Application Note

3 Fluke Corporation HVAC pressure applications with the Fluke 922
Combustion air blower pres-
sure differential: Category IV
direct vent gas appliances typi-
cally use a combustion air blower
to ensure airflow through the
vent system and appliance before
ignition. In most cases, a pres-
sure differential switch monitors
pressure differential across an
orifice. Greater pressure differen-
tial indicates more airflow, lower
differential indicates less airflow.
In this case, two tees are needed
to tap into both the high and
low pressure sensing tubes con-
nected to the pressure differen-
tial switch. This is an important
value to benchmark during appli-
ance installation. Pressure differ-
ential depends on the length of
connected vent and the applica-
tion. If this value is benchmarked
when installed, degradation
to differential created by heat
exchanger, condensate or vent
problems is more likely to be
detected before a failure occurs.
If not benchmarked, there’s no
way to evaluate degradation.
Follow guidelines from the
manufacturer.
Fan effects on combustion
pressures: When checking stack
draft, over-fire draft or combus-
tion air inducer/blower pressure
differential, an abrupt change
in pressure when the blower
starts may indicate a compromise
between the flue passages and
the air passages. Fan pressure
can easily be 15 times or more
greater than flue passage pres-
sure (-0.03” wc draft vs 0.5”
ESP), so abruptly changing draft
pressures commonly indicate
heat exchanger leaks. Gradual
changes in draft pressures most
likely mean combustion air sup-
ply problems or vent problems,
especially when accompanied
by rising CO and CO
2
, and fall-
ing O
2
levels in the combustion
products. This can be caused
by insufficient make-up air or
return leaks, especially in spaces
that are considered confined
(less than 50 cu ft per 1,000
Btu input) or tight construction.
Use the Fluke 922 to compare
equipment room pressure to
outside pressure before and dur-
ing equipment operation. Simply
adding a small supply register in
Airside devices: Pressure
drop across airside devices is
published by manufacturers and
is necessary for design con-
siderations, including selecting
equipment for satisfactory blower
performance and for duct design
considerations. Once installed,
these same performance tables
are useful in estimating airflow
and determining end of life for
filters. Evaporators, for instance,
will have pressure drops with
associated cfm’s for a clean dry
coil and for a clean wet coil.
High performance filters will
have pressure drops with associ-
ated cfm’s for a clean filter, and
pressure drops with associated
cfm’s for that same filter when it
has reached the end of its life.
The Fluke 922 can be used
with a choice of pressure sens-
ing devices to check pressure
drop across an airside device.
If the approach to and from the
device is straight, a Pitot tube
can be placed on each side of
the device and the difference in
total pressure readings will result
in pressure drop. The tube con-
nected to the upstream side Pitot
tube is connected to the 922
“+” port, the tube connected to
the downstream side Pitot tube
is connected to the “-” port, the
pressure reading displayed will
be the device pressure drop. If
the approach is not straight to
the device, then use the static
sensing side of the two Pitot-
Static tubes, or use two static
pickups connected to the meter
in the same manner.
the equipment room can often
compensate for this engineering
design deficiency.
VAV (Variable Air Volume)
control: Pressure sensors are
often used to control VFD (vari-
able frequency drive motor) RPM.
For instance, a supply duct pres-
sure sensor can modulate the
RPM of a VFD, based on pressure
changes caused by a combina-
tion of open and closed zone
dampers. Exhaust VFD blowers
may modulate RPM based on
pressure differential between the
occupied space and outdoors. In
each of these cases, the VFD var-
ies RPM to maintain a relatively
consistent pressure or pressure
differential. The Fluke 922 can
be used to monitor or check the
calibration of these sensors.
CAVB (Constant Air Volume
with Bypass) damper control:
Equipment with a constant RPM
blower may need to simulate
VAV in the supply duct, when
zoning is used. The control is
similar to a true VAV system. A
pressure sensor monitors supply
and modulates a bypass damper.
The damper relieves pressure
from the supply to the return,
maintaining a consistent supply
static pressure. The Fluke 922
can monitor or check the calibra-
tion of these sensors.
Pressure differential defrost
control: Some enthalpy wheels
use a differential pressure sen-
sor to determine if a defrost cycle
is needed during cold weather.
A pressure differential increase
(indicating a frost build up) will
cycle the outdoor air intake
blower off , so it can use the
indoor air to defrost the wheel.
The Fluke 922 can be used to
monitor pressure differential,
check the calibration of the sen-
sor, or test defrost operation.
Some air-to-air heat pumps use
a pressure switch to monitor
pressure drop across the outdoor
coil to determine if frost build-up
warrants a defrost cycle. In that
circumstance, the Fluke 922 can
be used to monitor defrost pres-
sure differential, since restricted
airflow may indicate a frosted
coil.
Confirming pressure drop across the air filter bank.