Installation Guide
36
Checking Static Pressure
Figure 40
C
IRCULATING
A
IR
& F
ILTERS
DUCT WORK - AIR FLOW
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 and 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.
CHECKING DUCT STATIC
Refer to your furnace rating plate for the maximum ESP (external duct static) rating.
Total external static refers to everything external to the furnace 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.
Excessive external static pressure will result in insufficient air which 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 your total external static pressure.
Example:
static reading from return duct = -0.1" W.C.
static reading from supply duct = +0.3" W.C.
total external static pressure on this system = 0.4" W.C.
NOTE: Both readings may be taken simultaneously and read directly
on the manometer if so desired. If an air conditioner coil or Elec-
tronic Air Cleaner is used in conjunction with the furnace, the read-
ings must also include these 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, registers, under-
sized and/or oversized poorly laid out duct work.
The temperature rise of the furnace must be within the temperature
rise range listed on the furnace rating plate.
N
EVER
ALLOW
THE
PRODUCTS
OF
COMBUSTION
,
INCLUDING
CARBON
MONOXIDE
,
TO
ENTER
THE
RETURN
DUCT
WORK
OR
CIRCULATION
AIR
SUPPLY
.
WARNING