Components

Air handler

The indoor assembly containing the blower, the evaporator coil, the filter rack, and the drain pan. In a ducted system it moves conditioned air into the supply ducts and pulls room air back through the returns.

Also called AHU, fan coil unit, indoor unit

Direct answer

What is an air handler?

An air handler is the indoor half of a system: a cabinet holding the blower, the evaporator coil, the filter, and the drain pan, which pushes conditioned air into the ducts.

Air handler: how it works

One blower inside the cabinet draws room air through the returns and the filter, across the coil, and out into the supply ducts. Everything the system does to the air happens in this box, so the cabinet is also where filter changes, condensate drainage, and airflow measurement are carried out.

Where you meet the term

  • On a matched system rating, where the air handler model is half of the pair the published efficiency belongs to.
  • In an airflow measurement, where external static pressure is read across this cabinet.
  • In a water leak, where the drain pan and its trap sit inside or below the unit.

How to check it

  1. Find the filter rack and check the direction arrow. A filter fitted backwards restricts more than it should. Check the arrow against the direction the blower pulls, not against the shape of the rack. Owner check
  2. Look under the cabinet for staining. Rust marks or a water trail below the unit point at the drain pan or the trap rather than at the refrigerant circuit. Owner check
  3. Read external static pressure across the cabinet. A manometer with probes either side of the air handler shows what the duct system is asking of the blower. Technician
Read this twice

Air handler: what it is not

  • An air handler is not the same as a furnace. A furnace burns fuel and supplies the blower for a coil above it; an air handler contains its own blower and any heat is electric.
  • Fan coil unit and air handler are the same idea at different scales, which is worth knowing when reading commercial documentation.

Air handler questions

What is the difference between an air handler and a furnace?

A furnace generates heat by burning fuel and its blower also serves the cooling coil above it. An air handler contains the blower and coil in one cabinet and provides heat, if at all, electrically or from a heat pump.

Where is the filter in an air handler?

In a rack at the return side of the cabinet, or in a return grille elsewhere in the house. The manual for the unit shows the location and the size the rack takes.

Why does my air handler drip in the loft?

The drain pan under the coil collects condensate, and if the drain line, its trap, or the pump is obstructed, the water backs up. A secondary pan and float switch are fitted for exactly this reason.

Where this definition comes from

Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.

  • ASHRAE TerminologyASHRAE · standards body publicationIndustry reference definitions for heating, ventilation, air conditioning, and refrigeration vocabulary.