29
C
IRCULATING
A
IR
& F
ILTERS
D
UCT
WORK
- A
IR
F
LOW
Duct systems and register sizes must be properly designed for the CFM and external static pressure rating of the furnace. Design the
ductwork in accordance with the recommended methods of “Air Conditioning Contractors of America” Manual D.
Install the duct system in accordance with Standards of the National Board of Fire Underwriters for the Installation of Air Conditioning,
Warm Air Heating and Ventilating Systems. Pamphlets No. 90A and 90B.
A closed return duct system must be used, with the return duct connected to the furnace.
NOTE:
Ductwork must never be attached
to the back of the furnace.
For upflow installations requiring 1800 CFM or more, use either two side returns or bottom return or a
combination of side /bottom. Flexible joints may be used for supply and return connections to reduce noise transmission. To prevent
the blower from interfering with combustion air or draft when a central return is used, a connecting duct must be installed between
the unit and the utility room wall. Never use a room, closet, or alcove as a return air chamber.
C
HECKING
D
UCT
S
TATIC
Refer to your furnace rating plate for the maximum ESP (ex-
ternal duct static) rating.
Total external static refers to everything external to the fur-
nace cabinet. Cooling coils, filters, ducts, grilles, registers
must all be considered when reading your total external static
pressure. The supply duct pressure must be read between the
furnace and the cooling coil. This reading is usually taken by removing the “A” shaped block off plate from the end on the
coil; drilling a test hole in it and reinstalling the block off plate. Take a duct static reading at the test hole. Tape up the test
hole after your test is complete. The negative pressure must be read between the filter and the furnace blower.
Too much external static pressure will result in insufficient air that can cause excessive temperature rise. This can cause
limit switch tripping and heat exchanger failure.
To determine total external duct static pressure, proceed as follows;
1. With clean filters in the furnace, use a draft gauge (inclined manometer) to measure the static pressure of the return
duct at the inlet of the furnace. (Negative Pressure)
2. Measure the static pressure of the supply duct. (Positive Pressure)
3. The difference between the two numbers is .4” w.c.
N
EVER
ALLOW
THE
PRODUCTS
OF
COMBUSTION
,
INCLUDING
CARBON
MONOXIDE
,
TO
ENTER
THE
RETURN
DUCT
WORK
OR
CIRCULATION
AIR
SUPPLY
.
WARNING
