A furnace that runs but blows cold air is the most common heating call we take in October and November, and about half the time the homeowner fixes it before we arrive. So this is the order we would check things in, from the ones that cost nothing to the ones that need a technician. Work down the list. If you get to the end and the air is still cold, call us and tell us where you stopped. That alone saves fifteen minutes of the diagnostic.
First: is it actually cold, or just not hot?
Put your hand over a supply register. Furnace air should feel distinctly warm, roughly 40 to 60 degrees warmer than the air going into the return. If it is lukewarm rather than cold, the burners may be lighting and shutting off early, which is a different problem from not lighting at all. If it is room temperature or colder, the burners are not running and the blower is moving unheated air.
Also note when it is cold. Cold for the first minute after the furnace starts is normal on most units. The blower is timed to start before the heat exchanger is fully warm. Cold for five minutes is not normal.
The six checks, in order
- The thermostat fan setting
- The filter
- The pilot light or igniter
- The flame sensor
- The condensate drain, on high-efficiency furnaces
- The gas supply
1. The thermostat fan setting
This one is embarrassing and it is also the single most common cause. If the fan is set to ON instead of AUTO, the blower runs constantly whether or not the burners are lit. Between heating cycles it pushes room-temperature air through the ducts, and that air feels cold on your skin. Switch it to AUTO. If the cold air stops between cycles, you are done.
While you are there, confirm the mode is HEAT, not COOL, and that the set temperature is above the room temperature. Furnaces get switched to cool by accident more often than anyone admits, usually by a thermostat schedule that nobody remembers setting.
2. The filter
A clogged filter starves the furnace of airflow. The heat exchanger overheats, the high-limit switch trips and shuts the burners off as a safety measure, and the blower keeps running to cool everything down. From inside the house, the furnace appears to be blowing cold air. What is actually happening is that it heated for two minutes, tripped, and is now purging.
Pull the filter and hold it up to a light. If you cannot see light through it, replace it. Then set the thermostat to heat and watch for a full cycle. If the burners stay lit for ten minutes or more, the filter was the problem. If the furnace has been doing this for weeks, the limit switch may have weakened from repeated trips and can fail on its own; that is a small repair.
If the furnace does this every few minutes, running briefly and stopping, read our article on a furnace that keeps turning on and off. The filter is only the first of six causes.
In the 1950s and 60s ranch homes around Arden-Arcade, Citrus Heights and Carmichael, the filter slot is often a one-inch return grille that clogs in six weeks in winter. A three-month schedule is not enough there.
3. The pilot light or igniter
Older furnaces, roughly pre-1995, have a standing pilot light. Look through the small window on the burner compartment. If you see no flame, the pilot is out. Follow the relighting instructions on the label inside the panel, and if it will not stay lit, the thermocouple is likely worn. That is a routine part.
Newer furnaces use a hot surface igniter, a ceramic element that glows orange to light the gas. Watch the burner compartment when the furnace calls for heat. You should hear the inducer fan start, then see the igniter glow, then hear the gas valve click and the burners light. If the igniter never glows, or glows and no gas comes, stop. That is a technician job, but it is a quick one and the part is inexpensive.
If the burners do light but with a bang, that is delayed ignition, and it is covered in our guide to furnace noises. Do not keep running it.
4. The flame sensor
If the burners light for three to five seconds and then shut off, then try again a minute later, and again, the flame sensor is the most likely cause. It is a thin metal rod that sits in the flame and confirms to the control board that the gas actually lit. When it is coated in oxidation it cannot read the flame, and the board shuts the gas as a safety precaution.
Cleaning it is a five-minute job for us and it is often the whole repair. Some homeowners do it themselves with fine steel wool, and if you are comfortable turning off the gas and power to the furnace, it is a reasonable thing to try. If you are not, do not force it.
5. The condensate drain, on high-efficiency furnaces
If your furnace has white PVC pipes coming out of it rather than a metal flue, it is a condensing furnace, 90% AFUE or higher. These make water as they run, and that water drains through a trap and a line. When the trap clogs with sediment, water backs up, a pressure switch trips, and the furnace refuses to light or shuts off mid-cycle. The blower may keep running and the house gets cold.
This is common in the attic furnaces in Natomas, Elk Grove and the newer parts of Roseville, where the drain line runs a long way and nobody looks at it. Clearing the trap fixes it. A furnace that is shutting off this way and is in an attic should not be left alone, because a backed-up condensate line ends up as a ceiling stain.
6. The gas supply
Rare, but worth ten seconds. Is the gas valve on the pipe to the furnace turned parallel to the pipe? Do the stove burners light? If PG&E has done work on your street, or if a gas bill went unpaid, the supply may be off. A furnace with no gas will go through its start sequence, fail to light, and run the blower to purge, over and over.
The problems that look like cold air but are not the furnace
Sometimes the furnace is fine and the house is still cold. Three things to consider before you assume the equipment has failed:
- Disconnected ducts. If one room is cold and the rest of the house is fine, a flex duct in the attic or crawlspace has likely pulled loose. The furnace is heating the attic. This is a duct repair, not a furnace repair, and it is more common than a failed part in homes over thirty years old.
- Closed or blocked registers. Furniture pushed over a floor register in a Land Park bungalow will do it. So will a damper someone closed in summer and forgot.
- A heat pump in defrost. If you have a heat pump rather than a gas furnace, cold air for a few minutes on a frosty morning is normal. The unit reverses briefly to melt ice off the outdoor coil. If it lasts longer than ten minutes, or happens constantly, call.
The one thing you should not ignore
If the furnace runs, the air is warm, but you notice a smell like exhaust, soot around the furnace cabinet, or a carbon monoxide alarm that has gone off, stop using the furnace and open windows. Those are the signs of a cracked heat exchanger or a blocked flue. It is not a cold-air problem, but people find it while looking for one.
We take a carbon monoxide reading at the registers on every furnace visit. If a heat exchanger is cracked, we show you the crack, we shut the gas to the unit, and we do not talk you into repairing it. That is the one furnace job we turn down.
When to call and what it costs
If you have worked through the thermostat, the filter and the gas supply, and the furnace still blows cold, it is time. The three most common repairs from this point are a flame sensor cleaning, a hot surface igniter, and a pressure switch or clogged condensate trap. All three are short visits.
If the furnace does nothing at all, no blower and no click, start with our seven checks for a furnace that will not turn on.
Our diagnostic is a flat $69 across the Sacramento area and it comes off the repair. Same-day service in Roseville, Rocklin, Sacramento, Citrus Heights, Folsom and Elk Grove. If you are in Roseville, see furnace repair in Roseville; in the city, furnace repair in Sacramento. We also have dedicated pages for Rocklin, Folsom, Elk Grove and Citrus Heights. Everything else is on our furnace repair page.
And if the furnace is past fifteen and this is not its first repair this year, our guide to when to replace a furnace walks through the four questions we ask before quoting either option.
If the furnace is not doing anything at all, no blower, no click, the list is different. Start with the breaker and the door switch, then read our checks for a system that will not turn on; the electrical side is the same for heating and cooling. And if the furnace is past fifteen years old and this is the second repair this year, our guide to furnace brands explains what we would put in and why.
Frequently asked questions
Why does my furnace blow cold air for a minute when it starts?
That is normal. The blower is timed to start before the heat exchanger is fully warm so it does not overheat. Thirty to ninety seconds of cool air at the start of a cycle is by design. Five minutes is not.
My furnace blows warm air, then cold, then warm again. What is that?
Usually the high-limit switch tripping from low airflow. Check the filter first. If the filter is clean, the return may be undersized for the furnace or the blower may be failing, both of which are common in older Sacramento-area homes on their second or third furnace.
Can I clean the flame sensor myself?
Many homeowners do. Turn off the gas and the power to the furnace, remove the single screw holding the sensor, polish the rod lightly with fine steel wool, and reinstall. If you are not comfortable with any of those steps, it is a short and inexpensive visit for us.
Is a furnace blowing cold air dangerous?
Cold air itself is not. The danger signs are different: an exhaust smell, soot on the cabinet, or a carbon monoxide alarm. If you notice any of those, stop using the furnace and call.
How much does it cost to fix a furnace blowing cold air?
Our diagnostic is $69 and it is credited toward the repair. A flame sensor cleaning, an igniter or a pressure switch are all inexpensive parts and short visits. A failed blower motor or control board costs more, and on a furnace past fifteen years we price that next to a replacement so you can decide.