An air handler — also called an air handling unit (AHU) — is the indoor component of a split HVAC system that circulates conditioned air throughout your home. It houses the blower motor, evaporator coil, filter, and in electric systems, heating elements. The air handler works in tandem with an outdoor unit (either a central air conditioner or heat pump) to deliver cooled or heated air through your duct system. Without the air handler, the refrigerant cycle in the outdoor unit has nowhere to transfer its energy inside the home.

The Components Inside an Air Handler

Each component inside an air handler has a specific role in the conditioning process:

  • Blower motor and fan: The heart of the air handler — this is what moves air through the entire system. Newer air handlers use variable-speed ECM (electronically commutated motor) blowers that adjust airflow automatically, improving comfort and efficiency over single-speed motors.
  • Evaporator coil: A refrigerant-filled heat exchanger. During cooling, refrigerant in the coil absorbs heat from indoor air, cooling it. During heating with a heat pump, the cycle reverses. The coil is also where moisture condenses out of the air (dehumidification) during cooling season.
  • Air filter: Positioned in the return air stream before the coil to protect internal components from dust buildup. A clogged filter is the single most common cause of air handler problems.
  • Electric heating elements (in all-electric or heat pump systems): Resistance heat strips that provide supplemental or backup heat when the outdoor heat pump can’t deliver sufficient capacity — relevant in Colorado winters when outdoor temperatures drop significantly.
  • Condensate drain pan and drain line: Collects and removes the water that condenses on the evaporator coil. A blocked drain line is a common service issue — water backs up into the pan and can overflow into the utility space.

Air Handler vs. Furnace: What’s the Difference?

In a gas heating system, the furnace serves as the air-moving component — it has its own heat exchanger, burner, and blower. An air handler is used instead of a furnace in all-electric homes or in homes with heat pumps as the primary heat source. The key distinction: a furnace generates heat through combustion; an air handler does not generate heat itself (it either transfers heat via refrigerant through a coil, or applies electric resistance heat). If you have a gas furnace with an added central AC coil on top, that coil is often called a “cased coil” or “A-coil” — not technically a full air handler, since the furnace is doing the blowing.

Air Handlers and Colorado’s Climate

In Denver-area homes, air handlers are most commonly paired with heat pumps, which are increasingly popular as natural gas costs have risen and heat pump technology has improved. Modern cold-climate heat pumps maintain effective heating down to 0°F and below — well within the range of what Denver winters regularly deliver. At 5,280 feet altitude, refrigerant operating pressures differ from sea-level specifications, which is why it’s important to have any heat pump or air handler serviced by technicians familiar with high-altitude equipment calibration. Altitude affects both refrigerant charge and system performance in ways that matter at tune-up time.

Common Air Handler Problems

The most frequent air handler issues Done technicians encounter include clogged filters restricting airflow and causing the evaporator coil to freeze, blocked or overflowing condensate drain lines, failing blower motors (often preceded by humming or grinding noises), dirty evaporator coils reducing heat transfer efficiency, and refrigerant leaks at the coil connections. Most of these issues are detectable during a routine tune-up before they cause a complete system shutdown.

When to Call a Professional

Filter changes are the only air handler maintenance a homeowner should do themselves. Everything else — coil cleaning, refrigerant handling, electrical component testing, drain line clearing — requires a licensed HVAC technician. Refrigerant work specifically requires EPA 608 certification by federal law. If your system is underperforming, making unusual noises, or showing ice on the coil, schedule a service call before operating the system further.

Done services air handlers across the Denver metro as part of routine maintenance and repair. Learn more at AC maintenance, heat pump services, or AC repair if something isn’t right.