Basics of unit heaters

Here's a primer on self-contained heating devices fueled by gas, oil, electricity, steam or hot water. Key topics discussed include energy source, delivery patterns, and placement.

By Plant Engineering Staff July 6, 2001

Plant Engineering – January 2001


Reference file : Basics of unit heaters

Sidebar 1: Air delivery patterns – vertical delivery unit heaters

Sidebar 2: Air delivery patterns – horizontal delivery unit heaters

Sidebar 3: Locating unit heaters

Sidebar 4: Quick comfort heating estimation chart

Sidebar 5: Suggested piping arrangements Joseph L. Foszcz, PE/CPE, Senior Editor, Plant Engineering Magazine

Key concepts

Energy sources

Delivery patterns


These self-contained heating devices are fueled by gas, oil, electricity, steam, or hot water ( Fig. 1 ). Each unit has a heat exchanger and air-moving fan or blower to deliver heated air to a space. They are individually suspended over the area to be heated.

Unit heaters are usually spaced evenly to give good air coverage with their air patterns complimenting each other. They can be equipped with turning vanes, which direct the discharge air in a desired path, primarily toward the floor.

A circular air pattern is the most effective arrangement. Heat is delivered along the walls where radiation cooling and infiltration occur, resulting in a greater level of comfort throughout a plant. When units are located in the center of a building, radiation cooling and infiltration are much more noticeable.

For larger spaces, two or more unit heaters should be installed, depending on heating requirements. If one unit is temporarily inoperative, there is adequate heat capacity for most applications to still use the space.

Unit heaters are usually controlled by individual, wall-mounted thermostats. Central control panel zone systems can also be used. The advantage of individual controls is that heaters can be individually operated to suit space heating requirements.

If circumstances of building use do not permit perimeter installation of unit heaters, random location may be necessary. When installed in this manner, unit heaters may not attain optimum air throw patterns and it may be necessary to consider infrared heating.

Gas and electrically-fueled units can provide infrared heat. This form of heating makes the floor and other permanent objects in the building heat sinks. They re-radiate or convect the heat, providing additional sources of heat distribution. This action creates a very comfortable and exceptionally dry environment without heating the air in the space.

When installing infrared units, be certain the infrared rays are not directed against outside walls. This placement results in excessive fuel costs, since the heat radiated into the wall is lost to the outdoors.

Plant Engineering magazine extends its appreciation to Chromalox, McQuay Intl., and Reznor for the use of their material in the preparation of this article.

Sidebar 1: Air delivery patterns – vertical delivery unit heaters

Due to their downward air discharge, vertical units ( see illustration ) are particularly desirable for heating areas with high ceilings and where craneways and other obstructions dictate high mounting of heating equipment. Unit heaters can provide distinctively different patterns to meet specific heat-throw and heat-spread requirements.


2: Air delivery patterns – horizontal delivery unit heaters

These units are widely used for general air heating ( see illustration ). Horizontally positioned louvers can be adjusted to lengthen or shorten heat throw and/or decrease or increase the mounting height. Adjustable vertical louvers, when used in combination with horizontal louvers, permit complete directional control of heated air.

H M Max. mounting height, ftT Heat throw @ H M’ ftH L Lower mounting height, ftT L Heat throw @ H L , ft










Locating unit heaters

Use as few units as possible to give proper heat distribution and coverage to the area. The number depends on the heat throw of the individual units. Arrange the units to minimize piping, wiring, and installation costs.

More than any other factor, improper mounting height is the cause of most unsatisfactory unit heater installations. When mounted higher than recommended, improper heat distribution results, and comfort is reduced.

Horizontal delivery units should be located so the airstream wipes the exposed walls of the building with parallel or angular flow, without blowing directly on the walls. Heaters should be spaced so each one supports the airstream from another. This arrangement sets up a circulatory air movement, producing a blanket of warm air along a cold wall.

Locate units so their airstreams have minimum interference from machinery, partitions, columns, and other obstacles.

Unit heaters installed in a building exposed to a prevailing wind should be located to direct a large portion of heated air along the windward side of the building.

Large expanses of glass or large doors frequently opened should be blanketed by long-throw unit heaters.

In buildings with high ceilings, vertical delivery unit heaters equipped with proper air distribution devices are recommended to produce comfort in central areas of the space to be heated. Horizontal units are used for heating peripheral areas in these buildings.

Horizontal units should not blow directly at workers. Airstreams should be directed down aisles, into open spaces on the floor, or along exterior walls.

When only vertical delivery units are used, they should be located to blanket exposed walls.

Several unit heaters can be operated by a single thermostat. In large, open spaces, where various activities take place, zonal heating, using individual thermostats, improve comfort and reduce fuel costs.

Unit heater fans may be operated during warm weather to maintain air circulation. Equip units with fan switches for this purpose.

A number of location patterns are available. To view a variety of suggested locations, click on the types below:

Monitor-type building

Mill-type building

Warehouse building/narrow building

Sidebar 4: Quick comfort heating estimation chart

Sidebar 5:Suggested piping arrangements

Hot water systems — Horizontal heater

Hot water systems — Vertical heater

Steam systems — High-pressure steam

Steam systems — Low-pressure steam

Plant Engineering