Why Is My Furnace Blowing Cold Air?

1. Check the thermostat fan setting

If the fan is set to On instead of Auto, the blower runs all the time, including between heating cycles when the burners are off. That pushes room-temperature air through the vents, and it feels cold on your hand. Set the fan to Auto and the system to Heat, a few degrees above the room temperature.

Air can also feel cool for a short time at the start and end of a cycle. Many furnaces wait for the heat exchanger to warm up before the blower starts, and keep the blower running for a bit after the burners shut off to pull out the last of the heat. Cool air for a minute is normal. Cool air for the whole cycle is not.

2. Check the filter

A badly clogged filter starves the furnace of air. The heat exchanger overheats, a safety switch called the high limit shuts the burners off, and the blower keeps running to cool things down. The result is cool air, often with the furnace starting and stopping over and over.

Pull the filter and hold it up to a light. If you can't see light through it, replace it with the same size, which is printed on the frame. Then give the furnace a few minutes. If the heat comes back, the filter was likely the cause.

3. The burners aren't lighting or won't stay lit

If the blower runs but the burners never light, or they light and go out after a few seconds, the furnace isn't making heat. The usual causes:

• A dirty flame sensor. It has to prove there's a flame within a few seconds or the furnace shuts the gas off. A coated sensor is one of the most common winter repairs.
• A failed ignitor. Most modern furnaces use a hot surface ignitor that glows to light the gas. They crack and wear out.
• No gas to the furnace. Check that the gas valve near the furnace is open (handle in line with the pipe) and that other gas appliances work.
• A lockout. After several failed tries, most furnaces lock out for a while. Write down the blinking light pattern you can see through the small window in the furnace door. It helps us diagnose it.

4. High-efficiency furnaces: drain and vent pipes

If your furnace vents through two white plastic pipes, it's a high-efficiency (condensing) furnace. It makes water as it runs and drains it away. If the drain or trap clogs, a pressure switch stops the furnace from lighting, and you may get cold air or no heat at all. See Furnace leaking water for more.

Those same pipes outside have to be clear. Snow drifts, ice, leaves and even bird nests can block them. Clear anything you can reach from the outside without taking anything apart.

5. If you have a heat pump, it may be normal

Heat pumps blow warm air, but it isn't as hot as a gas furnace's, so it can feel cool on your hand. During a defrost cycle the air can feel cooler for a few minutes too. That's normal. Our guides on heat pump aux heat and heat pump not heating explain what to expect and what isn't normal.

6. Leaky or disconnected ducts

If most rooms are warm but one or two get cool air, the problem may be in the ductwork, not the furnace. A duct that has come loose in the attic or crawlspace dumps heat where you don't need it, and the room at the end of that run gets air that has cooled off on the way.

When to stop and call

Call for service if the filter is clean, the settings are right and the air is still cold, or if you notice any of these:

• The furnace locks out again after you reset it once.
• A burning, electrical or hot-plastic smell. Turn the furnace off at the thermostat.
• A bang when the burners light.
• Any smell of gas. Leave the house first and call your gas utility or 911 from outside.

How HVAC Doctor can help

Call or text (859) 684-3259. One of the owners, Ahmad or Ammar, answers 24/7, and nights, weekends and holidays cost the same as a weekday. Mention our online special and the diagnostic is $39 (regularly $79), credited to the repair if you go ahead, and you get a firm price before any work starts. Most Lexington calls are same-day.

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