The heat is on, the fan is running, and what's coming out of the vents is cold. Like the cooling version of this problem, the fact that air is moving at all rules out a lot — the blower is fine, so the issue is on the heating side.
This is the single most common cause and it isn't a fault at all. With the fan switch set to ON instead of AUTO, the blower runs continuously — including during the long stretches between burner cycles. You feel room-temperature air and assume the furnace has failed.
Switch the fan to AUTO. The blower will then only run when there's actually heat to move. If the air turns warm within a few minutes, that was it.
A clogged filter doesn't just reduce airflow — on a furnace it can shut the heat down entirely. When too little air moves across the heat exchanger, the exchanger overheats and a safety device called the high-limit switch cuts the burners while leaving the blower running.
The result is a furnace blowing cold air on and off in a cycle. That safety is doing exactly what it should: an overheated heat exchanger is how cracks form, and a cracked heat exchanger is a carbon monoxide problem, not just a repair bill. Replace the filter and see whether the pattern stops.
On most modern furnaces the burners light, run for a few seconds, then shut off, and the blower keeps going. That pattern almost always points at a dirty flame sensor. The sensor's job is to confirm there's actually a flame; when it's coated, it can't confirm, and the furnace shuts the gas off as a safety measure.
It's a small part, it's usually cleaned rather than replaced, and it's one of the more common heating calls we run. On an older furnace with a standing pilot, a pilot that won't stay lit points at a thermocouple doing the same job.
Either way, that's where the DIY ends. Anything past the filter and the thermostat means opening a gas appliance, and that isn't the place to learn.
If you smell gas, leave the house first and call the gas company from outside. Not us, not later — that call comes before anything else.
A furnace that repeatedly trips its limit switch, a burner flame that's yellow rather than blue, soot around the unit, or a carbon monoxide alarm all mean stop using it and get somebody out. Those aren't comfort problems.
We're open 24 hours and no-heat calls get priority. Tucson nights get cold enough that a house without heat is a real problem, even if the days don't feel like it.
A dirty flame sensor is one of the most common heating repairs and one of the cheapest. Call or text — we run 24/7.
