Derating for High Altitude, Propane LP GAS Input Rate

Example:

Assume it took 29 seconds for one cubic foot of gas to flow and heating value of 1,000 Btu/cu.ft.

	1,000 x 3,600
Gas Input =		= 124,138 Btu per hour
	29

If you left no other pilots on, this is the furnace gas input.

8.If you left water heater, dryer or range pilots on, allow for them in calculating correct furnace gas input. A quick way is to allow 1,000 Btu per hour for a water heater, 500 Btu per hour for dryer and 500 Btu per hour for each range burner pilot.

Example:

If you left gas water heater, dryer, two range burner pilots and one oven pilot on, allow:

Water heater pilot	1,000		Btu per hour
Dryer pilot	500		Btu per hour
2 range burner pilots	1,000		Btu per hour
1 range oven pilot		500	Btu per hour
	3,000		Btu per hour

Subtracting 3,000 Btu per hour from 124,138 Btu per hour measured above equals 121,138 Btu per hour. This would be the correct furnace gas input after allowing for pilots left on.

9.Manifold pressure may be adjusted within the range of 3.2 inches W.C. to 3.8 inches W.C. to get rated input ± 2 percent. See Section 13, "Manifold Pressure Adjustment." If you cannot get rated input with manifold pressure within the allowable range, you must change orifices. See Section 17, "Changing Main Burner Orifices."

PROPANE (LP) GAS INPUT RATE

WARNING: Propane (LP) gas installations do not have gas meters to double-check input rate. Measure manifold pressure with an accurate manometer. Failure to accurately adjust pressure could cause heat exchanger failure.

1.Make sure you have correct pilot orifice and main burner orifices.

2.Gas supply pressure must be between 11 and 13 inches W.C. for propane (LP) gas. See Section 12, "Gas Supply Pressure and Pilot Adjustment."

3.Start furnace following "Operating Instructions" on front door.

4.Let furnace warm up for 6 minutes.

5.Adjust manifold pressure to 10.0 inches W.C. ± 0.3 inches W.C. See Section 13, "Manifold Pressure Adjustment."

WARNING: Do not set Propane (LP) manifold pressure at 11.0 inches W.C. It could cause heat exchanger failure.

NOTE: NOx reducing screens must be removed from each tube of California NOx furnaces when converting to propane (LP) gas. See Figure 20.

Figure 20. California NOx Screen Removal.

SECTION 15. DERATING FOR HIGH ALTITUDE

At higher altitudes, the density of the air is lower. Therefore, for proper combustion, the quantity of gas burned in the furnace must be reduced. This is called derating. Furnaces must be derated when installed at altitudes greater than 2,000 feet above sea level.

Derating the furnace by adjusting the manifold pressure lower than the range specified in the Section 13, "Manifold Pressure Adjustment" is considered to be an improper procedure.

At elevations above 2,000 feet, the furnace must be derated 4% for each 1,000 foot above sea level. It is the installer’s responsibility to see that the furnace input rate is adjusted properly. Derating must be achieved by reducing the size of the main burner orifices. Choose the proper orifices for your altitude and gas heating value from orifice size Table 7 or 8. In Canada, high-altitude models may be available that are rated for 2,000 to 4,500 feet above sea level.

SECTION 16. DETERMINING CORRECT ORIFICE SIZE

See Table 7 or Table 8 to determine correct orifice size for your gas type, gas heating value, and altitude. Natural gas and propane (LP) gas orifice kits are available through your supplier. Individual orifices are also available in convenient lot sizes. To ensure proper performance, use only manufacturer supplied orifices. To determine which orifices are installed in furnace, see rating plate --located on blower door.

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Installer’s Information Manual