Why your furnace or heat pump is blowing cold air, what to check in 5 minutes, and when to call a tech.
A furnaceblowing cold air almost always comes down to 1 of 7 things: the thermostatsetting, a dirty filter, lost power or gas, a failed igniter, a dirty flamesensor, a tripped safety switch, or, if you have a heat pump, a normal defrostcycle. You can check the first 3 yourself in about 5 minutes.
In Houston,furnaces sit idle most of the year. The first cold front is when worn ignitersand dirty flame sensors show up, all at once, across the whole city.
• Thermostat. Setit to Heat, and set the fan to Auto, not On. With the fan on On, the blowerruns between heating cycles and pushes room temperature air that feels cold onyour hand.
• Air filter. Aclogged filter chokes airflow, the furnace overheats, and a safety switch shutsthe burners off while the blower keeps running. That reads as cold air. Replacethe filter and give the system 15 minutes.
• Power and gas. Mostfurnaces have a switch near the cabinet that looks like a light switch. Makesure it is on. On a gas furnace, the gas valve handle should run parallel withthe pipe.
If you smell gas at any point, leave the home and call911 or your gas utility from outside.
• Failed igniter. Mostmodern furnaces light the burners with a hot surface igniter. When it cracks,the furnace tries a few times, then the control board locks it out. The blowermay still run, so you get cold air.
• Dirty flamesensor. The burners light, then shut off within a few seconds. Theflame sensor is a thin metal rod that proves the flame is there. A light coatof buildup is enough to make the furnace think it is not.
• Tripped safetyfloat switch. In many Houston attics, the AC coil sits right on topof the furnace and shares its drain. If that drain backs up, the float switchshuts the whole system down, heat included.
• Overheating thatkeeps coming back. If a new filter does not fix it, the cause isusually a blocked return, closed vents, or a blower motor losing strength. Thefurnace keeps tripping its high limit switch to protect itself.
If your homehas a heat pump, some cool air is normal. Heat pump air comes out warm, nothot, often in the 90s, which can feel cool against your hand compared with agas furnace.
On cold, dampmornings, the outdoor coil frosts over. The system reverses for a few minutesto melt it, and you may see steam rising off the outdoor system. That defrostusually lasts under 10 minutes. Many systems turn on backup heat during defrostto soften it.
Not normal:cold air for more than 15 minutes straight, an outdoor system frozen solid, oran outdoor fan that will not spin. Switch to Emergency Heat (EM Heat) on thethermostat to stay warm. It runs electric heat strips only, so it costs more torun, and it is meant for a day or 2 while you get the heat pump repaired.
Why does my furnace blow cold air for the first fewseconds? Most Houston ductwork runs through the attic. The airsitting in those ducts is cold on a winter morning, so the first puff is cool.It should turn warm within a minute.
Can I clean the flame sensor myself? It meansopening the burner compartment with the power off. If you are not comfortabledoing that, a tech handles it during the diagnostic.
Will maintenance prevent this? It catchesdirty filters and drain problems before the first cold front. Maintenance is$109 for the first system and $59 for each additional system.
Still blowing cold? Call 281-222-9591.
Phones answered 24/7. 2-hour arrival windows. $99 diagnostic.
Beede AC & Heating has repaired furnaces and heat pumps across West Houston since 2010.
© 2026 Beede AC & Heating LLC. All rights reserved.
Electrical services by Beede Electrical LLC, TECL 93748.
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