Basics of unit heaters
| Plant Engineering – January 2001
Reference file : Basics of unit heaters
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.
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.
Locating unit heaters