By Hood Service Company · Corsicana, TX · Updated
The short version
- Most quick fixes are thermostat settings, batteries, a dirty filter or a tripped breaker.
- Never relight a pilot, adjust a gas valve or open the burner area yourself.
- If you smell gas, leave first, then call your gas utility or 911 from outside.
- A furnace that starts and then shuts down is often tripping a safety control, which needs diagnosis.
What should you do first if you smell gas?
Leave the house first. Then, from outside, call your gas utility or 911. Don't flip switches, adjust the thermostat or try to find the leak on your way out.
The same goes for a carbon monoxide alarm: get everyone outside and call 911 from there. Go back in only when emergency responders say it's safe, and have a technician inspect the furnace before it runs again.
What should you check when the furnace isn't heating?
Work through the thermostat, the filter, the breaker and the condensate drain. These safe checks solve a fair share of no-heat problems, and they tell a technician where to start if they don't.
The HVAC troubleshooter walks through these checks one symptom at a time.
- Thermostat: set to Heat, a few degrees above room temperature, with fresh batteries. If it runs a schedule, make sure a program isn't overriding you.
- Filter: a clogged air filter can make the furnace overheat and shut itself off. Replace it if it's dirty.
- Breaker: look for a tripped breaker for the furnace. Reset it once; if it trips again, leave it off and call.
- Condensate drain: high-efficiency furnaces produce water. Look at the drain line and any condensate pump for kinks, clogs or standing water.
- Vents and returns: keep supply registers open and return grilles clear.
Why does the furnace blow cold air?
Cold air usually means the blower is running but the burners aren't lighting or staying lit. Common causes are ignition problems, a dirty flame sensor, a tripped safety switch or the thermostat fan set to On.
Check the fan setting first. On On, the blower runs between heating cycles and moves unheated air, so set it to Auto. If the air stays cold with the fan on Auto and the thermostat calling for heat, the problem is in the ignition or safety controls, and that's work for a technician.
Why does the furnace start and then shut off?
A furnace that lights, runs a few minutes and shuts down is often tripping a safety control. Restricted airflow, a blocked vent or drain, or a dirty flame sensor are frequent causes.
Safety controls stop the furnace when something is wrong, such as overheating from a clogged filter or a blocked exhaust vent. Replace the filter and look at the drain, but don't keep resetting the system. If short cycling continues, have it diagnosed. A cracked heat exchanger is one of the conditions a technician checks for, because it can let combustion gases mix with your home's air.
Which noises and smells are warning signs?
A brief dusty smell at the first cold-weather start-up is normal. A smell of rotten eggs or burning, loud bangs at ignition, screeching or rattling are not. Turn the system off and call.
- Dusty smell on the first cold morning: dust burning off inside the furnace; it should fade quickly.
- Rotten-egg smell: a possible gas leak. Leave first, then call your gas utility or 911 from outside.
- Bang or boom at start-up: often delayed ignition; have it checked before using the furnace more.
- Screeching or grinding: often a blower motor or bearing problem.
- Soot or scorch marks: a combustion problem; turn the furnace off and call.
When should you schedule a heating repair?
Once the safe checks are done and the furnace still won't heat, schedule a repair. Anything involving gas, ignition, burners, the heat exchanger or a breaker that keeps tripping needs a trained HVAC technician.
Book heating repair service and tell the technician what you already checked. A fall heating tune-up before the first cold snap can catch many of these problems early, and Hood Comfort Club members get one every fall; see our maintenance plans.
When should you call a technician?
- Any gas smell or carbon monoxide alarm, after you've left the house and called from outside.
- The furnace lights, runs briefly and shuts off again and again.
- Air from the vents stays cold with the thermostat on Heat and the fan on Auto.
- You hear booming at ignition or see soot or scorch marks around the furnace.
- A breaker trips again after one reset.
Frequently asked questions
Should my furnace be producing water?
If it's a high-efficiency condensing furnace, yes. It produces condensate that drains away through a line or a small pump. Water pooling around the furnace is not normal and usually means a clogged drain or a failed pump.
How often should I change my furnace filter?
It depends on the filter type, pets and how much the system runs. Check it monthly during heavy use and replace it when it looks dirty. Our guide to air filters and MERV ratings explains the options.
Can a thermostat stop a furnace from heating?
Yes. Dead batteries, the wrong mode, a program overriding your setting or a failed thermostat can all stop the call for heat. Replacing the batteries and confirming the settings is a safe first step; anything beyond that is work for a technician.