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.