using a heating value of approximately 1,000 Btu/hr, and should not normally require change. Most cases will not require a change at altitudes up to 7,500 feet. For use at altitudes in excess of 4,500 feet, refer to the following table for maximum input rates based on altitude. Contact your local gas utility for the heating value of the gas. Refer to National Fuel Gas Code ANSI Z223.1 and the following section to calculate firing rates.
| Maximium Firing Rates At Altitude | | |
| | | | | | |
| Model (Kbtu/hr) | | | | |
Altitude (ft) | 45 | 70 | 90 | | 115 | 140 |
5,000 | 39,000 | 58,500 | 78,000 | | 97,500 | 117,000 |
7,500 | 35,000 | 52,500 | 70,000 | | 87,500 | 105,000 |
10,000 | 34,000 | 51,000 | 68,000 | | 77,000 | 92,400 |
* Firing rates may be lower than listed in table
It is important to check and adjust the input rate of the furnace to prevent an overfiring situation. Overfiring can cause premature heat exchanger failure. The input is controlled by the supply pressure, orifice size, manifold pressure and heating (calorific) value of the gas.
The supply pressure must be measured with this and all other gas burning appliances in operation. The supply pressure must be adjusted to the pressure range stated on the series and rating plate. Applications for altitudes in excess of 4,500 feet usually require an orifice change. Alternately standard de-rate for altitude from National Fuel Gas Code ANSI Z223.1 of 4% per 1000’ may be taken. Refer to the most recent version of ASNI Z223.1 for correct gas orifice. The orifices must be selected using the table below. The furnace derate is 4% for each 1,000 feet above sea level. This table is based upon a heating value of approximately 1,000 Btu/ft3
| ALTITUDE | NAT. GAS |
| ORIFICE SIZE |
| |
| | |
| 0 - 2,000 | #43 |
| 3,000 | #44 |
| 4,000 | #44 |
| 5,000 | #45 |
| 6,000 | #45 |
| 7,000 | #46 |
| | |
The input to the furnace must be checked AFTER reorificing.
For altitudes above 4,500 feet, refer to appropriate section of the National Fuel Gas Code, ANSI Z223.1. To calculate the input of the furnace for installations in altitudes over 7,000 feet, use the following formula:
Corrected Input = Series & Rating Plate Input - (Altitude X .04) X (Series & Rating Plate Input / 1000)
Example:
Corrected input for a 90,000 Btu/hr. appliance installed at an altitude of 6,000 ft. utilizing natural gas with a heating value of 1,000 Btu/ft3 is determined by-
Corrected Input = 90,000 - (6,000 X .04) X (90,000 / 1,000) Corrected Input = 90,000 - (240 X 90)
Corrected Input = 90,000 - 21,600 Corrected Input = 68,400
Using the orifices sized as shown in the table for 6,000 feet (#45), a meter time of 52.6 seconds is measured. The actual firing rate of the furnace is:
FIRING RATE
Input = 1,000 (heating value of the gas) X 3600 (constant) / 52.6 (meter time for 1 ft3 of gas)
Input = 3,600,000 / 52.6 Input = 68,400 Btu/h
METER TIME IN MINUTES AND SECONDS FOR NORMAL INPUT RATING
OF FURNACES EQUIPPED FOR USE WITH NATURAL GAS
| INPUT | M ETER | HEAT | HEAT | HEAT | HEAT |
| | SIZE | VALUE | VALUE | VALUE | VALUE |
| Btu/hr | FT3 | | 900 | 1,000 | | 1040 | 1,100 |
| | | M IN. SEC. | M IN.SEC | M IN. SEC. | M IN. SEC. |
| 40,000 | 1 | 1 | 21 | 1 | 30 | 1 | 33 | 1 | 39 |
| 10 | 13 | 30 | 15 | 00 | 15 | 36 | 16 | 30 |
| |
| 60,000 | 1 | 0 | 54 | 1 | 00 | 1 | 03 | 1 | 06 |
| 10 | 9 | 00 | 10 | 10 | 10 | 24 | 11 | 00 |
| |
| 80,000 | 1 | 0 | 41 | 0 | 45 | 0 | 47 | 0 | 50 |
| 10 | 6 | 45 | 7 | 30 | 7 | 48 | 8 | 15 |
| |
| 100,000 | 1 | 0 | 32 | 0 | 36 | 0 | 37 | 0 | 40 |
| 10 | 5 | 24 | 6 | 00 | 6 | 14 | 6 | 36 |
| |
| 120,000 | 1 | 0 | 27 | 0 | 30 | 0 | 31 | 0 | 33 |
| 10 | 4 | 30 | 5 | 00 | 5 | 12 | 5 | 30 |
| |
In Canada, the series and rating plate input for the furnace apply to installations up to 4,500 feet (1370m) above sea level. Canadian certification applies to the installations of up to 4,500 feet above sea level. Installations above 4,500 feet are subject to acceptance by the local authorities having jurisdiction.
Do not derate the furnace by adjusting the manifold pressure to a lower pressure than specified on the furnace rating plate. The combination of the lower air density and a lower manifold pressure will prohibit the burner orifice from drawing the proper amount of air into the burner. This may cause incomplete combustion, flashback, and possible yellow tipping.
In some areas the gas supplier may artificially derate the gas in an effort to compensate for the effects of altitude. If the gas is artificially derated, the appropriate orifice size must be determined based upon the BTU/ft3 content of the derated gas and the altitude. Refer to the National Fuel Gas Code, NFPA 54/ANSI Z223.1, and information provided by the gas supplier to determine the proper orifice size.
Some models require a pressure switch change due to the reduced air density above certain altitudes. Refer to the following table to determine proper pressure switch settings for your application.
A different pressure switch may be required at high altitude regardless of the BTU/ft3 content of the fuel used. Contact your distributor for appropriate altitude ranges and pressure switch kits.
Pressure Switch Changes
Furnace input Kbtu/hr | Altitude (ft) Switch Setting (inches w.c.) |
45 | 8000' | 0.55 |
70 | NR | |
90* | 8000' | 0.6 |
115 | NR | |
140 | NR | |
* 904BNA does not require a switch change
PROPANE GAS CONVERSION
WARNING
POSSIBLE PROPERTY DAMAGE, PERSONAL INJURY OR DEATH MAY OCCUR IF THE CORRECT CONVERSION KITS ARE NOT INSTALLED. THE APPROPRIATE KITS MUST BE APPLIED TO INSURE SAFE AND PROPER FURNACE OPERATION. ALL CONVERSIONS MUST BE PERFORMED BY A QUALIFIED INSTALLER OR SERVICE AGENCY.
