The furnace is running. You can hear the blower, and air is coming out of the vents. It just is not warm. On the first cold nights of November this is one of the calls we take most, and the good news is that the cause is often something small.
The useful thing to know first: cold air means the blower is working but the burners are not heating the air, or are not staying lit long enough to do it. The list of reasons is short. Two of them you can check in the next five minutes for free.
One rule before anything else. If you smell gas, leave the house first and call Enbridge Gas (formerly Dominion Energy) at 1-800-767-1689, or 911, from outside. If a carbon monoxide detector goes off, do the same. Our carbon monoxide and furnace safety guide covers the warning signs.
Check these two first
1. The thermostat fan setting. Most thermostats have a fan setting of AUTO or ON. On AUTO, the blower runs only while the furnace is heating. On ON, the blower runs all the time, so between heating cycles the vents blow room-temperature air that feels cold against your hand. Set the fan to AUTO and the system to HEAT, and set the temperature a few degrees above the room. If the air turns warm within a few minutes, that was it.
While you are there, check that the thermostat is actually set to HEAT. A thermostat left on COOL from September, or a new smart thermostat set up for the wrong type of system, will run the blower without ever lighting the burners.
2. The filter. A clogged filter is the most common cause of a furnace that lights and then blows cool air. With too little air moving across it, the heat exchanger overheats, and a high-limit switch shuts the burners off to protect it. The blower keeps running to cool things down, and that is the cold air you feel. Pull the filter and hold it up to a light. If you cannot see light through it, replace it and give the furnace an hour.
If the air is warm after these two, you are done. If not, keep going.
Three more you can look at
3. The vent pipes outside (high-efficiency furnaces). If your furnace has two white PVC pipes leaving the house, go outside and look at both ends. Leaves, a nest, or drifted snow and ice on the intake or exhaust will make the furnace shut its burners off within minutes, and on some models the blower keeps running. Clear them carefully by hand.
4. The condensate drain (high-efficiency furnaces). A high-efficiency furnace makes water and drains it through a small tube, often to a floor drain or a small pump. If that line clogs or the pump fails, a safety switch stops the burners. Look for water around the furnace or a pump that is full and not running. Our furnace won’t turn on checklist walks through the drain and the other basics step by step.
5. The gas supply. Check that the gas valve on the line going into the furnace is turned on, with the handle parallel to the pipe. If other gas appliances in the house also stopped working, the problem is the supply, not the furnace, and that is a call to Enbridge Gas first.
Two that need a technician
6. A dirty flame sensor or a failing igniter. If you hear the burners light and then go out a few seconds later, over and over, the flame sensor is often coated and cannot prove there is a flame, so the control board shuts the gas off as a precaution. A weak igniter can fail to light the burners at all. Both are quick fixes on a service call, and after a few failed tries most furnaces lock out completely. If your furnace is doing this in short bursts, our guide to furnace short cycling explains the pattern.
7. Ductwork losing the heat. If the air is warm at the vents near the furnace and cool at the far end of the house, the furnace may be fine and the ducts may be the problem. Ducts that run through a cold attic, crawl space, or garage and have come apart at a joint can lose much of their heat before it reaches the room. A technician can find the gap; our duct cleaning and ductwork page covers what we do.
Why not just keep resetting it
A furnace that lights and then shuts its burners off is usually a safety switch doing its job. Reset it once after you change the filter. If it keeps happening, the switch is reacting to something real: heat that is not leaving the cabinet, a flame it cannot see, or exhaust that is not leaving the house. Resetting it ten times hides that until the night it quits for good.
What it costs to find out
Our furnace repair diagnostic is a flat $69, and it is waived if we do the repair. All makes and models, whether or not we installed it, and we tell you what we found before we fix anything. When you call, tell us what you hear: burners that never light, burners that light and go out, or burners that run while the air stays cool. Each one points to a different part, and it helps us bring the right one.
If the furnace is old and the repair is a big one, we will show you both paths. Our guide to repairing or replacing a furnace explains how we decide, and a replacement is finished in one day with a free 5-year labor warranty. The decision stays yours.
Still blowing cold air after the thermostat and the filter? Call or text (801) 678-5748, Monday through Friday, 8 AM to 8 PM, in English or Spanish.