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Why Is My Furnace Blowing Cold Air?

March 13, 2022

It’s a chilly night in Phoenix, and you switch on the heat, expecting some warm comfort. Instead, cold air is coming out of your vents. If your furnace is blowing cold air when the heat is on, there are four problem areas we can troubleshoot:

  • Your thermostat is on the wrong settings.
  • Your fan limit switch is on “manual override.”
  • Your air ducts are leaking.
  • Your furnace is overheating.

How do you know which issue is causing cold air to come out of your vents when you switch on the heat? Patrick Riley technicians will walk you through each diagnostic step so you can pinpoint why your vents are blowing cold air when the heat is on. We’ll also let you know when a professional is needed to solve the problem.

Live in the Phoenix area and need a technician now? We can send a Patrick Riley Heating technician over immediately.

Check Your Thermostat Settings

How you’ll know this is your issue:

  • Your thermostat is set to COOL
  • Your fan never stops running


Two thermostat issues that could cause your furnace to blow cold air include:

  1. Your thermostat is still set to COOL not HEAT

    It sounds silly, but many residents in Phoenix leave their thermostat set to COOL year round and forget about their HEAT button altogether. Homeowners may think the cold air blowing from their vents indicates a problem with their furnace, when really their HVAC systems are responding properly.
  2. Your thermostat fan setting is set to ON not AUTO

    Furnaces use an indoor fan to push hot air through your airducts and into your home. This fan has two modes of operation: ON and AUTO.

    If your furnace fan is set to ON, the fan runs continuously, regardless of whether your furnace is producing heat. Once a heating cycle ends and the furnace burners cool off, the fan continues to run. Because your furnace is no longer heating, the air being pushed into your home is cold.

What to do:

Check your thermostat is set to HEAT and your fan is set to AUTO.

thermostat set to heat and fan set to auto

Your fan limit switch is on “manual override”

How you’ll know this is your issue:

  • The air coming from your vents is first chilly, eventually turns warm, but then gets chilly again before the fan cuts off completely between heating cycles.


Your fan limit switch is located just under your furnace hood and is in charge of telling your furnace fan when to turn on and shut off. The fan limit switch allows the furnace burners to warm up completely before turning the fan on and should keep the fan running until the burners cool off to prevent damage or overheating. If you are experiencing cold air coming from your vents when the heat is switched on, your fan limit switch may be on the wrong setting or may have a shorted wire.

bad fan limit switch

What to do:

Read our post Why does my furnace keep running? to learn how to check the fan limit switch setting. If the fan limit switch is set to AUTO, a professional can inspect the fan limit switch for shorted wires.

Your air ducts are leaking

How you’ll know this is your issue:

  • Your heating and cooling systems struggle to reach your desired temperature.
  • You have higher-than-normal utility bills.
  • Certain rooms or areas are stuffy or don’t cool/heat properly.
  • Your home is excessively dusty.
  • You see ductwork with kinks, holes, or loose connections.


Leaks in your ductwork mean that cold, unconditioned air from inside your attic gets sucked into your ductwork. The cold attic air mixes with the warm air from your furnace, causing the air coming from your vents to feel considerably cooler than it should.

What to do:

Have a technician inspect your air ducts for leaks to recommend the best solution. A professional may opt to repair single leaks if the problem seems isolated. If leaks are extensive, consider sealing your ducts.

Your furnace is overheating

How you’ll know this is your issue:

  • Your furnace blows hot air, that air quickly turns cold, and then the furnace shuts off altogether.
  • This “hot then cold air” sequence happens over and over, and your home doesn’t warm up.


An overheated furnace means the heat exchanger in your unit gets too hot. As a built-in safety measure, your fan limit switch (discussed above) shuts off the burners but keeps the fan running until the heat exchanger cools down, thus blowing cold air even when the burners are not producing heat. Once the heat exchanger cools down, your furnace will try to warm your house once again, repeating this cycle as the heat exchanger continues to overheat.

What to do:

  1. Check your air filter. If the filter looks like the one on the right in the picture below, replace it. A clogged filter restricts the amount of cool air circulating through your furnace system and causes the unit to quickly overheat and shut off.

    check your air filter
  2. Check that all supply vents are open. Supply vents (pictured below) push warm air into your home. If you close too many, it increases pressure inside the duct system. This extra pressure strains the furnace blower, slowing it down. When the blower slows down, it struggles to push cool air over the heat exchanger, causing it to overheat.

    supply vents open

When do I need help from a Patrick Riley Heating Technician?

You checked your thermostat and fan limit switch settings, changed your air filter, and opened your supply vents. Still feeling cold air blowing from your vents when the heat is on? It’s time to get professional assistance.

Call a Patrick Riley Heating Technician to inspect the unit for more complex issues like leaking ducts or shorted wires.

Related reading:

Why choose Patrick Riley Heating Technicians?

Patrick Riley Heating service technicians pride themselves on being accurate and professional. We take the time to assess the issue and teach you how to troubleshoot before recommending replacement. Pricing will be discussed up front, and recommendations will be appropriate for your home. You expect warmth when you switch on the heat, and you expect honesty when you call for furnace repairs. We can help with the furnace and guarantee honesty.

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By Ty Lindsay

Ty Lindsay is the Director of Field Operations at Patrick Riley | Isley’s and a 15-year veteran of the plumbing and HVAC trades. In 2010, Ty earned his Journeyman’s plumbing license. He became a Master Plumber five years later and earned his Journeyman HVAC technician’s license that same year. Ty’s breadth of knowledge in plumbing and HVAC includes both residential and commercial work. He’s been a loyal member of the Patrick Riley | Isley’s team since 2016.