It's 14 degrees outside. Your thermostat says 58. The furnace is silent. Your phone is in your hand and you're about to call us.
Wait three minutes first.
We're going to drive out either way — we always do — but probably half the emergency calls we get between December and February are things we fix in about ninety seconds with nothing more than a flashlight and a finger press. That means a $225 emergency trip charge for a problem the homeowner could have cleared themselves.
This is the checklist we walk through with people on the phone before we dispatch a truck. If you get to the end and nothing here works, definitely call — something real is wrong. But try these first.
1. Is the thermostat actually set to heat?
Start here. We're not insulting you. We've walked into houses where a toddler flipped the switch, a guest "adjusted" it, or the thermostat got bumped during cleaning. Modern smart thermostats can also reset themselves after a power blip.
- Is the display on? If it's totally dark, the batteries may be dead (on battery-powered models) or it's lost power.
- Is it set to HEAT, not COOL or OFF or AUTO?
- Is the target temperature higher than the current room temperature? (If current is 68 and target is 66, the furnace will not run.)
- Is "HOLD" or "SCHEDULE" override mode off if you're trying to override it?
If the thermostat display is dark and it takes batteries, try fresh batteries before anything else.
2. Is the furnace switch flipped on?
On the side of every furnace, there's a switch that looks exactly like a standard light switch. Sometimes it's mounted on a joist next to the furnace instead of on the cabinet itself.
It is not a light switch. It cuts power to the furnace.
We cannot tell you how many calls we get where someone bumped this switch with a broom while storing Christmas decorations in the utility room, or flipped it thinking it was a light, or had a kid flip it during a game of tag. The furnace was never broken. It was just unpowered.
Look at the side of your furnace. If there's a switch in the off position, flip it on. Wait 60 seconds. See if the furnace responds.
3. Is the breaker tripped?
Walk to your electrical panel. Find the breaker labeled "Furnace" or "HVAC." If the switch is in the middle position (neither clearly on nor off), it's tripped.
- Firmly flip it all the way OFF.
- Wait 10 seconds.
- Firmly flip it all the way ON.
Don't half-push it — breakers need to fully reset. If the breaker trips again immediately, STOP. That's an electrical problem that needs professional diagnosis. Don't keep flipping it. That's the kind of thing that starts a fire.
4. Is the filter absolutely destroyed?
Pull the filter out. If it looks like a gray sponge you can't see light through, that's your problem.
A heavily clogged filter chokes off airflow to the furnace. When the furnace can't move enough air, it overheats. When it overheats, the high-limit safety switch cuts the burners to protect itself. The blower may still run (pushing cool air), but there's no heat.
Replace the filter with a new pleated MERV 8-11. Turn the furnace off for five minutes to let the overheat switch reset. Turn it back on.
If the filter is black with dust, you might need to leave the furnace off for 10-15 minutes for the limit switch to cool down before it will let the burners fire again.
5. Is the gas on?
Walk to your gas meter outside. Is the main gas valve turned on? The handle should be parallel to the pipe when open, perpendicular when closed.
Also check any individual shutoff on the gas line near the furnace itself — there's usually a yellow handle you can see right next to where the gas enters the furnace.
If you smell gas anywhere in this process, STOP, leave the house, and call the gas company from outside. Don't mess with anything. Don't flip light switches. Just get out.
6. Is the condensate drain backed up?
High-efficiency furnaces (90%+ AFUE) produce water as a byproduct of combustion. That water drains out through a small PVC pipe. If the pipe is clogged, the furnace has a safety switch that shuts off the burners to prevent water damage.
This is incredibly common and constantly trips people up. Symptoms:
- Furnace turns on briefly, then shuts off without heating much
- Blower runs but no heat
- Possible clicking or gurgling from near the furnace
What to do: Look at the white PVC drain line coming out of the furnace. Follow it to where it exits. Look for standing water in any clear traps along the way. Most furnaces have an accessible cleanout cap. Remove it, clear any crud you find, and pour a cup of white vinegar down to kill algae.
If you're not comfortable doing this, we can do it in about 10 minutes on a service call. But it's worth trying first because it's so common.
7. Is the flue vent blocked?
The furnace exhausts combustion gases through a pipe that exits your house — either through the roof or through the side wall for high-efficiency units.
In winter, that vent can get blocked by:
- Snow drifted against the wall of the house. This is the #1 winter cause. A bad storm drifts snow up against the side of your house and buries the vent.
- Ice buildup from condensation freezing around the exit.
- Bird nests left from spring that you didn't notice.
- Insulation blown into the wrong place during a recent attic insulation job.
A blocked flue causes the furnace's pressure switch to trip and shut the burners down — a safety feature so carbon monoxide doesn't back up into your house.
What to do: Walk outside. Find the vent pipe. If it's buried in snow, carefully clear a 3-foot circle around it. If there's ice blocking the opening, use warm (not boiling) water to melt it gently. If you see anything stuck in the pipe, clear it.
After clearing the vent, the furnace may need to cycle power once to reset the pressure switch. Flip the furnace switch off, wait 30 seconds, flip it back on.
8. Is the flame sensor dirty?
This is one you probably can't fix yourself, but you can recognize the symptom so you know what to ask for.
Symptom: the furnace fires up, you hear the burners light for a few seconds, then they shut off. The blower may run for a minute. Then the whole cycle repeats.
What's happening: the flame sensor is a small metal rod that sits in the path of the burner flame. It tells the control board "yes, the burners actually lit." Over years, it gets coated with carbon and stops sensing the flame even when it's there. The control board thinks there's no flame, shuts the gas off for safety, and tries again.
Fix: A technician pulls the sensor out, gently cleans it with fine emery cloth or a Scotch-Brite pad, and puts it back. It takes about five minutes on a service call and costs whatever a diagnostic visit costs. If you're mechanically inclined and want to try it yourself, it's doable with basic tools, but we can't guide you through it in a blog post because the sensor location varies by furnace model.
This is probably the single most common cause of "my furnace runs for a minute and then stops" calls we get.
9. Is the ignitor cracked?
Older furnaces use a pilot light. Newer ones (post-2000 or so) use an electric hot surface ignitor — a small ceramic rod that glows orange when the furnace calls for heat, ignites the gas, and then shuts off.
Symptom: the furnace tries to start, you hear the draft inducer motor run, you hear a click-click-click, but the burners never light. The furnace tries a few times, then gives up and shuts down for safety.
The ignitor is almost always the cause of this specific pattern. They're cheap ($30-80 for the part) but they crack from thermal cycling after 5-10 years and eventually stop glowing.
You cannot fix this without replacing the ignitor, which means opening the furnace. Unless you know what you're doing, this is a service call. But if you describe the symptom to us on the phone ("it tries to start and clicks but never lights"), we can usually bring a replacement ignitor on the first visit and have it working in 20 minutes.
10. Is the drain pan float switch tripped?
On AC systems that share the cabinet with the furnace, there's a condensate drain pan with a float switch. If the pan fills with water (because the AC drain line is clogged), the float rises and shuts off the HVAC system to prevent overflow. Some systems shut off ONLY the AC. Some shut off the whole system including the furnace.
In winter, this can catch people off-guard. You're not running the AC, so you assume the drain pan issue doesn't matter. But if the pan still has water in it from the end of summer or from a leak, the float switch can be stuck in the "off" position and the furnace won't start.
Look at the drain pan under or beside the furnace. If there's water in it, find the drain line, clear the clog, and dry the pan. The switch should reset once the water drains.
If none of this works, call.
You've done your part. Whatever's wrong is beyond "swap a filter and reset the breaker." That's fine — that's what we're for. Call us at (801) 766-8585. A real technician picks up, and we'll either walk through a few more diagnostic questions on the phone or dispatch a truck depending on how urgent it is.
On an emergency winter call, we typically aim to be at your door within 90 minutes during business hours, and within 3-4 hours outside of business hours. We keep parts trucks stocked with the most common failures — igniters, flame sensors, pressure switches, gas valves — so most of our winter emergency calls get resolved on the first visit.
The short version, in order
- Thermostat — on, heat, target temp higher than room temp
- Furnace switch — on
- Breaker — reset once
- Filter — replace if destroyed
- Gas valve — open
- Condensate drain — clear any water block
- Flue vent outside — clear of snow/ice/debris
- Flame sensor — dirty (if furnace runs then stops)
- Ignitor — cracked (if furnace clicks but won't light)
- Drain pan float — stuck
If you want to print this and stick it on the inside of your utility room door, go ahead. We'll still show up when you call. We'd just rather save you a trip charge when we can.