Application Note

3 Fluke Corporation When start-up inspections pay off for HVAC
mocouple Adapter. This adapter
interfaces the standard Type K
thermocouple mini-pin connector
to the standard dual banana plug
inputs found on DMMs and clamp
meters. This will allow me to use
my pipe clamp probes, piercing
probes, immersion probes, sur-
face probes and air probes with
the Fluke 902.
The next measurements I take
are important benchmark tests
because a definitive initial value
is not specified: flame signal and
combustion air blower/inducer
pressure differential. Both of
these are safety devices and
have specified minimum values.
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PO Box 1186, 5602 BD
Eindhoven, The Netherlands
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©2006 Fluke Corporation. All rights reserved.
Printed in U.S.A. 8/2006 2743037 A-EN-N Rev A
Fluke. Keeping your world
up and running.™
Once these minimum values are
reached, the furnace will no
longer operate. By benchmark-
ing the operating values, service
technicians can reference these
new installation values and
compare them to values read
during maintenance checks to
see if performance has degraded
and if action needs to be taken.
Flame signal drop-out values
and pressure differential make/
break values are provided by
the manufacturer, but without
knowing the changes that may
have occurred since installa-
tion, one cannot be sure what
action, if any, needs to be taken.
Checking current on a residential furnace.
So I connect my clamp meter
in series with the flame sensor
lead, set the meter on micro-
amps and record 0.8 microamps.
This is 0.64 microamps above
the published drop out value
of 0.16 microamps. With direct
vent furnaces such as this one,
flame signals generally are not
as likely to degrade as with fur-
naces that get their combustion
air from within the structure. It
all depends on contaminants that
may or may not be in the com-
bustion air supply.
Pressure differential created by
the combustion air blower will
vary from installation to installa-
tion depending on vent diameter
and equivalent length. These
values need to be benchmarked
at the time of installation so ser-
vice techs on future visits know
what is “normal” for this furnace
in this house. The manufacturer
just provides pressure switch
make/break settings. So I “tee”
into the pressure switch tubing
with my incline manometer and
measure pressure differentials of
0.9” WC on low fire and 1.5” WC
on high fire. On future visits, ser-
vice techs will know that low fire
pressure differential should be
0.4” WC above the break setting
and high fire pressure differential
should be 0.65” WC above the
break setting.
My Fluke 902 is proving to be
a good choice for an “everyday”
meter on today’s job so far. On to
the next task...