The short answer
If your furnace in Northern Virginia won’t turn on, check six things first: the thermostat is on heat and set warmer than the room, its batteries, the furnace power switch, the breaker, the filter, and that the front panel on the furnace is pushed all the way closed. Those six fix a real share of first-cold-morning calls. If all six check out and it still won’t run, it needs a technician, and the blinking light on the control board will usually tell them where to start.
If you smell gas, skip the list. Leave the house, then call the gas company and 911 from outside. If a carbon monoxide alarm is going off, the same goes (what to do when the CO alarm sounds).
The six checks
1. The thermostat. Set to Heat, not Cool or Off, and at least three degrees higher than what the room reads now. If the screen is blank or dim, swap the batteries even if it says they’re fine. A thermostat that clicks and nothing happens is a different problem (thermostat not working).
2. The furnace switch. There’s a light switch on the wall or the ceiling near the furnace, often with a red cover plate. It looks like a light switch, so people flip it off by accident. Make sure it’s on.
3. The breaker. Look for one labeled furnace, air handler or HVAC. A tripped breaker sits in the middle, not fully on. Push it fully off, then back on, once. If it trips again, leave it off and call.
4. The filter. A filter that’s packed solid chokes the airflow, the furnace overheats, and a safety switch shuts it down. Change it, wait a few minutes, try again.
5. The front panel. The furnace has a door switch that cuts power when the panel is off or loose. If someone changed the filter and set the panel back crooked, the furnace won’t run. Push it until it seats.
6. The drain, on a high-efficiency furnace. If your furnace vents through white plastic pipe, it makes water, and a clogged drain line or a full condensate pump trips a float switch that stops the furnace. Water on the floor next to it is the clue.
The $67 Heating Check
We check your furnace, heat pump or boiler before winter. Through December 31. See the offer
Read the blinking light
Most furnaces built in the last 25 years have a small window on the lower panel with a light that blinks a code. Count the blinks and look for the chart on the inside of the door. Write it down or snap a photo before you turn the power off and on, because a reset clears it. That one number saves the tech time, and it saves you money.
What it usually is when the six don’t fix it
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| What you notice | Usually |
|---|---|
| Blower runs, no flame, then it shuts down | A dirty flame sensor or the igniter |
| It clicks, tries to light, and quits after a few tries | Igniter, gas valve or a venting safety |
| Lights for a few seconds, then shuts off | Flame sensor, almost every time |
| Nothing at all, no light on the board | Power, a blown fuse on the board or the transformer |
| Runs, but the air coming out is cold | See blowing cold air in winter |
| Starts and stops every few minutes | See furnace short cycling |
A dirty flame sensor is the most common one we see on the first cold morning of the year. The furnace sat all summer, a little residue built up on a thin metal rod, and the furnace can’t confirm it has a flame, so it shuts the gas off to be safe. Cleaning it takes a tech a few minutes. Don’t sand it yourself with the gas still on.
The fix for next year
Run the heat for an hour on a mild afternoon in October, before you need it (the first cold night test run), and have it checked once a year. Our heating check is $67 per system, through December 31: furnace, heat pump or boiler. On a furnace, for $229 (regularly $467) we also pull the blower wheel and clean it blade by blade, fit a new filter, set your dampers for winter and tune it. No heat right now? Call (703) 997-9757 and we’ll come out.
Book it
Book the heating check, call (703) 997-9757, or book online from goswiftpro.com/cookiesoffer and a box of warm cookies comes with the visit. Rated 5.0 stars across 1,138 reviews on Google, Angi, Yelp, the BBB and Facebook, Virginia Class A license 2705192276.
Questions people ask
Why won’t my furnace turn on?
Most often the thermostat (wrong mode, dead batteries), the furnace power switch or breaker, a clogged filter, or a loose front panel. If those check out, a dirty flame sensor or a failed igniter is the usual cause.
Many furnaces don’t have one. Turning the furnace switch off for 30 seconds and back on resets the control board, but note any blinking code first, because the reset clears it.
Why does my furnace turn on and then shut off right away?
If it lights and goes out within a few seconds, the flame sensor is usually dirty and the furnace can’t confirm a flame. A technician cleans or replaces it.
What should I do if my furnace won’t turn on and I smell gas?
Don’t touch the furnace or any switch. Leave the house and call the gas company and 911 from outside.
