HVAC
No heat? Checks to make before you call for service
If you smell gas or a carbon monoxide alarm is sounding, get out and call from outside. Otherwise, check that the thermostat is on heat and set above room temperature, the furnace switch is on, the furnace door is seated, and the filter isn't clogged. If heat doesn't come back, call for service and mention anyone at risk from the cold.
The first cold night of the year is when furnaces that sat idle all summer get asked to work, and some of them refuse. HVAC phones light up by dinnertime. A few of those calls are true emergencies. Plenty of others turn out to be a thermostat on the wrong setting or a switch someone bumped, and you can sort that out yourself in about ten minutes.
Safety first: a gas smell or a CO alarm
Safety first: If you smell gas (a rotten egg or sulfur smell), don't touch light switches, the thermostat or anything that could make a spark. Get everyone out, then call your gas utility or 911 from outside. If a carbon monoxide alarm is going off, get everyone out and call 911 from outside. Stay out until the fire department or the utility tells you it's safe.
Carbon monoxide has no smell, so the alarm is the only warning you get. Headaches, dizziness or nausea in several people at once, especially symptoms that ease when you step outside, are a reason to leave even with no alarm. Don't try to open windows and stay put. Just go.
No alarm and no smell? Good. Move on to the next question.
Who's in the house tonight?
A cold house is a nuisance for a healthy adult and a real risk for a newborn, an older parent, or someone with a heart or lung condition. If that describes your household, say so in the first minute of the call. Good HVAC companies move those homes to the front of the line on a cold night.
While you wait, keep everyone in one room with the door closed. If you use a space heater, give it three feet of clear space from bedding, curtains and furniture, plug it straight into a wall outlet rather than an extension cord, and don't leave it running unattended. Never heat a home with an oven, a grill, a camp stove or a generator. Those produce carbon monoxide, and they're how a no-heat night becomes an ambulance call.
The five checks, in order
None of these need tools, and none involve opening anything that's screwed shut. Go in this order, because the first two solve a surprising number of no-heat calls on their own.
- Thermostat on heat, and set above the room. Look for HEAT, not COOL or OFF. Raise the set point three or four degrees above the temperature the display shows. Then give it a few minutes, because many systems pause before starting and some take a while to blow warm air.
- A blank or dim thermostat screen. Try fresh batteries. Plenty of thermostats rely on batteries even though they're wired to the furnace, and a dead display looks exactly like a dead furnace.
- The furnace switch and breaker. Near the furnace, on the wall or the side of the unit, there's a switch that looks just like a light switch. People turn it off by mistake all the time, thinking it runs a basement light. Make sure it's on. Then look in your breaker panel for a tripped breaker marked furnace or HVAC; reset it once by pushing it fully off, then on. If it trips again, leave it off and call.
- The furnace door. Most furnaces have a safety switch behind the front panel that keeps the unit from running when the panel is loose. If someone changed the filter and set the door back crooked, the furnace won't fire. Seat it firmly the way it came off.
- The filter. A filter packed with dust starves the furnace of air. It can overheat, shut itself down on a safety limit, cool off and try again, which looks like heat that comes and goes. If yours is gray and caked, swap it.
Two more if you have a newer furnace or a heat pump
High-efficiency furnaces vent through plastic pipes, usually white, that run out the side of the house or through the roof. Drifting snow or piled leaves can block those pipe ends, and the furnace shuts down to protect you. If you can safely reach them from the ground, clear snow or debris away from the openings by hand. Don't reach inside the pipes.
The same furnaces make water as they burn, and it drains through a small tube or into a little pump. If that pump fills up or the drain clogs, a float switch stops the furnace. Water pooled around the base is the clue. Fixing it usually takes a tech, but it's worth mentioning when you call.
Heat pumps have their own quirks. In cold weather the outdoor unit runs a defrost cycle now and then, and it can steam or go quiet for a few minutes. That's normal. Ice that covers the whole outdoor coil and stays, or cool air from the vents for more than a short stretch, is not.
What the checks tell you
| What you notice | What it often means | Next move |
|---|---|---|
| Thermostat screen is blank | Dead batteries or no power reaching the thermostat | New batteries; call if it stays blank |
| Complete silence from the furnace | Switch off, breaker tripped, or door panel loose | Check those three; call if nothing changes |
| Blower runs but the air stays cold | The burners aren't lighting, or a heat pump fault | Call for service and describe exactly this |
| Starts, then shuts off within minutes | Often airflow, venting or a flame sensing problem | Change the filter, check vent pipes, then call |
| Breaker trips again after one reset | An electrical fault somewhere in the system | Leave it off and call today |
General guidance only. Furnaces differ, so read the label on your unit and your owner's manual.
When to stop and call
Pick up the phone if the checks don't bring heat back within about 15 minutes, if a breaker trips twice, or if the unit is banging, screeching or rumbling. A faint dusty smell on the first run of the season is common and fades; a strong burning or electrical smell is not. Don't remove the burner cover, don't jump wires at the thermostat, and don't keep resetting things hoping one sticks. On some furnaces, repeated restarts trigger a longer lockout and hide the clues a tech needs.
When you call, have these ready. It shortens the call and helps the company decide how fast to send someone.
- What you already checked. Thermostat, switch, breaker, door, filter. Nobody has to walk you through it twice.
- What the system is doing. Silent, blowing cold, starting and stopping, or making a noise.
- Who's home. Babies, older adults, anyone sick or on medical equipment.
- The indoor temperature. Read it off the thermostat right before you dial.
- Brand and model if you can see them. The label inside or on the furnace usually shows both.
For HVAC pros: sharing this with customers
Send this to customers in early fall, before the first cold snap, not the night of it. People remember a checklist they read on a quiet afternoon. A customer who already checked the switch and the filter makes your call shorter and your dispatch decision easier, and the one who fixes it herself remembers who showed her how.
The order here matches the questions a good dispatcher asks: gas and CO, then vulnerable people, then the basics. If you're still deciding who covers those late calls, see setting up an after-hours on-call rotation, and the HVAC hub has more on the calls that spike each season. Customers on a maintenance plan should have their furnace checked in the fall, well before this night comes.
Common questions
Is it safe to relight a furnace pilot light myself?
Only on an older furnace that has a standing pilot, and only by following the step-by-step instructions printed on the unit. Many modern furnaces have no pilot at all and light with an electronic igniter. Never attempt it if you smell gas; leave and call from outside. A pilot that won't stay lit needs a technician.
Why does my furnace smell like burning the first time I turn it on?
Dust settles on the burners and heat exchanger over the summer and burns off during the first few runs of the season. A faint dusty smell that fades within an hour or so is common. A strong or lasting burning smell, anything that smells electrical, or visible smoke means shut the system off and call a technician.
How cold is too cold to wait until morning for furnace repair?
There's no single number. It depends on who lives there, how fast the house is losing heat, and the overnight low. If it's well below freezing outside, the indoor temperature is dropping quickly, or someone at home is very young, elderly or ill, tell the HVAC company and ask for the soonest visit. Frozen pipes in outside walls are another reason not to wait.
Should I switch my heat pump to emergency heat?
Emergency heat runs the backup electric heat strips and skips the outdoor unit. It's meant for times when the heat pump itself has failed, and it usually costs more to run. If the outdoor unit is iced over solid or not running at all, emergency heat can keep the house warm until a technician arrives. Tell them you switched it.