Know your system · Heating

Furnace won't ignite: safe checks before you call

Short version: if the furnace clicks but never fires, start with four visible checks - the thermostat setting, the power switch and breaker, the filter, and the gas valve position. Everything past those, from the igniter to the flame sensor to the burners, is a pro job. If you smell gas, skip all of this: get everyone out and call your gas utility or 911 from outside. Do not operate switches, open windows, or go back in.

Check 1: the thermostat

Make sure it is set to "HEAT" and the set temperature is above the current room temperature. Someone in the house may have changed it.

Check 2: power

A gas furnace still needs electricity for the blower, the igniter and the control board. You can confirm it has power without opening anything or testing with a meter.

• The furnace switch. It usually looks like an ordinary light switch, either on the wall beside the furnace or mounted on the furnace itself. Make sure it is "ON".

  

  

• The control board light. Many furnaces have a small window in the lower door, also known as a sight glass. It shows a light on the control board to indicate the power is on, and the light may blink in certain patterns to indicate something is wrong. Look through the window only. Do not take the doors off. Some boards have a small display instead of a single light. On most boards, error codes start with an "E". Other codes show a setting: "tP1", for example, means the blower is on tap speed 1, its lowest speed for circulating air.

  

• The breaker panel. Look for a tripped breaker labeled for the furnace. Do not reset any breaker more than once. If it trips again, leave it off and call a pro. A breaker that trips again needs to be checked out for your safety.

  

If the panel is wet, damaged, overheating, buzzing, sparking or smells burnt, stay away and call a pro. If you cannot tell which breaker is the furnace's, do not guess or reset it. Do not test anything with a meter. If you are not comfortable making these checks, call a pro instead.

Check 3: the filter

A badly clogged filter can restrict airflow. Trane names a dirty filter as one possible cause of ignition trouble, not a diagnosis from this symptom alone. The filter is usually in one of two places: in a slot at the furnace, or behind a return air filter grille on a wall or ceiling. Before changing an accessible filter, turn the system off at the thermostat and make sure the fan is set to AUTO, not ON. Do not use the furnace switch or the breaker for this. To check it, do a light test: hold the filter up to a light beside a new one. If light passes through easily, air can too. If the light looks dim, or does not come through at all, replace it. Color alone does not decide its condition. Use the correct size and an equipment-compatible type. The size printed on the old filter can be wrong. To find the correct size, measure the space the filter fits into, look for a label at the furnace or grille, or ask a pro. Do not reach inside the furnace. How long a filter lasts is different in every home. Check it each month to learn how fast yours gets dirty; if nothing else, changing it every 3 months is a good rule of thumb. FilterPing prepares reminders you can add to your own calendar.

Check 4: the gas valve

The furnace's gas shutoff is usually a lever or knob on the pipe a few feet from the unit, and a lever-type handle in line with the pipe usually indicates an open valve. Do not open a closed gas valve unless you know why it was closed. Contact a professional for assistance if you are not sure.

If it still will not light: more to consider

Many furnaces today light with a hot surface igniter, and some, usually package units, use a spark igniter. Older furnaces often have a standing pilot instead. Trane says the ones it made since 2010 use an igniter, but the changeover date varies by maker, so check the label or manual. When an igniter furnace will not fire, the usual suspects are a failed igniter, a dirty flame sensor (a safety part that shuts the gas off if it cannot see a flame), or dirty burners. All three are common, fixable - and all three mean opening the furnace, which is pro territory. Trane is blunt about it: DIY burner work can even void a warranty. If yours is still under warranty, check the end date before anyone opens it up. An older furnace with a standing pilot has its own relighting steps, printed on the unit. Follow them exactly. If you are in doubt at any step, stop and call a pro. If the pilot goes out again after you relight it, leave it off, call a pro, and make sure your CO alarms work.

When to call

Call a qualified professional if the breaker trips again after one reset or if the furnace still will not light after the visible checks above. Stop before attempting a reset if the panel shows any of the warning signs listed above or you cannot clearly identify the correct breaker. If anything about the gas side is uncertain, stop and call a professional. A professional fall check can find problems before the first cold night. If something does not seem right, or you are not comfortable with a check, trust your instincts and call a pro.