High altitude derate and orifice change

What is going on

Input is based on sea level and is only changed above 2,000 ft.

Check in this order

1. Up to 2,000 ft, do not change the input rating.
2. At 2,000 ft or higher, reduce input 4% for each 1,000 ft above sea level.
3. Check input by clocking gas flow and multiplying by the local heating value.
4. Change input by manifold pressure first upflow natural gas 3.0 to 3.7 in. W.C.; downflow natural gas 3.0 to 4.5 in. W.C.
5. If the target input cannot be reached with pressure, change the orifices. LP installations always need an orifice change.
6. Above 4,000 ft, a pressure switch change may be needed; use the BAYHALT high altitude accessory kit listed in product data.
7. Recheck input rate and temperature rise after any orifice change.
8. Install above 2,000 ft in line with local code, or the National Fuel Gas Code / CSA 149.1 and the listed high altitude conversion kit.

More furnace guides

• Abnormal conditions and protection
• Adjust gas valve manifold pressure
• Air for combustion and ventilation in non-direct vent installs
• Airflow adjustment check
• Airflow and temperature rise check
• Bottom return filter rack installation (upflow)