Natural gas at high altitude: derate and orifice check

What is going on

No high altitude pressure switch change is needed. The input must be derated 4 percent per 1,000 ft above 2,000 ft. Factory orifices are sized for a sea level natural gas heating value of 1100 BTU per cubic ft, so a regional lower heating value may make an orifice change unnecessary except at extreme altitudes. Installation above 2,000 ft must follow local codes or the National Fuel Gas Code (ANSI Z223.1/NFPA 54), or CSA B149.1 in Canada.

Check in this order

1. Find the local natural gas heating value (BTU per cubic ft) from the gas supplier.
2. Calculate the gas flow per hour sea level input per burner divided by the regional heating value (example: 14000 / 900 = 15.56 cu ft per hour).
3. Use the National Fuel Gas Code Annex E tables (E.1.1(a) for sea level orifice, E.1.1(d) for elevation de-rate) or the supplemental orifice chart (Table 14) to pick the orifice.
4. Reduce the sea level input by 4 percent per 1,000 ft and confirm the orifice size for that reduced input.
5. Change the burner orifice only if the calculation shows it is needed. Re-check the input rate after any change.

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