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 12)
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.
SET TEMPERATURE RISE
Temperature rise must be within limits specified on furnace
rating plate. Recommended operation is at midpoint of rise
range or slightly above. Failure to follow this caution may
result in primary and secondary heat exchanger failure.
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
exchangers 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 tem-
perature from supply-air temperature to determine air tem-
perature rise.
3. Adjust temperature rise by adjusting blower speed. Increase
blower speed to reduce temperature rise. Decrease blower
speed to increase temperature rise.
Disconnect 115-v electrical power before changing speed tap.
Failure to follow this warning could result in personal injury.
4. To change blower motor speed selections for heating mode,
remove blower motor lead from control HEAT terminal. (See
Fig. 31.) Select desired blower motor speed lead from 1 of the
other motor leads and relocate it to HEAT terminal. See Table
Fig. 59—Burner Flame
A89020
BURNER FLAME
BURNER
MANIFOLD
Fig. 57—Redundant Automatic Gas Control 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. 58—Orifice Holes
A93059
BURNER
ORIFICE
EXAMPLE:
100,000 Btuh input furnace installed at 4300 ft.
Derate Furnace Input Rate
Furnace Input Rate X Multiplier = at Installation
at Sea Level Factor Altitude
100,000 X 0.91 = 91,000
Table 11–Altitude Derate Multipler for U.S.A.
ALTITUDE
(FT)
PERCENT
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 factors are based on midpoint altitude for altitude range.
47
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