30
Isolate this unit from the gas supply piping system by
closing its external manual gas shutoff valve before
pressure testing supply piping system with test pressures
equal to or less than ½ psig (3.48 kPA).
WARNING
If the gas furnace is installed in a basement, an excavat
-
ed area or confined space, it is strongly recommended to
contact a certified contractor to install a gas detecting
warning device in case of a gas leak.
CIRCULATING AIR & FILTERS
Duct Work - Air Flow
WARNING
Never allow the products of combustion, including carbon
monoxide, to enter the return duct work or circulation air
supply.
Duct systems and register sizes must be properly designed
for the CFM and external static pressure rating of the
furnace. Ductwork should be designed 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.
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.
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 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, registers, undersized 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.
.
Checking Static Pressure
(80% Furnace Shown, 90% Similar)
Figure 46
