HPAC Magazine

What’s new in air handlers

The AHU has undergone many changes since gravity warm air furnaces were the norm.

February 1, 2016   By Ian McTeer

If you could be an HVAC system component for a day, which one would you be? Maybe an electronically controlled fan motor, since they are all the rage these days; or, I know: a thermostat! When a start-up thermostat company sells for $3.2 billion dollars, you have got to know where all the glamour is these days. I am betting one component you would not want to be is an air handling unit (AHU). Tucked away in attics, crawl spaces, closets, alcoves, tight mechanical rooms, roof tops and above dropped ceilings, this hard working machine is often the most neglected part of a system.

By the time I started working in the industry, many residential buildings had gravity (octopus) warm air furnaces. Some were still burning coal, but oil-fired versions were the most popular. The gravity furnace graduated into an AHU, however, these gigantic cast iron behemoths, along with their component large round ducts, took up too much space and were replaced by considerably smaller oil and gas furnaces.

Eventually, the concept of perimeter heating (supplying warm air to the perimeter of the room) took over from gravity systems. Original forced warm air heating system designs typically placed the air handler at, or near, the centre of the building. This approach allowed equal lengths of trunk duct running each way from the AHU.This minimized friction and dynamic losses caused by elbows, abrupt transitions and offsets.

By the late 1970s, basements had become “unfinished recreation rooms.” Builders and homebuyers wanted the AHU out of sight and out of mind. AHUs were often more window dressing than an efficient heating system. Invariably, these systems had too many changes in direction using elbows with square throats (and no turning vanes) or a very small throat radius.


Today, there is a cornucopia of air handlers available for residential and commercial applications. Gas and oil-fired AHUs can be part of a multi-zoned approach, but they suffer a bit in terms of flexibility. They must be close to an appropriate vent termination point and sometimes multi-poised furnaces are difficult to install and service.

The latest AHUs forming the indoor section of conventional heat pumps can be used almost anywhere in any configuration. The last AHU I worked with had a modular cabinet, metal skin with plastic interior, blower in the bottom part of the cabinet, aluminum coil, serial port ECM, electronic expansion valve, communicating controls, and could accept electric heating elements or a hydronic coil (as shown in Figure 1). It could be part of a single zone or multi zone system when combined with an outdoor unit having capacity control. Such units are, typically, very tall. Many have the blower in the upper portion of the cabinet requiring all return air through the bottom. In such cases, this kind of AHU should be installed horizontally which, in my view, limits its flexibility in some situations.


I am very impressed by the inverter drive, variable refrigerant flow heat pumps made by several manufacturers. Initially, the air handler options were self-contained “free delivery” wall mounted or ceiling cassettes that permitted multiple zoning capabilities. Some systems can heat and cool at the same time, all the while producing maximum heating performance to well below 0C. Now, ducted air handlers are available.

Anton Wolmarans, general manager of Multi HVAC Inc. explains, “their air handling units are specifically designed for use with the manufacturers’ single and multiple connection variable refrigerant flow (VRF) systems. Heat exchangers are formed and installed in a vertical ‘v’ shape to provide maximum surface area and efficiency while reducing total unit height. In addition to this, all fan coil units feature linear electronic expansion valves (LEV) with integrated control that provide accurate superheat or sub cooling control in case of heat pump operation.”

Wolmarans further explains that, “a unique feature is the addition of standard supply air sensors that adjust the LEV and compressor pumping frequencies in response to supply air conditions to prevent cold drafts in both cooling and heating modes, while thermo ON and OFF is controlled by either an internal return air sensor placed up stream of the unit’s fresh air intake or in the room thermostat. The DC blower static pressure can be adjusted electronically through the system controller to match design air flow with duct system pressure. Other features include blower wheels that are designed to provide quiet operation by offsets in the blade positioning on the wheels and internal condensate pumps to make disposal of condensate easy in any ceiling height application.” See Figure 2.


The SDHV air handler is part of a flexible system that can be used to heat and cool homes or commercial buildings. The air handler works in conjunction with one supply plenum (typically spiral duct) and supply tubing delivering up to 40 cfm per outlet. Six outlets per nominal ton will usually suffice. The SDVH air handler can be part of an all-electric heat pump system featuring an inverter drive outdoor unit or may be combined with a hot water coil fed by a wall-hung boiler.

There is a chilled water option available. Many SDHV air handlers have a factory-installed boiler relay, chiller relay, EAC/HRV control, humidifier compatibility, potable water circulation, EC motor, soft start and zoning capability. Figure 3  shows a unit with a blower coil, refrigerant coil and a hot water coil.

Small duct, high velocity system means a heating and cooling product that contains a blower and indoor coil combination that is designed for, and produces, at least 1.2 inches of external static pressure when operated at the certified air volume rate of 220-350 CFM per rated ton of cooling; and when applied in the field, uses high velocity room outlets generally greater than 1000 fpm, which have less than 6.0 sq. in. of free area.1 Currently, Unico and SpacePak manufacture air handlers capable of meeting the DOE definition.

SDHV air handlers can be installed just about anywhere eliminating the need for bulky ductwork. The flexible two-inch tubes are unobtrusive and produce a room air mixing effect called “aspiration” as the supply air is forced through the supply outlet diffuser.


Properly specified, installed (including commissioning) and maintained system are the key to energy efficiency. Unfortunately, many products promising flexible advantages are horribly abused in reality; none more so than those using flexible ductwork. These products have specific rules and can easily become non-PSIM in the hands of amateurs.

I will leave you with the words of Chandler von Schrader of the U.S. EPA as quoted in the New York Times recently: “Studies have shown that many HVAC contractors don’t follow industry standards when installing equipment. Proper equipment sizing, having matching equipment, ensuring proper air flow, and performing appropriate diagnostics are concepts every HVAC contractor should know and follow. For those contractors that are following their industry’s standards, they can be undercut by competition who take shortcuts.”

Ian McTeer is an HVAC consultant with 35 years experience in the industry. He was most recently a field rep for Trane Canada DSO. McTeer is a refrigeration mechanic and Class 1 Gas technician.

1Definition courtesy of US Department of Energy