Furnace runs or calls for heat but the house stays cold

Trane's first check is human error: the thermostat must be set to Heat and the temperature in the house must be lower than the setpoint. Some thermostats bring the furnace on automatically while others need a switch or button. Replace batteries in a battery-powered thermostat, look for loose wiring on a hard-wired one, and read any error code shown on a programmable thermostat.

A gas furnace still uses electricity for the blower, ignition and control board. Look for a switch that resembles a light switch on the side of the furnace, on a nearby wall or on a ceiling joist, and make sure it is on. Check the breaker panel. Trane says to switch a tripped breaker back on once and to call for service if it trips again, because that means an electrical problem. Another Trane guide says not to reset a tripped breaker at all, because it most likely tripped for a problem in the furnace. If in doubt, leave it off and call.

Change the filter if it looks dirty or you cannot remember when it was changed. Trane's guidance is every 1 to 3 months depending on the filter, and Carrier lists a clogged filter and closed vents as the first quick fix for weak or uneven heat. Move furniture and curtains off registers and return grilles. Restricted airflow makes the furnace overheat and shut itself off.

Note whether the inducer fan starts, whether you hear clicking with no flame, whether the blower runs, and what the status light does. Trane says not to take the front panel off while the furnace is running, and that a blower door switch cuts power and gas when the door is removed. Some furnaces have a light you can see from outside; on others it is on the control board behind the panel, which is a technician's view.

Trane says the gas shut-off is usually within a few feet of the furnace, on a black or copper pipe, with a knob or lever, and that it should be in the ON position. When in doubt, contact a professional. A rotten-egg smell means leave the house and call the gas company; do not investigate further.

Trane notes that furnaces built before about 2010 may have a standing pilot light rather than electronic ignition. If the pilot is out, relight it following the lighting instructions on the furnace itself. If it will not stay lit, stop and call for service.

Record the color, whether it flashes, how many long and short flashes, and any displayed code. For a Carrier furnace the code is two digits: the first is the number of short flashes and the second the number of long flashes. Match it on this site's Carrier or Lennox code list, or, for any other brand, take a photo or video of two full repetitions for your technician. Write down the furnace brand and model number too.

Trane's rule is simple: if the furnace has power, the thermostat is set to Heat and the filter is clean and it still will not run, call an HVAC company. Lockouts, ignition failures, pressure-switch faults, gas-valve problems and control-board faults are all technician work. Tell the person booking the visit what lights or codes you saw, plus the brand and model.