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Suspended Unit Heaters

This type of unit is a forced warm air furnace that normally hangs or is "suspended" from the ceiling joists in a factory or warehouse. The sole purpose of this style of unit is to heat a space to a preset temperature controlled by a thermostat. These units come in all sizes depending upon the amount of heat required and use a propeller style fan blade to circulate the air within the space. Unit heaters (U/H) are from 70% efficient for older units to 82% efficient for newer models. They have a chimney to vent the products of combustion to the atmosphere. There are specific regulations governing the application and use of this style of heater under the B-149A gas code book, but it has been the most common form of heating factories and warehouses for years. If there was a selling feature of U/H's it would be that they were and are relatively inexpensive to install or replace. The drawbacks to using U/H's are as follows.

  • The units are not as efficient as other equipment on the market
  • The fan generates a considerable amount of noise
  • The fan uses more hydro than alternate systems of heating
  • The fan circulates whatever dust and dirt is in the air
  • This style of unit takes up a considerable amount of ceiling space
  • Can be easily struck by fork lift trucks and damaged
  • Exhausts air through the chimneys even when not in use
  • Require more maintenance than alternate means of heating
  • Most use an open flame that means they cannot be used in some applications
  • Some have a closed flame or sealed combustion at a hefty premium cost
  • Heats only air so if a shipping door opens all the heat escapes and must be reheated
  • Cost of repair parts is higher than alternate forms of heating
  • Because hot air rises all the heat is at the ceiling versus where it is needed at the floor
The above may make the writer appear biased against unit heaters, but these are the facts and I do not understand why engineers are still specifying this form of heat in new buildings. For property investors, this style of unit limits the tenants that are suitable as renters of the space. Open flame unit heaters cannot be used in applications where there is sawdust, flour dust (most air borne dust can be explosive in certain concentrations) or volatile vapors in the air. This style of heater is also subject to failure of the heat exchanger in wet or corrosive atmospheres and flame failure in negative pressure conditions.

There is two types of heat exchangers available in this style of heater. The original design was called a "clam shell" also commonly used in residential furnaces that looks similar in shape to an inflated letter envelope. The other style that is of newer design is a tubular design made out of similar tubing to an automobile exhaust pipe. The clam shell style was subject to severe, thermal metal expansion and contraction resulting in "metal fatigue" and subsequent failure of the exchanger.

The standard warranty on the heat exchanger of a unit heater is 10 years. Some lower quality manufacturers may only have a 5 year warranty. The warranty provides only the new heat exchanger after the one year labor warranty expires and is void if the exchanger is faulty due to corrosion or rust. This type of failure results from chemicals or vapors in the air that are beyond the control of the manufacturer.


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