Furnace won't turn on this fall? What to check before calling an HVAC pro
A step-by-step look at what's safe to check when a furnace won't start, and when carbon monoxide risk means it's time for a licensed HVAC pro.
Furnaces that sat idle all summer often show their problems the moment you ask them to run for the first time this fall. Furnaces are part of what CPSC grouped into its winter warning about fire and carbon monoxide risk from home heating equipment, and CPSC separately notes that CO poisoning often shows up in colder months, right when fuel-burning furnaces go back into heavy use. Most no-start problems have a simple cause. This guide covers what you can safely check yourself, and where the line is for calling a licensed HVAC pro.
What's going on
A furnace that won't turn on usually traces back to one of a few things: the thermostat isn't calling for heat the way you expect, airflow through the system is blocked, or the unit missed maintenance it needed. CPSC points out that carbon monoxide poisoning from heating systems can show up when a furnace is installed wrong, poorly maintained, has blocked venting, or gets misused, which is part of why furnaces get treated more carefully than, say, a dead light switch. CPSC also describes CO as a silent killer that can't be seen, smelled or tasted yet can turn deadly within minutes. None of that means every furnace that won't start is dangerous, but it's why the checks below stop well short of anything involving gas, wiring or venting.
What you can safely check or fix
- Test your smoke and carbon monoxide alarms first, and confirm there's one on every level of the home and outside each sleeping area.
- Swap in fresh batteries if your alarms don't already run on sealed 10-year batteries, per CPSC guidance.
- Check the furnace's air filter and clean or replace it if it's dirty; the Department of Energy lists filter care as routine furnace and heat pump upkeep.
- Walk through any ductwork you can reach, in the basement, attic or crawlspace, and look for sections that have come apart or show obvious damage. DOE says heating ducts in poor repair can waste up to 60% of the heat at the registers.
- Compare the thermostat's current schedule with when your household actually wants heat. DOE recommends a programmable thermostat set to the times people are home and asleep.
Where to stop and call a pro
If those checks don't get the furnace running, or if anything smells off near the unit, the next call is to a licensed HVAC technician, not a deeper look inside the furnace yourself. CPSC recommends a professional inspect fuel-burning heating systems, including furnaces, once a year, and DOE recommends the same yearly professional maintenance for a furnace or heat pump. EPA guidance on carbon monoxide also points to professional inspection of fuel-burning appliances as the way to catch leaks a homeowner can't see on their own.
Stop here: Any work on the furnace's gas supply, its burner or ignition system, its electrical wiring, or its venting and chimney connection needs a licensed HVAC pro. CPSC's warning on carbon monoxide ties exactly those systems, installation, maintenance, venting and use, to CO poisoning risk.
What to tell the pro
When you call, describe exactly what you already checked (alarms tested, filter inspected, ducts looked over, thermostat schedule confirmed) and what the furnace did or didn't do when you asked it to run. Mention roughly how old the furnace is and when it was last professionally inspected, since both CPSC and DOE point to once-a-year inspection as the baseline. If anyone in the house has had a headache, dizziness, weakness, nausea, vomiting, sleepiness or confusion since the furnace last ran, say so right away, those are the CO poisoning symptoms CPSC lists, and the right first move if CO poisoning is suspected is to get outside to fresh air and call 911, not to wait for a technician to arrive.
Placing a carbon monoxide alarm
A carbon monoxide alarm is a backup, not a substitute for a furnace that's properly installed and maintained, according to EPA. EPA recommends an alarm certified to the UL 2034 or IAS 6-96 standard, placed in the hallway near every separate sleeping area. EPA also notes that elevated indoor CO can come from fuel-burning appliances that are poorly maintained, wrongly installed, or backdrafting, not only from equipment that's obviously broken.
Quick answers
Why won't my furnace turn on?
It's often the thermostat, blocked airflow, or missed maintenance. CPSC notes that CO poisoning from heating systems can occur when a furnace is installed wrong, poorly maintained, has blocked venting, or is misused, which is also why furnace problems deserve more caution than most no-start issues.
What can I check myself before calling an HVAC pro?
You can safely test your smoke and CO alarms, check or replace the furnace filter, look over accessible ductwork, and compare the thermostat's schedule to your household's routine; DOE lists filter care, duct repair and thermostat scheduling as basic furnace upkeep.
What if I feel sick near the furnace?
Get outside to fresh air right away and call 911. CPSC lists headache, dizziness, weakness, nausea, vomiting, sleepiness and confusion as CO poisoning symptoms, and calls CO a silent killer that can turn deadly within minutes.