Heater Blowing Cold Air? Furnace and Heat Pump Causes and Fixes

Check these five things first

A lot of "heater blowing cold air" calls turn out to be a setting or a filter. Check these before anything else:

• Thermostat mode: it should say Heat, not Cool or Off. Raise the setpoint a few degrees above room temperature.
• Fan setting: set the fan to Auto. On "On," the blower runs between heating cycles and pushes room-temperature air that feels cool.
• Batteries and screen: a blank or dim thermostat can stop calling for heat. See how to reset your thermostat.
• Air filter: a clogged filter can make a furnace overheat and shut its burners off while the fan keeps blowing.
• Power and gas: check the furnace switch (it looks like a light switch near the unit), the breaker, and that the gas valve to the furnace is open.

Why a furnace blows cold air

A gas furnace should blow clearly warm air within a minute or two of starting. If it does not, these are the usual causes:

• Normal warm-up: many furnaces run the burners briefly before the blower starts, and the first few seconds of air can be cool while the ducts warm up. Give it two or three minutes.
• Dirty flame sensor: the burners light, then shut off after a few seconds because the sensor cannot confirm the flame. The blower keeps running cool air. This is one of the most common furnace calls. Read what a flame sensor does.
• Overheating (high-limit switch): a clogged filter or closed vents trap heat, the safety switch shuts the burners off, and the fan keeps running to cool the furnace down.
• Ignition or pilot problems: a failed hot-surface igniter, or a pilot light that is out on an older furnace.
• Clogged condensate drain: high-efficiency furnaces have a drain line. If it backs up, a safety switch can stop the burners.
• Gas supply: a closed valve or a gas service problem.

Flame sensors, igniters, limit switches and gas parts should be tested by a licensed technician. See furnace repair in San Antonio.

Why a heat pump blows cold air

Many San Antonio homes heat with a heat pump instead of a furnace. Heat pumps behave differently, so some "cold air" is normal:

• Air that feels cooler than a furnace: heat pump air is warm but not hot. It can feel cool on a wet hand even when the house is warming. Check whether the room temperature is rising.
• Defrost mode: in cold, damp weather the outdoor coil frosts over. The heat pump briefly reverses to melt it, and you may see steam outside and feel cooler air inside for a few minutes. That is normal.
• Outdoor unit not running: if the indoor fan runs but the outdoor unit is silent, a capacitor, contactor or fan motor may have failed.
• Reversing valve or thermostat setup: if the system blows cold air steadily in Heat mode, the valve that switches between heating and cooling may be stuck, or the thermostat may be set up for the wrong type of system.
• Low refrigerant: a leak reduces heating just as it reduces cooling.

If the EM or AUX light is on, read EM heat vs. heat. For a heat pump that will not warm the house, see heat pump repair.

When the ducts are the problem

If warm air comes out of some vents but others blow cool, look at the ductwork. A duct that has come loose or torn in a hot or cold attic can dump heated air into the attic and pull attic air in. Rooms at the end of long duct runs feel it first. See duct repair and sealing.

When to stop and call

• You smell gas: leave the house and call your gas company from outside. Do not flip switches.
• A carbon monoxide alarm sounds: get everyone out and call 911.
• A breaker trips again after one reset: leave it off.
• The burners light and shut off over and over, or the furnace makes banging, squealing or grinding noises.
• The settings and filter are fine and it still blows cold air.

Cowboy's answers 24/7 with our own dispatch on Blanco Road. See heating repair in San Antonio or call (210) 495-7771.

Frequently asked questions

Why is my heater blowing cold air?

Start with the thermostat: it should be on Heat with the fan on Auto. Then check the filter, the furnace switch and breaker. If those are fine, a furnace may have a dirty flame sensor, an overheating shutdown or an ignition problem, and a heat pump may be in defrost or have an outdoor-unit problem.

Why does my furnace blow cold air and then warm air?

A short burst of cool air at start-up is normal while the ducts warm up. If the air goes warm and then turns cold again within minutes, the burners are likely shutting off early, often from a dirty flame sensor or overheating.

Is it normal for a heat pump to blow cold air in heat mode?

For a few minutes during defrost, yes. Heat pump air also feels cooler than furnace air. Steady cold air in Heat mode is not normal and should be checked.

Can a dirty air filter make my heater blow cold air?

Yes. A clogged filter can make a furnace overheat, which shuts the burners off while the blower keeps running. Replace the filter and give the system time to reset.

Should I turn off my heater if it is blowing cold air?

If you smell gas, the furnace is short cycling, or it makes loud noises, turn it off and call. Otherwise, check the thermostat and filter first.

Related service: heating repair in San Antonio · furnace repair · heat pump repair. Cowboy's AC answers 24/7 at (210) 495-7771.