HVACopediaRheem · Furnace troubleshooting
Example: checking orifice size at high altitude (natural gas)
What is going on
Worked example of orifice sizing for natural gas at altitude, using a 900 BTU/ft3 regional gas. These numbers are for the example only. Do not reuse them on another site, use the local heating value and the manual tables.
Likely causes
- Not a fault call. Reference example showing how altitude derating changes the orifice.
Check in this order
- Calculate Q = I / H: 14000 / 900 = 15.56 ft3 per hour (I = sea level input per burner, H = heating value).
- Find the sea level orifice for 3.5 in. w.c. in NFGC Table E.1.1(a): #49.
- Find the orifice at 5000 ft using NFGC Table E.1.1(d) with 4% derate per thousand ft: #51.
- Find the orifice at 8000 ft using the same method: #52.
- On a real job, get the heating value from the gas utility, not from this example.
- Orifice changes and gas input adjustment are for a licensed gas technician.
Applies to: Natural gas orifice sizing above 2,000 ft
More furnace guides
Take it to the job
All guides, error codes and tips work offline in the HVACopedia app.
Source: 96V series installation instructions, p. 80. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.