Direct Vent Manifold Pressure Check - American Standard *UD Series Operation Manual

Direct Vent Manifold Pressure Check

The 40" Direct Vent furnaces (single and two stage)
reference the burner box inlet static to provide proper
gas valve regulation. The burner box static pressure
varies with the different inlet vent pipe lengths
and ambient air conditions. Therefore, the gas valve
regulator must be able to reference the box pressure
to maintain a constant manifold setting. The gas
valve vent port has a barbed hose fitting which
connects with a "tee" fitting to the burner box and
combustion air switches.
When setting or checking the gas valve outlet
manifold pressure, the manometer must be con-
nected to both the gas valve outlet pressure tap
and the burner box pressure hose, see figures below.
Note: The final manifold pressure will be higher
than the reading indicates if the burner
box hose is not connected. The actual
manifold pressure will be off by the same
amount as the burner box static pressure.
Correct Method of Checking Direct Vent Manifold
Pressure with Burner Box Referenced is shown below.
See
Inset
Example: 3.50" W.C. gas valve outlet pressure tap
The burner box pressure hose does not have to
be connected to set the outlet manifold pressure
if the burner box front cover is removed.
Separate the tube
at the unit Tee and
reconnect with a
short piece of field
supplied tube and
another Tee with
the "U" Manometer
attached.
Be sure to reconnect
burner box tubing
to original position
after testing the
manifold pressure!
Field supplied barb fitting
with manometer tube is
attached to the "Outlet
Pressure Tap" on the outlet
side of the gas valve.
32
reading,
–.15" W.C. burner box inlet pressure hose
not connected.
3.65" W.C. actual manifold pressure instead
of desired 3.5 in. W.C.