It is the first cold night of the year. You nudge the thermostat up, wait for the familiar click and whoosh from the hallway closet, and nothing happens. A furnace not turning on in Las Vegas catches people off guard because the heater has been sitting idle since March. Dust has settled on everything, a battery has gone flat somewhere, or a switch got bumped while the holiday boxes went into the garage.
The good news is that most “my furnace won’t turn on” calls come down to a handful of simple causes, and several of them you can check yourself in ten minutes without touching anything dangerous. Work through the list below in order. Each step rules something out, and by the end you will either have heat or a clear idea of what to tell the technician.
Start at the thermostat, because that is where most false alarms live
Before you blame the furnace, make sure it is actually being asked to run.
Confirm the thermostat is set to Heat, not Cool or Off, and that the fan is set to Auto. On a lot of Las Vegas thermostats the mode has not been touched since spring, and someone may have switched it to Off during the last warm week. Then raise the setpoint at least five degrees above the room temperature. Many furnaces will not fire for a one degree difference.
Next, look at the display. A blank or dim screen on a battery-powered thermostat means dead batteries, and that alone stops the whole system. Replace them even if you think they are fine. If the thermostat is hard wired and blank, the problem is upstream, so keep going.
Give it time. After a change at the thermostat, a furnace can take three to five minutes to start its ignition sequence. Standing next to it tapping the screen will not speed that up.
Check the power in three places
A furnace needs electricity even if it burns gas. The blower, the control board and the igniter all run on it, and a gas furnace with no power is just a metal box.
First, the breaker panel. Look for a breaker labeled furnace, heat, air handler or HVAC. If it sits in the middle position, it has tripped. Push it fully to Off, then back to On. If it trips again right away, stop. A breaker that will not stay on is telling you there is an electrical fault, and resetting it over and over is how boards get cooked.
Second, the furnace switch. Most furnaces have a light switch mounted on or beside the unit, and it looks exactly like a room light switch. In closets and attic spaces it gets flipped off by accident all the time. Make sure it is on.
Third, the access panel. Furnaces have a door safety switch that cuts power when the blower compartment door is open or not seated. If the panel was removed to change a filter and did not click back into place, the furnace will play dead. Press the door firmly until it locks.
A clogged filter can stop a furnace outright
You might not expect a dirty filter to keep a furnace from starting, but modern furnaces have a high limit switch that shuts the burner down when the heat exchanger gets too hot. Starved airflow from a packed filter is the most common way that happens, and after a few cycles some furnaces lock themselves out entirely.
Pull the filter and hold it up to a light. If you cannot see light through it, replace it. In the Las Vegas Valley, dust, desert pollen and a summer of running the AC all day can load a filter far faster than the box suggests. The U.S. Department of Energy's building science program notes that a dirty or clogged filter can dramatically reduce airflow, increasing furnace run time and wear on the motor, which is why filter checks sit at the top of every maintenance list (bsesc.energy.gov).
Put in a new filter, set the thermostat back to Heat, and wait a few minutes. If the furnace went into a lockout, you may need to turn the furnace switch off for 30 seconds and back on to reset it.
Look for a flashing light and read the code
Open the small viewing window on the lower panel of the furnace. Most units made in the last two decades have a status LED on the control board. A steady light usually means the board is happy and waiting for a call for heat. A blinking light is a diagnostic code, and the pattern (two short blinks and a pause, for example) matches a legend printed on the inside of the panel door.
Note the pattern before you do anything else. Even if you decide not to go further, that code tells a technician whether the furnace is seeing a pressure switch problem, a flame sensor fault, a limit trip or an ignition failure, and it can turn a diagnostic visit into a same-visit repair.
Listen for what happens, then stop at the burner
Turn the thermostat up once more and listen. The sequence tells you where the furnace is giving up.
If you hear nothing at all, no hum, no click, the problem is still power, thermostat or the control board. Go back over the first two sections.
If you hear a small motor spin up for about 30 seconds and then everything stops, that is the inducer fan. The furnace is trying to prove its vent is clear before it lights, and a pressure switch is refusing to close. Blocked flue, a stuck switch or a cracked hose are the usual reasons, and all of them are a technician's job.
If you hear the inducer, then a click, then a faint whoosh that dies within seconds, the burner lit and the flame sensor could not confirm it. On a furnace that sat all summer, a flame sensor coated with a thin film of oxidation is extremely common. Cleaning it is a five minute task for a technician. It is not a good first project for a homeowner, because it sits next to the burners and the gas valve.
If you have an older furnace with a standing pilot, look for the small blue flame. A pilot that is out can be relit by following the instructions on the furnace label exactly, which means turning the gas control to Off and waiting at least five minutes for any gas to clear before you try. A pilot that keeps going out points to a weak thermocouple or a draft problem, and that needs a professional.
At this point, stop. Anything beyond the flame sensor, pilot and filter involves gas valves, igniters, limit switches and the heat exchanger. These are not things to improvise on, and damage from a DIY repair is the kind of thing a service contract can exclude.
When no heat is actually an emergency
Two situations change the plan from "troubleshoot" to "get out and call."
If you smell gas anywhere near the furnace, do not flip switches, do not relight anything, and do not run the furnace. Leave the house, then call Southwest Gas or 911 from outside.
If a carbon monoxide alarm sounds, or anyone at home has a headache, dizziness, nausea or unusual sleepiness that eases outdoors, treat it as CO exposure. The Consumer Product Safety Commission reports that more than 200 people in the United States die every year from accidental, non-fire carbon monoxide poisoning linked to consumer products, and recommends CO alarms on every level of the home and outside sleeping areas (cpsc.gov). A furnace that will not stay lit can be a furnace venting badly. Get everyone outside and call 911.
Where a home warranty fits when the furnace will not start
Once you have ruled out the thermostat, the breaker and the filter, the remaining causes (flame sensor, igniter, pressure switch, control board, blower motor, gas valve) are mechanical and electrical failures from normal wear. That is exactly what a home warranty is designed to handle.
Heating systems are covered in every Dwellness plan, including the Essential Plan, and you do not need to show maintenance records to buy coverage. When the furnace fails, you submit a service request online at any hour or call (702) 641-8888, pay the per-trade service fee, and a licensed local contractor is dispatched to diagnose and repair the covered breakdown. Dwellness approves 97 percent of claims and resolves more than 85 percent of them on the first visit. If the same repair acts up again within 30 days, a contractor comes back at no additional charge.
Details of what counts as a covered heating breakdown are on the heating home warranty coverage page, and the post on whether a home warranty covers a furnace walks through the usual exclusions, such as damage from a DIY repair or a unit that was already broken before coverage started.
A warranty will not help with the dead thermostat batteries or the dirty filter. Those are on you, and they are also the cheapest fixes on this list, which is why they come first.
Habits that keep a Las Vegas furnace starting on the first cold night
Our heating season is short, which is both good and bad. Good because the furnace works a fraction of what it would in Denver. Bad because it sits for eight months collecting dust, and a lot of "furnace not working" calls happen in the first week it is needed.
Run the heat for ten minutes in October, before you need it, so any problem shows up on a mild afternoon rather than a cold night. Change the filter at the end of the cooling season, since the summer is when it does the most work. Keep the area around the furnace clear so the access panels seat properly and the burner gets air. Vacuum the return grilles. And if the furnace is a decade or more old, have it looked at once a year; every Dwellness plan includes an HVAC maintenance plan for exactly this reason. The post on signs you need HVAC repair covers the warning signs worth acting on before winter.
Furnaces rarely fail without a reason. Dead batteries, a tripped breaker, a dirty filter and a tired flame sensor account for most of the no-heat calls in this valley, and the first three are yours to fix in minutes. For everything else, a plan that already covers the heating system means the only call you make is the service request. Request a free quote at dwellness.house or call (702) 509-7007.