Gas Furnaces:High-Altitude Adjustment and Changing Burner Orifices.

High-Altitude Adjustment

For elevations above 2000 ft, the Btu input should be reduced (derated) 4 percent for each 1000 ft above sea level. This adjustment is required because of the increase in air volume at higher elevations. Air expands at approximately 4 percent per 1000 ft of elevation. At elevations above 2000 ft, the volume of air required to supply enough oxygen for complete combustion exceeds the maximum air-including ability of the burners. Because the primary air volume cannot be increased, the gas flow (input) must be decreased (that is, derated).

Changing Burner Orifices

A gas-fired furnace is supplied with standard burner orifices (spuds) for the gas shown on the rating plate. The orifice size will depend on the calorific value of the gas (Btu per cubic foot) and the manifold pressure (3.5 in W.C. for natural gas and 11 W.C. for LP gas). The burner orifice size partially determines the firing rate by allowing only a predetermined volume of gas to pass. An orifice that is too large will allow too much gas to pass through into the burner. This condition sometimes results in overheating the heat exchanger or restricting the air-inducing ability of the burner.

The gas burner orifices supplied with a furnace are suitable for average calorific values of a gas at the listed manifold pressures. Table 11-2 lists the various orifice capacities for different drill sizes. The procedure for determining the suitability of a particular drill size is as follows:

1. Divide the total Btu input of the furnace by the total number of burner orifices in the unit.

2. From Table 11-2, select the burner orifice size closest to the calculated value (plus or minus 5 percent) for the gas used.

Courtesy The Trane Company

This procedure can be illustrated by determining the most suitable drill size for a 150,000-Btu-input, gas-fired furnace. This furnace has four burners, with two orifices per burner, and uses natural gas.