Worked example: natural gas orifice at 5,000 and 8,000 ft

What is going on

A worked example from the manual showing how the orifice drops as altitude rises. It uses a regional natural gas value of 900 BTU per cubic ft and 14,000 BTU per burner at sea level. The sea level orifice is #49 on the 3.5 in W.C. column. With the 4 percent per 1,000 ft de-rate, you land on #51 at 5,000 ft and #52 at 8,000 ft. The numbers are only an example, so use your own local gas value and the tables for a real job.

Likely causes

• Install above sea level where input has to be de-rated
• Checking the orifice size for a different altitude
• Training on how the NFGC tables are read
• Verifying that a furnace came with the right orifices for the site

Check in this order

1. Sea level input per burner 14000 BTU/h. Sea level heating value: 900 BTU per cu ft
2. Gas flow Q = 14000 / 900 = 15.56 cu ft per hour
3. Orifice at sea level (3.5 in W.C. column of NFGC Table E.1.1(a)): #49
4. Orifice at 5,000 ft (4 percent de-rate per 1,000 ft, NFGC Table E.1.1(d)): #51
5. Orifice at 8,000 ft (4 percent de-rate per 1,000 ft, NFGC Table E.1.1(d)): #52
6. These are example values only, confirm against the tables and your utility's gas value
7. Have a licensed gas technician make the change and verify input

More furnace guides

• Annual and start-of-season inspection
• Blower codes 66, 61 and 60 - what to look for
• Blower does not start after burners light
• Burners go out before the heat call ends / short cycling
• Canada only: derate high altitude by lowering manifold pressure
• Cement PVC vent joints