This post overlaps with a fuller one: Air Handlers Explained: What They Do and Where Yours Is.
An air handler is the indoor cabinet of a split system: a blower, a coil, and a filter rack that pull house air across the coil and push it into the ducts. If your Pasadena, CA home has a heat pump, or an air conditioner with no gas furnace, that cabinet is an air handler. If you have a gas furnace, the furnace does the same job with the AC coil sitting on top.
Air handler versus furnace
A furnace has burners and a heat exchanger and makes heat by burning gas. An air handler has no burners; on a heat pump its coil heats in winter and cools in summer, and some models carry electric strips as backup heat. Most 1950s-to-80s ranch homes in the San Gabriel Valley have a gas furnace in a hall closet, garage, or attic. Air handlers show up in all-electric homes, heat pump conversions, and ADUs. A rooftop package unit puts everything, blower included, in one outdoor cabinet.
What is inside and where it lives
Inside the cabinet: the blower, the coil, the filter slot, a drain pan with a condensate line, an optional set of electric heat strips, and the control board. Attic units get a secondary pan with a float switch. You will find the unit in a louvered closet, the attic, the garage, or, in some older Pasadena homes, the crawlspace. Follow the largest duct or the return grille.
What to check and when to call
Change the filter every one to three months in cooling season, watch that the condensate line drips outside when the AC runs, keep the closet door clear, and listen for rattles or a hum with no airflow. Leave the cabinet closed; the capacitor inside holds a charge. Call a licensed tech for weak airflow with a clean filter, ice on the coil, water in the pan, a blower that runs without heating or cooling, or a musty smell that points to a coil cleaning. When the outdoor unit is replaced, the indoor unit must match it, and California's energy code calls for a duct leakage test on an AC installation that replaces the air handler, condenser, or coil, unless the ducts were already sealed and documented, run under 40 linear feet, or contain asbestos.
Our diagnostic visit is $69, told to you before booking and credited toward the work. Call Boost Services Heating and Cooling at 818-277-5056 or contact us here.
The full guide: Air Handlers Explained: What They Do and Where Yours Is
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.