Furnace Not Heating? What to Check Before You Call

Furnace not heating? Check the thermostat first

Set the thermostat to Heat and raise the temperature a few degrees above what the room is now. It sounds obvious, but after a summer on Cool, or after someone changes a schedule, the setting is often the whole problem.

If the screen is blank or faint, put in fresh batteries. Smart and programmable thermostats can also be sitting in an away, eco or vacation mode that holds the house cooler than you expect, so look at the schedule as well. Then give the system a few minutes, since most furnaces run through a short startup sequence before warm air reaches the vents.

• Mode set to Heat
• Target temperature higher than the room’s current reading
• New batteries in a thermostat with a dim or empty screen
• Away, eco or vacation mode turned off

Confirm the furnace has power

Most furnaces have a power switch that looks like an ordinary light switch, sometimes with a red cover plate. It is usually mounted on or near the furnace, or at the top of the basement stairs, and it is easy to flip off by mistake when it sits beside a real light switch. Make sure it is on.

Next, check your electrical panel for a tripped breaker labeled furnace, heat or air handler. If it has tripped, switch it fully off and back on once. If it trips a second time, leave it off and call, because something in the system is drawing more power than it should.

Replace a dirty air filter

A furnace needs steady air flow to run safely. When a clogged filter cuts that flow, the furnace can overheat, and a built-in safety control shuts the burners down to protect it. You may hear the furnace start, run briefly and stop, or feel the blower pushing air that never gets warm.

Look for the filter in a slot where the return duct meets the furnace, or behind a large grille in a hallway wall or ceiling. If it is clogged with dust, swap in a new one of the same size with the arrow pointing toward the furnace. If reaching the filter means taking off the furnace’s front cover, leave that job to your technician.

Open the vents and clear space around the furnace

Walk the house and make sure supply vents are open and the large return grilles are not covered by furniture, rugs or holiday decorations. A furnace that cannot pull enough air back in struggles the same way it does with a dirty filter.

Keep the area around the furnace itself clear, too, with storage boxes, laundry and anything that can burn kept well away. If your furnace vents through white plastic pipes out a side wall, check outside after a storm: drifting snow or ice over those openings can shut the furnace down. Clear the snow away from the pipes by hand, but do not reach inside them.

• Heat registers open in the rooms you use
• No rugs, couches or boxes over the return grilles
• Storage and anything that can burn kept away from the furnace
• Outdoor vent pipes clear of snow and ice

Common reasons a furnace blows cold air or will not start

If the checks above do not bring the heat back, the cause is almost always inside the furnace, and only a technician can say for sure which part is at fault. Here is what the usual causes mean.

Connecticut winters are hard on heating equipment. The first cold snap of the fall is when many furnaces around Stamford, from Glenbrook to North Stamford, show a problem that has been building quietly, and a run of bitter nights leaves a struggling system little room to keep up. Homes across Fairfield County, from Ridgefield to Shelton, see the same pattern.

• Dirty flame sensor, so the burners light and then shut off
• Worn igniter that no longer lights the burners
• Safety lockout after several failed attempts to start
• Blocked vent or drain line tripping a safety switch
• Blower motor problem, so heat never reaches the rooms
• Thermostat wiring or control board fault

When to stop and call for heating repair

Stop troubleshooting once you have checked the thermostat, power, filter, vents and the space around the furnace. Everything past that point involves gas, flame, electricity and safety controls, and it calls for a licensed HVAC technician. Do not relight a pilot, reset the furnace over and over, or try to bypass a safety switch to get the heat back.

Heat problems in January do not wait for business hours, and Stamford HVAC Pros is available 24/7, including nights, weekends and holidays.

• A breaker that will not stay on after one reset
• Furnace starts, then shuts off over and over
• Loud banging, squealing or grinding from the furnace
• Burning smell that does not fade after the first run of the season
• Water around the base of the furnace
• A carbon monoxide alarm sounding (get out first, then call 911)