tripped switch or breaker, or a full condensate drain that has triggered a safety switch. For more on cold air from a furnace, read why your furnace may be blowing cold air.
Leave these to a professional: a failed ignitor, a dirty or failed flame sensor, a faulty pressure switch, control board errors, gas valve problems, and repeated short cycling.
Electric Furnace: No Heat or Lukewarm Air
Start with the breaker. Electric furnaces draw a lot of power, and they often use more than one breaker. Replace the filter and confirm that the thermostat is set to Heat. If the unit has a high-limit reset button that you can reach, you can press it once, but a unit that trips again needs service.
A technician should handle failed heating elements, a bad sequencer, control board faults, and a failed high-limit switch.
Heat Pump: No Heat or Outdoor Unit Iced Over
A heat pump can build up light frost in cold weather, and that is normal because the unit defrosts itself. Thick ice that stays for hours is not normal and points to a defrost problem or poor airflow. If the outdoor unit is iced over, switch the thermostat to Emergency Heat to keep the house warm until a technician arrives. Emergency Heat uses electric heating elements, so it can raise your power bill.
Clear snow, leaves, and debris from around the outdoor unit, and keep a few feet of open space on every side. In very cold weather, confirm that the auxiliary heat turns on when the heat pump cannot keep up.
Call a professional for defrost board or sensor faults, a low refrigerant charge, or a reversing valve problem.
Boiler or Radiator Heat: Some or No Heat
Confirm that the thermostat is calling for heat, that the boiler has power, and that the pressure gauge is in the range the manufacturer lists. If radiators are cold at the top but warm at the bottom, air is trapped inside, and bleeding them with a radiator key usually fixes it. If you have more than one heating zone, check that the zone valves are set correctly.
A professional should handle a failed circulator pump, a low-water cutoff problem, pilot or ignition faults, and expansion tank or pressure problems. Our guide to circulation pump troubleshooting explains the signs of a failing pump. Note that boilers and other hydronic systems are generally excluded from standard HVAC coverage, so check your agreement before you assume a repair is covered.
Portable Space Heaters: Basic Safety Only
Check that the outlet works, the heater is upright so the tip-over switch is not engaged, and the thermostat dial is turned up. Vacuum the intake grille if it is dusty. Do not use a heater with a frayed or damaged cord, do not run it while you sleep, and keep it 3 feet away from anything that can burn.
Uneven Heating or One Room Cold
Replace the filter and open every vent first. If one room is too warm, partially close its register to send more air to colder rooms. Look at any ductwork you can see for kinks, loose connections, or obvious leaks, and seal small gaps with foil tape made for ducts. Check whether a damper or zone control is stuck closed. Duct repairs and zoning faults need a professional.
Heater Runs Constantly or Short Cycles
A heater that never shuts off may have the thermostat fan set to On instead of Auto. Other causes include a very dirty filter, large air leaks in the home, a system too small for the house, or, on a heat pump, a low refrigerant charge.
A heater that turns on and off every few minutes is short cycling. It usually happens because the unit overheats from a clogged filter or blocked vents, or because of a dirty flame sensor or a pressure switch problem. Replace the filter and clear the vents. If the short cycling continues, schedule service.
When To Call a Pro
Call a licensed technician if you notice any of the following:
- A gas smell, soot, or a carbon monoxide alarm that keeps sounding. Leave the home first, then call.
- A breaker that trips more than once.
- Ignitor or flame sensor faults, control board error codes, or a heat pump that does not defrost.
- No improvement after you have tried the basic checks in this guide.
Even when nothing is wrong, have a professional service your heating system once a year, ideally before the heating season starts.
Preventive Care Checklist
- Replace the air filter on the schedule the manufacturer recommends, and check it monthly in winter.
- Keep outdoor units, intake pipes, and exhaust vents clear of snow, leaves, and debris.
- Clear the condensate drain line if your furnace has one.
- Test your carbon monoxide and smoke alarms every month.
- Schedule an annual tune-up. Our routine heating system care guide has more detail.
How a Home Warranty Helps When Your Heater Stops Working
A heating repair or replacement can be a large, unplanned expense. With eligible coverage, Liberty Home Guard can send a licensed professional to your home, and you pay a service fee for each approved claim instead of the full cost of the repair. If a covered system cannot be repaired, your agreement may provide for replacement.
HVAC Guard is built specifically for heating and cooling problems. It has no waiting period, and it allows pre-existing conditions. Heating system coverage is also part of Systems Guard. Water heaters are covered under Systems Guard and Total Home Guard for mechanical and electronic components, up to the limits in your agreement. Other plans generally begin 30 days after purchase.
It helps to know what a warranty does not cover. Coverage applies to normal wear and tear on eligible systems and components. Improper installation, lack of maintenance, and inaccessible items are generally excluded. Pre-existing conditions are allowed under HVAC Guard, but other plans may exclude them, so check your agreement. Certain parts are also excluded, including cracked heat exchangers, hydronic or steam systems and boilers, evaporator coils and drain pans, dampers, radiant heating, geothermal and water-source systems, outside piping, fuel storage tanks, fireplaces, and portable units. For water heaters, the tank itself and sediment buildup are often excluded.
Review your own agreement for waiting periods, exclusions, and coverage limits. You can read a sample policy to see the typical terms.
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