g. Refer to Table 16 for cu ft of gas per hr.
h. Multiply gas rate (cu ft/hr) X heating value (Btu/cu ft) using natural gas heating value from local gas utility/supplier.
EXAMPLE: (0—2000 ft altitude)
Furnace input from rating plate is 100,000 Btuh.
Btu heating input = Btu/cu ft X cu ft/hr
Heating value of gas = 975 Btu/cu ft
Time for 1 revolution of 2-cu ft dial = 70 sec
Gas rate = 103 cu ft/hr (from Table 15)
Btu heating input = 103 X 975 = 100,425 Btuh
In this example, the orifice size and manifold pressure adjustment is within
±
2 percent of the furnace input rate.
B.
Set Temperature Rise
CAUTION: UNIT DAMAGE HAZARD
Failure to follow this caution may result in overheating the heat exchangers or condensing flue gases in heat exchanger areas not
designed for condensate.
Temperature rise must be within limits specified on unit rating plate. Recommended operation is at midpoint of rise range or above.
Determine and adjust air temperature rise as follows:
1. Place thermometers in return and supply ducts as close to furnace as possible. Be sure thermometers do not see heat exchanger so that radiant
heat does not affect readings. This practice is particularly important with straight-run ducts.
2. When thermometer readings stabilize, subtract return-air temperature from supply-air temperature to determine air temperature rise.
Fig. 48—Redundant Automatic Gas Valve
A03142
REGULATOR
SEAL CAP
REGULATOR
ADJUSTMENT
SCREW
REGULATOR SPRING
(PROPANE - WHITE, 6 TURNS
NATURAL - SILVER, 10 TURNS)
GAS PRESSURE
REGULATOR
ADJUSTMENT
MANIFOLD
PRESSURE TAP
INLET
PRESSURE TAP
ON/OFF SWITCH
Fig. 49—Burner Flame
A89020
;;
;;
BURNER FLAME
BURNER
MANIFOLD
Table 15—Altitude Derate Multiplier for U.S.A.
ALTITUDE (FT)
% OF DERATE
DERATE MULTIPLIER FACTOR*
0—2000
0
1.00
2001—3000
4—6
0.95
3001—4000
6—8
0.93
4001—5000
8—10
0.91
5001—6000
10—12
0.89
6001—7000
12—14
0.87
7001—8000
14—16
0.85
8001—9000
16—18
0.83
9001—10,000
18—20
0.81
* Derate multiplier factor is based on midpoint altitude for altitude range.
—58—
