Homeowner switched the house from propane to natural gas.

The furnace and water heater are severely over-fired. Expect heat exchanger failure, sooting, CO production, and potential fire hazard.

Why: You cannot convert LP to natural gas by only adjusting manifold pressure. Orifice sizing is the primary determinant of input rate.

• Orifice Physics: LP orifices are ~40–50% smaller diameter than NG orifices for the same BTU input. NG requires ~2.5x the gas volume for the same heat.
• The Result: At 3.5" w.c. manifold pressure, the tiny LP orifices flow significantly more BTUs than the appliance's rated input (often 130–150%+ of nameplate).
• Furnace: Flame impingement on heat exchanger → rapid cracking/warping. High CO. Rollout risk. High limit tripping.
• Water Heater: Severe condensation in flue → acidic corrosion. Sooting. Flame rollout. T&P valve discharge from overheating.

Code Violations (IFGC / NFPA 54):

• IFGC §301.3 / NFPA 54 §9.1.2: Appliances must be installed per manufacturer's instructions. Conversion requires a listed conversion kit (orifices, spring, regulator adjustment, labeling).
• IFGC §403.5 / NFPA 54 §9.2.2: Input rate must not exceed nameplate rating.
• IFGC §304.1 / NFPA 54 §9.4.1: Combustion air requirements are based on actual input; now undersized.

Immediate Action:

1. Shut off gas to both appliances.
2. Do not operate.
3. Hire a licensed HVAC/plumbing contractor to install proper OEM conversion kits (or replace appliances if kits unavailable/uneconomical) and perform combustion analysis.
4. Verify gas pipe sizing for NG volume (IFGC §402 / NFPA 54 Ch. 6) – existing LP piping may be undersized.

This "plumber" created a lethal hazard. Report to state licensing board.

The homeowner’s appliances are now dangerously misconfigured — and failure is imminent.

What’s about to happen: The furnace and water heater were designed for propane (LP), which operates at ~10 in. w.c. manifold pressure. Natural gas requires ~3.5 in. w.c. — so setting the manifold pressure to 3.5 in. w.c. is correct for NG. However, the critical error is that the appliance orifices (jets) were not changed.

Propane orifices are significantly smaller than natural gas orifices because propane has higher energy density (~2,500 BTU/ft³ vs. ~1,000 BTU/ft³ for NG). Running LP-sized orifices on natural gas at 3.5 in. w.c. will result in:

• Severe overfiring: Too much gas flowing through too-small orifices → flame becomes large, lazy, yellow-tipped, and produces excessive carbon monoxide (CO).
• Incomplete combustion: Leads to sooting, heat exchanger cracking (furnace), and potential flame rollout.
• Water heater risks: Similar overfiring can damage the burner assembly, venting system, and produce lethal CO levels in the home.
• Immediate danger: CO poisoning risk is high — especially if the home is tightly sealed. Symptoms (headache, nausea, dizziness) may appear within hours. Long-term exposure can be fatal.

What must be done NOW:

1. Shut off gas supply to both appliances immediately.
2. Do not operate either appliance until corrected.
3. Contact a licensed HVAC/gas technician (not just a “plumber”) to: - Replace furnace and water heater orifices with NG-specific sizes. - Verify correct manifold pressure (3.5 in. w.c. for NG is right — but only with correct orifices). - Check venting, combustion air, and test for CO with a calibrated analyzer. - Perform a full safety inspection per IFGC Section 303 (combustion air) and Section 403 (appliance installation). - Verify safety controls (limit switches, rollout switches) are functional.

Code reference:

• IFGC 2021, Section 303.1: Requires adequate combustion air — compromised by sooting.
• IFGC 403.1: Appliances must be installed per manufacturer’s listing — which requires NG-specific orifices.
• NFPA 54 (National Fuel Gas Code): Mandates orifice conversion for fuel change.

Bottom line: This is not a “set and forget” adjustment. Operating LP orifices on NG is a life-threatening code violation. Turn off the gas and call a licensed pro today. Do not wait for symptoms — CO is odorless and deadly.