Why does my furnace turn on but not heat?

Why does my furnace turn on but not heat?

A gas furnace lights in steps and stops at the first one that fails: you hear it start, but a later safety check did not pass.

Payne's 2018 condensing furnace owner's manual and a 1999 Carrier troubleshooting guide for technicians describe the same order:

1. The thermostat calls for heat.
2. The inducer fan starts. This small motor pushes exhaust out the vent pipe.
3. The pressure switch closes. It proves the inducer is moving enough combustion air.
4. The ignitor heats up. Payne says the hot surface ignitor glows for about 20 seconds.
5. The gas valve opens and the burners light off the ignitor.
6. The flame sensor proves the flame. If it does not sense flame, the control shuts the gas off. On the furnaces in Carrier's 1999 guide, that window is 2 seconds.
7. The blower starts and sends warm air through the house. Payne says this can take up to 60 seconds after ignition.

Timings vary by model. "It turns on" usually means steps 1 and 2 happen and a later step fails.

What does each failure sound or look like?

You can often tell which step failed by watching and listening at the furnace.

What can I safely check when my furnace is not igniting?

The thermostat, the power, the filter and the outside vent pipes. Payne's manual lists all four; Trane's furnace not igniting page lists the first three. Stop at the first one that fixes it.

1. Thermostat. Set it to Heat, above the room temperature. Trane's furnace not blowing hot air page adds dying batteries; replace them.
2. The furnace switch. It looks like a light switch on or near the furnace. Make sure it is on.
3. The breaker. Look at the furnace breaker only if you can reach the panel without stepping into water near outlets, cords or the panel; otherwise stay out and call. If it has tripped, Trane says not to reset it: leave it off and call.
4. The filter. Payne says a dirty filter can make the furnace overheat and shut itself down, and to check it every four weeks. Turn the furnace switch off before you slide the filter out, and mind the sheet metal edges.
5. The door panel. Carrier's 1999 guide notes that on its furnaces, power reaches the controls only with the blower door in place. Seat a loose panel firmly; never tape the door switch in.
6. The vent pipes outside. A high efficiency furnace often has two plastic pipes on a side wall; Payne says they must not be obstructed. After lake effect snow or a drift, brush snow and ice off the pipe ends by hand. Enbridge Gas Ohio says to clear intake and exhaust vents after every storm.
7. The condensate drain. Trane says a clogged condensate drain line on a high efficiency furnace trips a safety that keeps the burners from lighting. Water around the furnace or a full or unplugged condensate pump is a reason to call. Never step into that water to reach the switch, an outlet or anything else.

Where to stop

Stop at the outside of the cabinet. Payne's manual says owners can clean and replace filters; all other work belongs to trained service personnel.

• Never open the burner compartment. Never remove, sand or clean a flame sensor, ignitor, gas valve or pressure switch. Trane says not to clean or replace the flame sensor yourself; it belongs in a furnace tune up. The burner area can be hot after a run.
• Never jump, bypass or tape a safety switch, including the door switch or a rollout switch.
• Never reset a tripped safety more than once. If it trips again, call.
• Never restart a furnace that has stood in water. Payne calls a flood damaged furnace extremely dangerous and says a qualified service agency should inspect it and replace wet gas controls and electrical parts.

Ignition faults, flame sensors, pressure switches, blower motors and control boards are all part of our furnace repair work.

Why is my furnace clicking but not turning on?

A furnace that clicks, hums, goes quiet and repeats is usually failing to light and starting over.

How many tries it makes depends on the maker (see lockout below). Note how far each try gets (inducer only, ignitor glow, or a brief flame) and tell the technician.

What does the blinking light on my furnace mean?

It is a status code. On many furnaces a small LED on the control board flashes a pattern for the current or last fault.

1. Read it through the window first. Payne says the light can be seen through a view port in the blower access panel.
2. Do not take the door off or cut the power yet. Payne says to record the code before removing the blower door or turning off power, and Carrier's 1999 guide says removing that door erases stored codes.
3. Write it down or film it for the technician.
4. Match it to the code chart only if the chart is on the outside. Payne keeps its legend in a booklet inside the main furnace door; leave that one to the technician. On the Carrier control in the 1999 guide, code 14 means ignition lockout; other makers use other codes.

What is furnace lockout, and should I reset it?

Lockout is a safety: after a set number of failed ignitions, the control stops trying. How many tries, and how it resets, depends on the maker:

• Trane. Trane's page on furnace lockout describes a soft lockout the first one or two times the control stops the ignition sequence, then a hard lockout the third time that needs a technician.
• Payne. Locks out after four failed attempts; the manual says to shut the furnace down and call your dealer for service.
• Carrier, 1999. The guide's furnaces try once plus up to 3 more times; the lockout resets itself after 3 hours or when power is briefly cut.

Follow your own furnace's manual. If it allows a reset, turn the furnace switch off and on once, after the checks above. If it locks out again, stop and call.

Why is my furnace blowing cold air?

Often it is the thermostat fan set to On instead of Auto, which Carrier's cold air guide says circulates cool air between heating cycles. Set it to Auto.

Air that stays cold means the burners are not lighting or are going out. If the burners light but the blower never comes on, Payne says to shut the furnace down and call for service.

What if you smell gas or a CO alarm goes off?

Leave first, then call. Never troubleshoot a furnace that smells of gas.

Gas smell. Enbridge Gas Ohio says natural gas is given a rotten egg or sulfur smell so you notice a leak. Get everyone out now. Enbridge says not to use phones, switches or electronics, not to smoke, and not to turn lights or appliances on or off. Treat a flashlight, a lighter and the garage door opener the same way: do not use them. Payne's manual adds: do not try to light any appliance. Once you are safely away, call Enbridge Gas Ohio's emergency line at 1-877-542-2630 or 911. On another gas utility, call its emergency line or 911.

Carbon monoxide. A furnace with a blocked vent or a fault can make carbon monoxide (CO), which you cannot smell. The CDC lists the symptoms as headache, dizziness, weakness, upset stomach, vomiting, chest pain and confusion. If anyone has them, alarm or no alarm, or if a CO alarm sounds, get everyone outside to fresh air and call 911. That is what the U.S. Consumer Product Safety Commission advises if you suspect CO poisoning, and what Enbridge says to do when an alarm sounds. Enbridge adds: do not go back in until a qualified professional, such as a firefighter, says it is safe.

When to call Raber HVAC

• Gas smell, a CO alarm or CO symptoms: everyone out, then 911 or your gas utility. Once it is safe, call us before the furnace runs again.
• No heat in freezing weather, or a furnace that keeps shutting itself off: call our 24/7 emergency line at (216) 888-4822. It also covers active water leaks and sewage backups. See what counts as an emergency. A cold house can freeze pipes on outside walls; see our frozen pipes guide.
• While you wait for heat, use an electric space heater only. The U.S. Fire Administration says to plug it directly into a wall outlet, never an extension cord or power strip, keep anything that can burn at least 3 feet away, and turn it off when you leave the room or go to bed. The CDC says never heat your house with a gas oven, never use a camp stove or burn charcoal indoors, and never run a generator inside the home or garage. Never use a grill or patio heater indoors or in the garage either.
• Lockout, a status code or burners that will not stay lit: call (216) 888-4822. The office is open every day, 8:00 AM to 10:00 PM.
• Anything that can wait a day: book online, pick a day and an arrival window, and the office calls to confirm.

Sources

• Payne Heating and Cooling Systems, Condensing Gas Furnace Owner's Manual (OG-9GFP-08, 2018)
• Carrier, Deluxe 2-Stage Gas-Fired Induced-Combustion Furnaces Troubleshooting Guide (58-3T, 1999)
• Carrier, Why Is My Furnace Blowing Cold Air?
• Trane, Furnace Not Igniting? Find Out Why and What to Do
• Trane, Furnace Not Blowing Hot Air?
• Trane, Solving Furnace Hard Lock Out Issues
• Enbridge Gas Ohio, Smell Gas?
• Enbridge Gas Ohio, Carbon Monoxide Safety
• CDC, Carbon Monoxide Poisoning Basics
• U.S. Consumer Product Safety Commission, Carbon Monoxide Information Center
• U.S. Fire Administration, Heating Fire Safety handout