Gas ConversionChart: Natural Gas Orifice vs Propane – A Complete Guide
When homeowners or technicians need to switch a gas‑fired appliance from natural gas to propane—or vice‑versa—the correct orifice size is the single most critical factor. A proper gas conversion chart ensures safe operation, optimal combustion efficiency, and compliance with local codes. This article walks you through the science behind orifices, explains how to read a conversion chart, outlines the step‑by‑step process for swapping gases, and answers the most common questions that arise during a conversion. By the end, you’ll have a clear roadmap for matching the right orifice to each fuel type and avoiding the pitfalls that can jeopardize performance and safety Easy to understand, harder to ignore..
Introduction – Why Orifice Size Matters
The orifice is a tiny opening that meters the flow of gas into the burner. Because natural gas (mostly methane) and propane (liquefied petroleum gas, LPG) have different energy densities and combustion characteristics, they require different orifice diameters to achieve the same heat output (BTU/hr). Using the wrong size can cause:
- Incomplete combustion – leading to soot, carbon monoxide, and wasted fuel.
- Excessive flame – which can overheat components and shorten appliance life.
- Flame roll‑out – a dangerous condition where the flame escapes the burner, increasing fire risk.
A reliable gas conversion chart bridges the gap between these two fuels, translating pressure, flow rate, and orifice dimensions into a language that technicians can apply on the job.
Understanding the Science Behind Orifice Sizing
Energy Content Differences
- Natural gas contains roughly 1,030 BTU per cubic foot.
- Propane packs about 2,150 BTU per cubic foot – more than double the energy per volume.
Because propane delivers more energy in the same volume, a smaller orifice is needed to meter the same amount of gas mass that a larger natural‑gas orifice would allow Surprisingly effective..
Pressure and Flow Dynamics
- Both fuels are typically supplied at ≈ 7‑10 inches water column (WC) static pressure for residential appliances. * When converting, the downstream pressure may shift slightly; the chart accounts for these variations by providing a range of orifice sizes that maintain the target BTU rating.
BTU Rating Formula
The basic formula used in most charts is:
[ \text{BTU/hr} = \frac{P \times \text{Orifice Area} \times \text{Conversion Factor}}{1,000} ]
where P is the pressure in inches WC, Orifice Area is the effective opening (in square inches), and the Conversion Factor incorporates the fuel’s heating value. This equation underscores why a smaller orifice is required for propane when the pressure is unchanged Which is the point..
How to Read a Gas Conversion Chart
Key Columns
| Column | What It Represents |
|---|---|
| Fuel Type | Natural Gas (NG) or Propane (LP) |
| Design Pressure | Typical supply pressure (inches WC) |
| Orifice Size (mm or #) | Diameter of the orifice opening |
| BTU Rating | Heat output at that size |
| Conversion Factor | Adjusted for fuel heating value |
Example Snapshot
| Fuel | Pressure (inches WC) | Orifice Size | BTU/hr |
|---|---|---|---|
| Natural Gas | 7.075 in (≈ 1.9 mm) | 10,000 | |
| Propane | 7.So 0 | 0. 0 | 0.055 in (≈ 1. |
The table shows that a 0.055‑inch orifice on a propane‑ready burner produces the same 10,000 BTU/hr output as a 0.075‑inch orifice on the same burner set for natural gas.
Using the Chart in Practice
- Identify the appliance’s rated BTU input.
- Locate the corresponding row for the fuel you plan to use.
- Select the orifice size that matches the BTU rating at your supply pressure.
- Verify the pressure after installation; adjust if necessary using a manometer.
Step‑by‑Step Conversion Process
Below is a concise checklist that technicians can follow when converting a burner from natural gas to propane (or the reverse). Each step references the conversion chart to ensure accuracy.
- Shut Off Gas Supply – Isolate the appliance and verify zero pressure with a manometer.
- Remove Existing Orifice – Use a proper orifice wrench; keep the old orifice for reference.
- Consult the Conversion Chart – Find the orifice size that delivers the appliance’s rated BTU at the current pressure.
- Install the New Orifice – Insert the selected orifice, ensuring it seats fully and is not cross‑threaded.
- Re‑pressurize and Test – Slowly open the gas valve, check for leaks with a soap‑solution, and measure the pressure. 6. Adjust Burner Air Shutter – Fine‑tune the air‑fuel mixture to achieve a blue, stable flame.
- Verify Flame Characteristics – The flame should be steady, silent, and fully envelope the burner ports.
- Document the Conversion – Record the orifice size, pressure reading, and date of conversion for future maintenance.
Common Pitfalls
- Skipping the pressure check – Even with the correct orifice, an unexpected pressure spike can cause over‑firing.
- Using a mismatched orifice material – Brass is standard; stainless steel may be required for high‑temperature applications.
- Neglecting air‑shutter adjustment – A proper air‑fuel ratio prevents soot and ensures complete combustion.
Scientific Explanation – Why the Numbers Work
When you replace a natural‑gas orifice with a propane‑specific one, you are essentially reducing the flow area by roughly 25‑30 %. This reduction compensates for propane’s higher heating value, ensuring that the same mass flow rate delivers comparable heat output. The underlying physics can be expressed as:
Worth pausing on this one.
[\frac{Q_{\text{propane}}}{Q_{\text{natural gas}}} = \frac{\text{Orifice}_{\text{propane
