Checking Gas Heat Input
For two stage and modulating furnaces, input rate
should be checked at full input and minimum input.
The intended input for each furnace is shown in
and on the unit rating plate. The table applies to
units operating on 60 Hz power only.
To determine the rate of gas
flow at Second Stage (full input)
About this task:
To determine the rate of gas flow at Second Stage
(full input):
1. Turn off all other gas appliances connected to
the gas meter.
2. Turn on the furnace and make sure the
thermostat is calling for Second stage (100%
input) heat.
3. Measure the time needed for one revolution
of the hand on the lowest increment dial on
the meter. A typical gas meter has a 1/2 or a
1 cubic foot test dial.
4. Using the number of seconds it takes for one
revolution of the dial, calculate the cubic feet
of gas consumed per hour. (See example
below).
5. If necessary, adjust the high pressure
regulator as discussed in the section Manifold
Gas Pressure Adjustment. Be sure not to
over-fire the furnace on second stage. If in
doubt, it is better to leave the second stage of
the furnace slightly under-fired. Repeat Step 1
to Step 5.
To determine the rate of gas
flow at First Stage (minimum
input)
1. Turn off all other gas appliances connected to
the gas meter.
2. Turn on the furnace and make sure the
thermostat is calling for first stage (minimum
input) heat.
3. Even when the thermostat is calling for first
stage heat, the unit will light on second stage
and will run on second stage for 1 minute.
Allow this one-minute time period to expire
and be certain the unit is running on first
stage.
4. Measure the time needed for one revolution
of the hand on the lowest increment dial on
the meter. A typical gas meter has a 1/2 cubic
foot or a 1 cubic foot test dial.
5. Using the number of seconds it takes for
one revolution of the dial, calculate the cubic
feet of gas consumed per hour (See example
below).
6. If necessary, adjust the low pressure regulator
as discussed in the section Manifold Gas
Pressure Adjustment. Be sure not to under-
fire the furnace on first stage. If in doubt, it
is better to leave the first stage of the furnace
slightly over-fired. Repeat Step 1 to Step 6.
Table 69: Gas Rate Cubic Feet Per Hour
Size of test dial
Seconds for one rev.
1/2 cu. ft 1 cu. ft
10 180 360
12 150 300
14 129 257
16 113 225
18 100 200
20 90 180
22 82 164
24 75 150
26 69 138
28 64 129
30 60 120
32 56 113
34 53 106
36 50 100
38 47 95
40 45 90
42 43 86
44 41 82
46 39 78
48 37 75
50 36 72
52 35 69
54 34 67
56 32 64
58 31 62
60 30 60
Note: To find the Btu input, multiply the
number of cubic feet of gas consumed
per hour by the Btu content of the gas in
your particular locality (contact your gas
company for this information as it varies
widely from area to area).
Installation Manual: YORK
®
Sun
™
Select YV28 to YV50
90
Johnson Controls Ducted Systems
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