When the indoor air handler does nothing, the house gets no heating or cooling at all, even if the outdoor unit is fine. The air handler is the blower, the coil and the controls, usually in a closet, garage or attic. Most of the time the cause is one of a short list, and several are easy to check.
Check these yourself
- The thermostat. Is it on, set to a mode, and calling for heat or cooling? Try setting the fan to ON. If the blower starts on fan-only but not in heat or cool, the problem is in the call from the thermostat, not the blower.
- The breaker. The air handler has its own breaker, often a double pole breaker if it has electric heat strips. Reset once. If it trips again, stop.
- The service switch. A light switch style switch on or near the unit. It is easy to flip by accident.
- The door switch. A safety switch behind the access panel cuts power if the panel is not seated. After a filter change, press the panel firmly into place.
- The float switch. A switch in the drain pan or on the drain line shuts the unit off when water backs up. Standing water in the pan means the drain is clogged. See air handler leaking water.
- The filter. A severely clogged filter will not usually stop the blower from starting, but it can trip limits and cause shutdowns.
What it usually is when those checks pass
- A blown 3 amp fuse on the control board. Usually caused by a short in the low voltage wiring. See an air handler blowing its fuse.
- A failed blower capacitor on units with a PSC motor. The motor hums but does not spin.
- A failed blower motor. Common on older units, and variable speed ECM motors can fail in their electronic module.
- A failed transformer, which removes the 24 volt power the whole control circuit needs.
- A failed control board or relay.
- Damaged thermostat wiring, often from rodents in attics.
Clues that point to the cause
- Board LED off entirely: no power to the board. Check the breaker, switch, door switch, fuse and transformer.
- Board LED blinking a code: the board is telling you something. Look for the code chart inside the door. See how to read blinking light codes.
- Humming but no air: a capacitor or seized motor. Turn it off.
- Outdoor unit running, indoor silent: the outdoor unit gets its signal, but the blower circuit has failed. Turn the system off, because an outdoor unit running without indoor airflow can freeze the coil.
Attic air handlers
In many homes in Natomas, Elk Grove and newer Roseville, the air handler sits in the attic. Be careful going up: stay on the framing, bring a light, and do not work on anything live. If the secondary drain pan under the unit is full, the primary drain has failed and water is close to your ceiling.
What we do
We trace power from the breaker through the switch, door switch, transformer, fuse and board to the blower, and test the motor and capacitor. That finds the failed part rather than replacing things in sequence. Our diagnostic is a flat $69 and it is credited toward the repair. If replacement is on the table, air handler replacement cost covers the numbers. See also heat pump not turning on.
Frequently asked questions
Why is my air handler not turning on?
The most common causes are a tripped breaker, a service switch turned off, the door safety switch not engaged, a tripped float switch from a clogged drain, a blown 3 amp fuse, or a failed blower motor or capacitor.
Why does my air handler hum but not blow air?
Usually a failed blower capacitor or a seized motor. Turn the system off to avoid overheating the motor.
Can a clogged drain stop my air handler?
Yes. Most air handlers have a float switch that shuts the unit off when water backs up in the drain pan or line.
My outdoor unit runs but the indoor air handler does not. Is that bad?
Yes. Without indoor airflow the coil can freeze. Turn the system off and have the blower circuit checked.
Does my air handler have its own breaker?
Yes, usually separate from the outdoor unit, and often a double pole breaker if it includes electric heat strips.
What does the light on my air handler board mean?
A steady or slow blink often means normal operation. A counted flash pattern is a fault code. Check the chart inside the access door.