No heat, weak heat, or a furnace that lights and shuts straight back down. Tell us what it is doing and we will get a tech out.
Serving Phoenix, AZ
# ROC 102297
Rated by BBB
and after hours availability
CMI-AIR repairs furnaces, heat pumps and packaged units across Phoenix and the surrounding Valley.
Almost every no-heat call we run in Phoenix has the same shape. The furnace worked fine in February. It sat untouched through a summer of the blower pushing desert dust across it. Then the first cold night in December arrives, the thermostat calls for heat for the first time in ten months, and nothing happens. That pattern is why the parts we replace most often here are not the expensive ones. They are the small ones that corrode and foul while the system is idle.
Call (602) 437-0066 or use the form. If the overnight low is [below 40] and you have no heat at all, say so, because that changes how the call gets prioritized.
Match what your system is doing to what it usually means. A starting point, not a diagnosis.
Cold air from the vents when the heat is calling usually means the burners are not lighting, or they lit and the flame sensor failed to confirm it so the control board shut the gas off. It can also be the blower running through the fan-off delay, which is normal for a minute or two, or a thermostat set to fan rather than heat. If the air stays cold beyond a couple of minutes, the burner side needs looking at.
A furnace that clicks and tries but never lights is usually dealing with a failed hot surface igniter, a dirty or corroded flame sensor, a gas valve problem, or a blocked flue triggering the pressure switch. In Phoenix the igniter and the flame sensor account for most of these, because ten idle months in a dusty cabinet is hard on both.
This is the flame sensor almost every time. The burner lights, the sensor cannot confirm the flame, and the board closes the gas valve as a safety measure. It retries a few times and then locks out. It is one of the cheapest and most common heating repairs there is, and it is not something to keep resetting your way through.
If nothing happens when the thermostat calls for heat, work outward from the simple things: the thermostat batteries and settings, the furnace power switch, the breaker, and the service door safety switch if a panel was recently removed. If all of those check out, it is usually the control board, the transformer, or the thermostat wiring.
If the burners fire but no air moves, the blower motor, the run capacitor or the blower control has failed. This one matters more than it sounds, because a furnace producing heat with no airflow to carry it away will trip its high-limit switch and shut down, and repeated overheating cycles are hard on the heat exchanger.
If the furnace is running properly and parts of the house still will not warm up, the problem is usually distribution rather than the equipment. Leaky or disconnected ducts in an attic, no return path in that part of the house, or a system that was never balanced. Same causes as uneven cooling in summer, same fix.
A dusty smell on the first run of the season is normal and should clear within an hour. It is dust burning off the heat exchanger after months of sitting. What is not normal is a sharp or chemical smell, a smell of gas, or a burning smell that persists past the first hour. Shut the system off and call.
A furnace that starts and stops repeatedly is often overheating and tripping its limit switch, usually because airflow is restricted by a clogged filter or a blocked return. A furnace that runs constantly without reaching temperature is either undersized, losing heat through ducts, or has a burner problem. Both are worth diagnosing rather than living with, because the short cycling case is hard on the heat exchanger.
Stop and call on this one. A gas burner should burn blue. A yellow or orange flame, soot marks, or a sooty smell can indicate incomplete combustion, and incomplete combustion produces carbon monoxide. This is a safety call, not a comfort call.
If you have a gas furnace, read this part.
A cracked heat exchanger can let combustion gases into the air your blower circulates through the house. You cannot see it from outside the cabinet and it does not announce itself. We test for it on every heating repair and maintenance visit.
If a carbon monoxide detector sounds, leave the house and call the gas company or 911 before you call us. If you do not have CO detectors, put one on every level of the home, and replace them on the manufacturer’s schedule, because the sensors expire even when the unit still powers on.
Symptoms worth taking seriously: headaches, nausea or dizziness that appear when the heat is running and clear when it is off, especially if more than one person in the house has them.
The tech runs the system through a full heat cycle and watches what fails, rather than guessing from the symptom. Gas pressure, ignition sequence, flame signal, temperature rise across the heat exchanger, blower amp draw, and the safety controls.
On any gas system this happens whether or not it relates to your complaint. Heat exchanger inspection, flue draft, and combustion. If we find a safety problem we will tell you plainly and we will not put the system back into service without saying so.
The failed part and the readings, not a verdict.
Every option that solves the problem, with the price on each, including the cheap one. Nothing happens until you approve it.
Most heating repairs finish on the first visit. Common failure parts ride on the truck. Afterward the tech re-runs a full cycle and confirms ignition, temperature rise and safe shutdown.
[CMI-AIR offers emergency heating repair in Phoenix at [hours]. [After-hours calls are/are not charged at a premium rate.]]
Phoenix winters are mild, so a night without heat is uncomfortable rather than dangerous for most households. It is not mild for everyone though. If there are infants, elderly residents or anyone with a medical condition in the house, say that when you call.
If you smell gas, do not wait for us. Leave the house and call the gas company from outside.
Four things worth checking, because any of them can turn out to be the whole problem.
Do not keep cycling a furnace that lights and shuts down repeatedly. Each attempt sends gas into the burner before it shuts off, and the system locks out for a reason.
Either the burners are not lighting, or they lit and the flame sensor did not confirm it so the gas shut off. Less commonly it is a thermostat set to fan instead of heat, or you are hearing the normal fan-off delay at the end of a cycle. If cold air keeps coming while the thermostat is calling for heat, it is the burner side and it needs a tech.
Flame sensors and igniters. Both are inexpensive, both foul or corrode during the ten months the system sits idle here, and both prevent the furnace from proving the burner lit. They account for a large share of the no-heat calls we run each December.
The flame sensor. The burner lights, the sensor fails to confirm the flame, and the control board closes the gas valve as designed. It retries a few times, then locks out. Cheap part, common failure, quick repair.
Once, to see whether it was a one-off. Not repeatedly. A furnace that locks out is doing it deliberately, and forcing repeated ignition attempts on a system with a gas or airflow fault is how a small repair becomes a bigger one.
Most finish in one visit, usually inside two hours including diagnosis. Blower motors, control boards and heat exchangers take longer and sometimes need a part ordered.
Yes. Heat pumps, gas furnaces and packaged units, all major residential brands, regardless of who installed the system. Heat pumps running in heating mode have their own failure pattern, mostly reversing valves and defrost controls, and they need a tech who works on them regularly.
Usually yes, and more often than with an air conditioner. A Phoenix furnace runs so few hours that it commonly reaches [15] to [20] years in serviceable condition, so a single failed component on a fifteen-year-old furnace is often worth fixing. The exception is a cracked heat exchanger, which is a safety issue and a replacement conversation rather than a repair.
Not usually. It is dust burning off the heat exchanger after months of sitting idle, and it should clear within an hour. A sharp, chemical or gas smell is different, and so is a burning smell that does not go away. Shut the system off and call for those.
CMI-AIR is the residential division of Consolidated Mechanical, a local, family-owned commercial air conditioning company with more than 30 years of experience.
Copyright © 2026 CMI-AIR Inc. All Rights Reserved. Designed by Verge Technologies | Marketing by Jives Media
