HVACopediaRheem · Furnace troubleshooting
Canada only: derate high altitude by lowering manifold pressure
What is going on
In Canada, unless local codes mandate an orifice change, altitude derate can be done by lowering manifold pressure instead. The table values assume 1000 BTU per cu ft natural gas and 2500 BTU per cu ft LP. The actual input must be measured on site to confirm a real 10 percent reduction.
Check in this order
- Confirm the installation is in Canada and that local codes do not require an orifice change.
- Find the altitude band and fuel type in the alternate derate table.
- Set the manifold pressure to the value shown for that band.
- Measure the actual input rate on site and confirm about a 10 percent reduction.
- Attach the manifold pressure change label (Figure 43, part 92-24399-01-01) to the front of the furnace cabinet in a visible spot.
Applies to: 90 Plus models, Canadian installations 2,000 to 4,500 ft
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Source: Rheem 96V series installation instructions, p. 82. 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.