Natural Gas to Propane Boiler Conversion: What Contractors Need to Know

Why Fuel Type Matters in Boiler Combustion

Weil-McLain boilers are designed to operate with specific combustion components and control settings based on fuel type. This is critical because propane and natural gas differ significantly in energy content and pressure requirements.

Propane contains approximately 2,500 BTUs per cubic foot, compared to 1,000 BTUs per cubic foot for natural gas. As a result, propane systems require less fuel flow to achieve the same heat output but operate at higher incoming gas pressures. Without the proper configuration, improper combustion, reduced efficiency, or equipment damage can occur.

To account for these differences, propane-fired boilers typically use orifice restrictions to limit gas flow and, in some high-efficiency models, a dedicated propane venturi to maintain correct air-to-fuel ratios.

Most Weil-McLain boilers ship factory-set for natural gas and must be converted to propane in the field using an approved propane boiler conversion kit. The following models are available from the factory in propane and do not require conversion:

• Ultra (80)
• CGi
• CGa

All other applicable models require conversion per the instructions included with the kit and the boiler manual.

Natural Gas to Propane Conversion: Installation Best Practices

When performing a natural gas to propane boiler conversion, follow these steps carefully:

• Verify all components listed on the conversion kit packing sheet are included
• Install every component in the conversion kit—do not omit parts
• For cast iron boilers, confirm the control module is propane compatible
• Apply all propane conversion labels included with the kit
• Update the control module to Propane or LP before firing, where applicable

Ultra boilers (except size 80) ship with a propane orifice and gas conversion label. Store these with the boiler for future use if needed.

Where required, complete all coarse gas valve adjustments outlined in the conversion instructions and boiler manual before firing. Failure to complete pre-adjustments may result in damage to the boiler.

Verify gas pressure with the boiler running:

• Natural Gas: ≥ 3.5” WC
• Propane (LP): ≥ 10.0” WC
  • Do not exceed 14” WC static pressure, as this can damage the gas valve

Using a combustion analyzer, set combustion values strictly according to manual specifications.

Failure to follow the conversion instructions and boiler manual can lead to improper combustion and unsafe operating conditions. You are working with flammable gases—improper installation may result in property damage, serious injury, or even loss of life.

Need a Propane Conversion Kit?

Propane conversion kits are available through authorized Weil-McLain distributors. Click here to find a distributor.

For full installation instructions and model-specific requirements, reference our Resource Library or specific boiler product page on weil-mclain.com. Also, check out our training videos that walk through the process with a few select boiler models.