What manifold pressure should a natural-gas furnace run, what about propane, and what actually has to change to convert one to LP?

Manifold Pressures

• Natural gas: 3.5 inches water column (w.c.) ±0.5" (Standard: ANSI Z21.47 / NFPA 54)
• Propane (LP): 10.5 inches w.c. ±0.5" (Same standards)

Conversion Requirements (NG → LP)

1. Orifices: Must be replaced with smaller-diameter LP orifices (LP has higher energy density per volume, requiring less fuel flow).
2. Manifold Pressure Regulator: Adjust or replace to step-down from 10.5" w.c. (LP) to the correct outlet pressure.
3. Gas Valve: May need adjustment or replacement if it has fixed internal stops for NG pressure settings.
4. Air Shutter: Requires adjustment to maintain proper air-to-fuel ratio (LP burns differently).
5. Labeling: Update nameplate/furnace label to reflect LP gas (required by code).

Critical Notes

• Never adjust orifices without proper sizing—improper sizing causes incomplete combustion, soot, or CO production.
• Leak testing required after conversion (soap solution or electronic tester).
• Professional installation required in most jurisdictions—conversion affects safety controls and venting.

Code Reference: NFPA 54 (2021) Section 7.3.4 requires appliances to be listed for the specific gas type and installed per manufacturer instructions.

Bottom Line: Conversion is not a DIY job—requires certified technician, proper orifice sizing, and combustion testing. Incorrect conversion risks fire, carbon monoxide poisoning, or furnace damage.

Standard Manifold Pressures (at the gas valve outlet):

• Natural Gas: 3.5" w.c. (Typical range: 3.2"–3.8" w.c.)
• Propane (LP): 10.0" w.c. (Typical range: 9.5"–11.0" w.c.)

Always verify the exact rating plate on the specific furnace. These are nominal values per ANSI Z21.47 / CSA 2.3.

What Must Change for LP Conversion (Kit Required):

1. Gas Valve: Replace spring/regulator assembly or install LP-specific valve (different regulation range). Do not just adjust the existing valve screw past its stop.
2. Burner Orifices: Replace every main burner orifice with smaller LP orifices (higher pressure + higher BTU/ft³ = smaller hole). Drill size is critical.
3. Pilot Orifice: Replace pilot orifice (if standing pilot or intermittent pilot).
4. Pressure Switch(es): Often require different trip points due to altered flue gas density/flow. Check kit instructions.
5. Flame Sensor/Ignitor Position: Occasionally requires repositioning for proper flame rectification/ignition with LP flame characteristics.
6. Gas Valve Inlet Pressure Tap: Verify inlet pressure after conversion (LP: 11"–13" w.c. typical max).

Critical Steps:

• Clock the Meter: Verify input rate matches nameplate after conversion. Do not rely solely on manifold pressure.
• Combustion Analysis: Mandatory. Check CO (<100 ppm air-free), CO₂, O₂, and stack temp. Adjust manifold pressure within rated range only if analyzer dictates.
• Labeling: Apply "Converted to LP" label on furnace and gas valve (NFPA 54 / IFGC §301.3, §623).

Warning: Improper conversion causes sooting, CO, delayed ignition, or heat exchanger failure. Use the OEM conversion kit only. If no kit exists, the unit cannot be converted.