Natural Gas vs Propane Conversion vs Dual Fuel: Method Decision Matrix
Why this matters
A customer with a natural-gas genset who just lost gas pressure in a regional event, or a propane customer who wants the convenience of utility gas, lands on the same question: convert to the other fuel, run a conversion kit, or install a dual-fuel system that can switch. Each path has different hardware, different derate, and different code consequences. Picking the wrong one leaves a unit that runs lean and overheats on the new fuel, or a dual-fuel install that violates the gas code because the two supplies were not isolated correctly. This matrix lays out the three methods, what each requires, and the field flow to choose between them.
The options
There are three real methods, and they are not interchangeable.
Method 1: Single-fuel conversion. The engine is permanently re-jetted or re-regulated to run one fuel instead of the other. Natural gas (NG) and propane (LP) have different energy densities and require different orifice sizing and regulator pressure. NG runs at lower pressure and lower BTU per cubic foot; LP runs at higher pressure and higher BTU. A conversion changes the carburetor/mixer orifice or demand-regulator setting so the engine meters the correct fuel. The unit then runs ONLY that fuel.
Method 2: Conversion kit (factory dual-fuel-capable carb). Many standby engines ship with a dual-fuel mixer and a selector that lets the installer set the unit for NG or LP at commissioning. This is not a field switch the customer flips; it is an installer-set position. The hardware supports either fuel, but the unit is committed to one at install per the supply plumbed to it.
Method 3: True dual-fuel system. Two separate fuel supplies (utility NG and an LP tank) are plumbed to the unit through a switchover that lets the engine run on the primary and fall back to the secondary when the primary fails. This is the path for customers who want NG economy with LP backup during gas outages. It requires two regulators, two shutoffs, and a code-compliant changeover that cannot feed both supplies into the engine at once.
Comparison
- Hardware cost and complexity: single conversion is lowest; conversion-kit set-at-install is low; true dual-fuel is highest (two supplies, changeover, two regulators).
- Power output: gaseous fuels derate the engine versus its diesel or gasoline rating. LP generally yields slightly higher output than NG because of higher BTU content. Confirm the derate table for the specific unit; running an NG-jetted engine on LP without re-jetting runs rich and sooty, while an LP-jetted engine on NG runs lean and hot.
- Fuel availability resilience: single-fuel ties the customer to one utility; true dual-fuel survives an NG outage by switching to the LP tank.
- Code and permit load: any fuel change requires correct regulator pressure and a confirmed appliance listing for the fuel; dual-fuel adds the changeover and dual-shutoff requirements of the fuel gas code.
When it applies
- Choose single-fuel conversion when the customer is permanently changing fuels (for example, the LP tank is being removed because NG service was just run to the property). Commit the engine to the new fuel and decommission the old supply.
- Choose the conversion-kit / set-at-install path when the unit already ships dual-fuel-capable and you are simply commissioning it on the supply that is present. No added hardware; you set the selector and verify the mix.
- Choose true dual-fuel only when the customer specifically wants automatic fallback (NG primary, LP backup) and the site supports both a utility-gas drop and an LP tank with code-legal setback. This is the right call for critical loads in regions where NG pressure sags during widespread outages.
Field decision flow
- Identify what the engine is jetted/set for today. Read the data tag and the demand-regulator/mixer position. Do not assume; an engine plumbed to NG may have been factory-set for LP.
- Identify the supply or supplies available at the site: NG meter and pressure, LP tank size and setback, or both.
- If the customer wants one fuel only, match the engine setting to that fuel: confirm orifice/mixer position, set the regulator to the fuel's required inlet pressure, and verify the derated output is still adequate for the connected load.
- If the customer wants resilience and both supplies exist, scope a true dual-fuel changeover: two regulators, two manual shutoffs, a changeover device that physically prevents both supplies feeding simultaneously, and a verified primary/secondary priority.
- Verify combustion on the final fuel: check for correct mixture (no black soot from rich, no overheating/lean surge), confirm rated frequency under load, and confirm the regulator holds pressure at full draw without lockup.
- Pressure-test and leak-check every new gas joint before energizing.
Fuel gas work carries fire and explosion risk. Only re-jet, re-regulate, or add a fuel supply per the engine listing and the fuel gas code, leak-test every joint with the supply pressurized, and confirm the unit is listed for the fuel it ends up running. An LP-jetted engine left on NG runs lean and can overheat; an NG-jetted engine on LP runs dangerously rich.
References
- NFPA 54, National Fuel Gas Code, regulator, pressure, and appliance-fuel requirements for natural gas and LP appliances.
- NFPA 58, Liquefied Petroleum Gas Code, LP tank, regulator, setback, and piping requirements.
- NFPA 37, Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines, fuel-supply provisions.
- Generator and engine manufacturer fuel-conversion instructions and the unit's NG/LP derate table.