Carrier 48DL Installation Manual download pdf (Page 68)

264975-BIM-B-1208

68 Johnson Controls Unitary Products

valve) clockwise to increase manifold pressure or

counterclockwise to decrease manifold pressure. Be

sure not to under-fire the unit on first stage.

9. Once pressure has been checked, replace the plastic

cap covering the HI and LO pressure adjustment screws.

NOTE: When using natural gas, the manifold pressure for

second stage (100% input) should be 3.5 IWG ±

0.3. The manifold pressure for first stage (60%

input) when using natural gas should be 1.5 IWG ±

0.3.

CHECKING GAS INPUT

NATURAL GAS

This unit has two stages of gas heat. The first stage is 60% of

the full fire input and is considered the minimum input for the

furnace. The intended input for each furnace is shown in the

table below. The following Table applies to units operating on

60 Hz power only.

To determine the rate of gas flow (Second Stage).

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 smallest 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 dis-

cussed in the section “Manifold Gas Pressure Adjust-

ment”. 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 Steps 1-5.

To determine the rate of gas flow (First Stage)

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 (60% 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 smallest dial on the meter. A typical gas meter

has a 1/2 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 dis-

cussed in the section “Manifold Gas Pressure Adjust-

ment”. 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 (greater than 60% input).

Repeat Steps 1-6.

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).

TABLE 69: GAS HEAT STAGES

# of Burner Tubes

2nd Stage

Input (100%

Btuh)

1st Stage

Input (60%

Btuh)

120,000 72,000

180,000 108,000

240,000 144,000

TABLE 70: GAS RATE CUBIC FEET PER HOUR

Seconds

for One

Rev.

Size of Test Dial

1/2 cu. ft. 1 cu. ft.

180 360

150 300

129 257

113 225

100 200

90 180

82 164

75 150

69 138

64 129

60 120

56 113

53 106

50 100

47 95

45 90

43 86

41 82

39 78

37 75

36 72

35 69

34 67

32 64

31 62

30 60

1 2 ... 63 64 65 66 67 68 69 70 71 72 73 ... 83 84

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Carrier 48DL Installation Manual Page 68

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