Your furnace is running, but the air coming out of the vents is cold. That combination narrows the problem down quickly — here is what it points to.
When a furnace runs but blows cold air, the burners have stopped firing while the blower keeps turning. SAVA HVAC & Appliance Repair has diagnosed this fault across Chicago and the North Shore since 2017, and it almost always traces to one of five things: a tripped high-limit switch caused by restricted airflow, a dirty or failed flame sensor, an igniter that has stopped glowing, an interrupted gas supply, or a thermostat fan left on "On" instead of "Auto." The distinction matters, because two of those are deliberate safety shutdowns — the furnace is protecting itself, and forcing it to keep running risks cracking the heat exchanger. A furnace that fails ignition repeatedly will eventually lock itself out completely and stop responding to the thermostat at all. SAVA is EPA Section 608 Certified, family-owned, and rated 4.9 stars across 150+ reviews.
This is the most common cause by a wide margin. When airflow across the heat exchanger is restricted, heat builds up inside the cabinet and the high-limit switch shuts the burners down to prevent a fire. The blower keeps running to carry that heat away — which is exactly why you feel cold air. The switch is doing its job; the restriction behind it is the actual fault, and it needs to be found and cleared.
The flame sensor confirms to the control board that a flame actually ignited. As it accumulates residue it stops conducting properly, the board assumes ignition failed, and it closes the gas valve within seconds as a safety measure. You will often see the burners light and then go straight back out, with the blower continuing afterward.
A hot-surface igniter glows to light the gas. When it cracks or burns out, the sequence starts — you hear the inducer motor and the click of the gas valve — but nothing ignites. Igniters are a wear part with a finite lifespan, and they typically fail at the start of the heating season when the furnace is asked to run for the first time in months.
If the gas valve has failed, the supply line has been shut off, or pressure is inadequate, the furnace goes through its full ignition sequence with nothing to burn. This also happens after utility work in the neighbourhood.
The simplest explanation of all: with the fan set to “On” rather than “Auto,” the blower runs continuously, including between heating cycles. Air keeps moving through the vents when the burners are idle, and it feels cold. If the air is only cold some of the time, this is worth ruling out first.
Less common, and the most serious. A crack can disrupt the burner flame enough to trigger repeated safety shutdowns. A cracked heat exchanger can also let combustion gases, including carbon monoxide, into the air your home breathes. Any furnace suspected of this needs to be shut down and inspected before it runs again.
The more you can describe, the better the chance we arrive with the right part already on the truck. It helps to know:
Heating faults rarely improve on their own, and in a Chicago winter a furnace that is struggling in November is a furnace that fails in January. If the house is not holding temperature, if a safety device keeps shutting the system down, or if anything about the sound or smell has changed, have it diagnosed rather than waited out.
SAVA is a family-owned, EPA Section 608 Certified HVAC company serving Chicago and the North Shore suburbs since 2017. We identify the exact fault before any work begins, explain what we found and what it will cost, carry common parts for same-day repairs when available, and give straight repair-versus-replace guidance. Learn more about our furnace repair service or emergency furnace repair.
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