Installation Guide
24
CIRCULATING AIR & FILTERS
DUCT WORK - AIR FLOW
Duct systems and register sizes must be properly designed for the
CFM and external static pressure rat ing of the furnace. Design the
ductwork in accor dance 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 upfl ow 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 con nections to reduce
noise transmission. To prevent the blower from inter fering
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.
CHECKING DUCT STATIC
Refer to your furnace rating plate for the maximum ESP (ex-
ternal duct static) rating.
N
EVER
ALLOW
THE
PRODUCTS
OF
COMBUSTION
,
INCLUDING
CARBON
MONOXIDE
,
TO
ENTER
THE
RETURN
DUCT
WORK
OR
CIRCULATION
AIR
SUPPLY
.
WARNING
Total external static refers to everything external to the furnace
cabinet. Cooling coils, fi lters, 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 fi lter and
the furnace blower.
Too much external static pressure will result in insuffi cient 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 fi lters 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.
Example:
static reading from return duct = -.1” w.c.
static reading from supply duct = .3” w.c.
total external static pressure on this system = .4” w.c.
NOTE: Both readings may be taken simultaneously and read
directly on the manometer if so desired. If an air conditioner
coil or Electronic Air Cleaner is used in conjunction with the
furnace, the readings must also include theses components,
as shown in the following drawing.
4. Consult proper tables for the quantity of air.
If the total external static pressure exceeds the maximum listed
on the furnace rating plate, check for closed dampers, regis-
ters, undersized and/or oversized poorly laid out duct work.
The temperature rise of the furnace must be within the tem-
perature rise range listed on the furnace rating plate.
Checking Static Pressure
Figure 22