The first cold night of the year is when a lot of heating systems show what has gone wrong over the summer. If cool air is coming out of the vents when the heat is on, the cause can be a thermostat setting, a five minute fix, or a fault that needs a technician. Which one depends partly on whether you have a gas furnace or a heat pump, so start by working out which you have.
A gas furnace burns natural gas or propane and usually sits in a closet, garage, attic or crawl space with a flue pipe running out of it. A heat pump has an outdoor unit that looks like an air conditioner and runs in winter as well as summer. Some homes have both.
If you smell gas, stop here
If you smell natural gas, do not troubleshoot. Leave the house, get everyone out, and avoid matches, flashlights and anything else that could spark. PG&E’s instruction is to call 911 first and then PG&E, from somewhere away from the house.
California law requires a carbon monoxide alarm in homes with a fuel burning heater or appliance, a fireplace or an attached garage. If yours is sounding, get everyone outside and call 911.
Checks you can make in five minutes (furnace or heat pump)
- Make sure the thermostat is set to Heat and not Cool or Off.
- If the thermostat fan is set to On rather than Auto, the blower runs all the time, including between heating cycles. Between cycles, the air coming out is room temperature, which feels cold. Set it to Auto.
- Check that the temperature has not been turned down and that a programmed schedule has not shifted, for example after the clocks change. Raise the setpoint a few degrees and see whether the system responds.
- Look at the air filter. A clogged one cuts airflow. On a furnace it can make the system overheat and shut the burners off while the blower keeps running. Replace it if it looks gray or packed.
- Open the supply vents. Closing a lot of them can make the system short cycle.
- Check that the breaker has not tripped and the furnace or air handler switch, which often looks like a light switch, is on.
If the air is still cold after all six, the cause is inside the equipment.
Gas furnace blowing cold air
It is still warming up
Many furnaces wait a short time after the burners light before the blower starts, so the first air is not ice cold. If it is still cold after a few minutes, keep reading.
The furnace overheated and shut off the burners
When a furnace runs too hot, usually because of a clogged filter or blocked airflow, a safety switch shuts the burners off. The blower can keep running and pushing unheated air. A new filter sometimes fixes it. If it keeps happening with a clean filter, the furnace needs to be checked, because overheating is hard on the heat exchanger.
The flame sensor or ignition has failed
If the furnace cannot confirm a flame, it shuts the gas off for safety. A dirty or failed flame sensor is a common cause, and so is a worn igniter on newer furnaces or a pilot that will not stay lit on older ones. These are jobs for a technician, because they involve the gas valve and burners.
A gas supply problem
If the gas supply to the furnace is interrupted, or the gas valve or burner is faulty, the furnace will not heat. Gas work is for a licensed technician.
A blocked condensate line on a high efficiency furnace
High efficiency furnaces produce water and drain it away through a small line. If that line clogs, the furnace can shut itself down. Some homeowners can clear it, but if you are not sure where it is or what you are looking at, leave it to a technician.
Leaky ducts
If the furnace is making heat but rooms stay cold, warm air may be escaping through leaking or disconnected ducts in the attic or crawl space before it reaches the vents.
Heat pump blowing cold air
It may be in defrost mode, and that is normal
In cold, damp weather, frost builds up on the outdoor coil. The heat pump clears it by switching into defrost for a while, and during that time the air from the vents can feel cool. Lennox says a defrost cycle usually lasts 5 to 15 minutes and is a normal part of how the system looks after itself.
Heat pump air feels cooler than furnace air
A heat pump moves heat rather than burning fuel, so the air from the vents is warm rather than hot. If the house is reaching the temperature you set, the system is working.
Heavy ice that does not clear
A light frost that comes and goes is normal. Heavy ice on the outdoor unit that is still there after a defrost cycle is not. Keep leaves, plants and stored items at least two feet away from the unit, and call a technician if the ice stays.
Low refrigerant
If a heat pump is low on refrigerant, usually because of a leak, it cannot move enough heat into the house. Finding and fixing a refrigerant leak takes a licensed technician with the right equipment.
When to call us
Call us if the air is still cold after the five minute checks, if the furnace keeps shutting off with a clean filter, if the outdoor unit is iced over, or if you are not sure what you are looking at. Book a furnace repair or a heat pump repair visit.
The diagnostic visit is $189, and we waive it if you go ahead with the repair. We do not charge by the hour. Once we know what is wrong, you get a written upfront price before any work begins. Most of the time we can reach your home within a few hours. Our technicians are NATE certified and factory trained.
If the system is old and the repair is large, multiply the age of the system in years by the repair cost. If the answer is more than $5,000, replacement is usually the better use of the money. If you are replacing a furnace, it is worth pricing a heat pump too. See AC and heat pump installation or furnace replacement. For other warning signs, read our guide to signs you need heating system repair.
Frequently asked questions
Why is my furnace blowing cold air?
The most common causes are a thermostat fan set to On instead of Auto, a clogged air filter that makes the furnace overheat and shut off its burners, a failed flame sensor or igniter, or a gas supply problem. Check the thermostat and the filter first. If the air is still cold, the furnace needs a technician.
Is it normal for a heat pump to blow cold air in winter?
For short periods, yes. In cold, damp weather the heat pump goes into defrost mode to clear frost from the outdoor unit, and the air can feel cool for a few minutes. Lennox says a defrost cycle usually lasts 5 to 15 minutes. Cold air that lasts longer, or heavy ice that does not clear, needs a technician.
Should the thermostat fan be on Auto or On?
Auto, for most homes. With the fan set to On, the blower runs between heating cycles and pushes room temperature air through the vents, which feels cold.
What should I do if I smell gas near my furnace?
Do not troubleshoot. Leave the house, get everyone out, avoid anything that could make a spark, and call 911 and then PG&E from a safe location away from the house.
How much does it cost to have a technician find the problem?
The diagnostic visit is $189, and we waive it if you go ahead with the repair. Once we know what is wrong, you get a written upfront price before any work begins.
