Furnaces give warning before they quit. The signals fall into three groups: things that mean book a service visit, things that mean call now, and things that mean shut it off.
Book a Service Visit
These indicate decline rather than failure, and a technician can usually resolve them in one trip.
Longer run times. The furnace takes noticeably longer to reach setpoint than it did last winter. Dirty burners, drifted gas pressure, or restricted airflow are the usual causes.
Higher gas bills with no change in habits or rates. Efficiency loss is gradual enough that households acclimate to it, and the bill is often the only evidence.
Weak airflow at registers. Check the air filter first. If it is clean, a dirty blower wheel is the next suspect — dust on the blades progressively cuts output while the motor itself is fine.
Uneven heating between rooms. Usually ductwork or insulation rather than the furnace, and duct sealing is the fix. But worth measuring rather than assuming.
It has been over a year since service. That alone is the signal, and most warranties require it.
Call for a Repair
These are failures, and delay makes them worse.
No heat after confirming the thermostat is set to heat, the breaker has not tripped, and the furnace door panel is fully seated — most furnaces will not run with a loose panel.
The blower runs but the air is cool. Typically an ignition problem: a failing igniter, a dirty flame sensor, or a gas supply issue.
Short cycling. On and off every few minutes indicates restricted airflow, a failing limit switch, or an oversized system. It is hard on ignition components.
Squealing or grinding. Bearings. Grinding means they are further gone, and continuing to run turns a motor replacement into more than that.
A loud hum with no blower. Usually a failed capacitor — inexpensive, and it takes the motor with it if ignored.
The blower will not shut off. A limit switch or fan control fault.
Shut It Off
A few symptoms mean stop using the furnace rather than schedule anything.
A persistent burning or electrical smell. A brief dusty smell on the first cycle of the season is normal. Burning insulation or hot metal is not.
Loud banging or booming at ignition. Gas is accumulating before it lights. This stresses the heat exchanger and can be dangerous.
Visible soot around the furnace or vents, which indicates combustion problems.
A yellow or orange burner flame. It should be steady blue. Yellow means incomplete combustion, which produces carbon monoxide.
Leave the House
Two situations are not repair calls until everyone is outside.
A gas smell — rotten eggs or sulfur. Get out without touching light switches or garage door openers, and call from outside.
A carbon monoxide alarm. Treat it as real every time, even if nobody feels ill. CO is colorless and odorless, and early symptoms resemble the flu.
What You Can Check First
Three things resolve a meaningful share of no-heat calls and take about two minutes.
Confirm the thermostat is set to HEAT and the setpoint is above the current room temperature, and replace the batteries if it uses them. Check that the furnace breaker has not tripped, and look for the service switch near the unit — it looks like an ordinary light switch and gets flipped by accident with real regularity.
Then look at the control board through the small window on the furnace. Most modern units blink a diagnostic code there, with the legend printed inside the door panel. Write down the pattern before you cycle the power, because resetting clears it and intermittent faults are the hardest to catch on a service call.
The Freeze Consideration
In the Hill Country, no heat during a hard freeze is more urgent than the temperature alone suggests, because supply lines here often run through unconditioned attics. A house losing heat during a sustained freeze risks burst pipes, and the water damage costs multiples of the furnace repair.
Preventing Most of This
Nearly every no-heat call we run in January was findable in October. Igniters, flame sensors, pressure switches, and capacitors all give warning if someone is looking.
Fall furnace maintenance catches them. If yours has already failed, that is furnace repair, and emergency service runs around the clock.
Contact 72 Degrees or call (830) 468-5316.
