Application Note

2 Fluke Corporation When start-up inspections pay off for HVAC
blower and measure 7.2 micro-
farads. It’s within 10 % of its
7.5 MFD rating, so it’s fine. I
measure and record 123 volts to
the furnace and 28 volts on the
transformer secondary. I connect
my manometers to the inlet and
manifold side of the gas valve. I
check the static gas inlet pres-
sure and record 6.5” WC (water
column). I’ll watch for a pressure
drop when the gas valve opens.
Electronics can perform errati-
cally, or not at all, if there is not
a good ground path back to the
source. To check for adequate
ground, I set the clamp meter
for volts ac and measure from
cabinet ground to the line volt-
age neutral connection on the
ignition control. With nothing
on the furnace operating, this
value should be less than 2 volts;
otherwise the ground circuit will
need to be improved. I measure
0.4 volts, so it’s okay.
Now it’s time to see some
operating values. I put the zone
panel in central mode and call
for maximum system air flow. My
incline manometer shows 0.35”
WC external static pressure with
a dry evaporator. I attribute the
reasonable static pressure to the
duct improvements we made.
I check the blower motor amp
draw. The manufacturer of the
electronically-commutated motor
specifies that amperage must be
taken with a true-rms meter. My
new clamp meter fits that speci-
fication, and measures 5.1 amps
on a motor rated at 7.7 amps full
load. Out of curiosity, I remove
the blower door panel and watch
my amps drop to 4.8 amps. If
this had been a PSC (permanent
split capacitor) motor, I would
have seen an increase in amps.
But these variable speed motors
only use as much torque and
RPM as they need to achieve
required air flow, so less duct
restriction means lower RPM at
less torque, thus less amperage.
I set my clamp meter to volts dc
and check the variable voltage
demand from the zone panel to
the furnace. A reading between
0 volts dc and 22 volts dc would
operate the blower proportionally
between its factory default
minimum air flow and the field
selected maximum flow. I read
22 volts dc; telling me the motor
is being commanded to the maxi-
mum air flow selected for this
system based on cooling capacity
(0 volts dc would command the
blower to its minimum flow.)
I remove the call for full oper-
ating air flow, unplug the hot
surface ignitor and measure
15 ohms through it. This particu-
lar ignitor should be between
11 and 20 ohms, so it’s okay. I
reconnect the ignitor and estab-
lish a call for a heating demand.
I connect my clamp meter across
the hot surface ignitor leads to
check ignitor volts. This ignition
control will reduce ignitor volt-
age by 6 % on each successive
ignition trial until it reaches a
“no light” voltage, then it steps
voltage back up 6 % and remains
there for the next 255 heat
cycles. The life of the ignitor
is extended if it uses only the
minimum voltage; higher voltage
produces greater ignitor temper-
ature. The ignition control “con-
ditions” the line voltage, which
requires that it be measured with
a true-rms meter. I measure 92
volts as the ignitor heats. If I had
measured this with a standard
averaging meter, my reading
would have been deceptively
low, probably around 50 volts. I
record all of these values on my
start-up form as I wait for the
gas valve to open.
When the gas valve opens,
the gas inlet pressure barely
changes, so the gas piping is
sized correctly. I clock the gas
meter at low fire and high fire
and calculate the BTU input to be
within 99 % of the furnace rat-
ing on each stage. This is based
on 1,070 BTUs per cubic foot of
gas. If this had been in the next
county where that utility sup-
plies gas at 1,025 BTUs per cubic
foot, the input would have been
about 95 % of the furnace rat-
ing and I would have increased
the manifold pressure from 3.5”
WC to 3.7” WC on high fire, and
from 1.7” WC to 1.8” WC on low
fire. No gas meter calculation fac-
tors have to be applied since the
elevation here is about 500 feet,
gas delivery pressure through
the meter is less than 9” WC,
and the meter is temperature
compensated. I find the side-
wall vent termination, check
the vent temperature with the
Type K bead thermocouple that
came with my clamp meter and
record that it is 105 °F. With
my combustion analyzer, I find
7.3 % CO
2
and 10 ppm CO. Both
of these values are well within
the specifications supplied by the
furnace manufacturer.
Back inside, I record a return
air temperature of 67 °F and
a supply air temperature of
137 °F for a temperature rise of
70 °F. For temperatures within
a duct, I prefer to use my Fluke
80PK-24 Air Temperature Probe
since it has a rigid wand and I
can be certain of its placement
within the duct. In order to use
the entire arsenal of Type K
thermocouples that I have col-
lected over the years with my
Fluke 902 HVAC Clamp Meter, I
purchased a Fluke 80AK Ther-
Testing the flame rectification circuit on a residential furnace.