Start Gas Rates: Why We Calculate

If you are an apprentice or a seasoned gasfitter moving into industrial Type B appliances, you’ve probably heard the term "Start Gas Rate." You also know that regulatory inspectors—whether it’s Energy Safe Victoria (ESV), Building and Energy WA, or the SA Office of the Technical Regulator (OTR)—are absolutely stringent with the calculations behind it.

But what exactly is the start gas rate, what are we trying to prevent, and why do the standards demand such rigorous calculations before you can even think about handing over a Certificate of Compliance? Let’s break down the engineering jargon into plain English, look at the legal requirements, and explore the three methods used to keep everyone safe.

The Concept: The "Danger" of Ignition

When you fire up a Type B appliance (like an industrial oven, boiler, or furnace), the most dangerous part of the entire operation is the ignition sequence. When the gas safety shut-off valves open, raw, unburnt gas feeds into the combustion chamber. In a perfect world, the piezo or pilot lights it instantly. But what if the spark fails? What if the flame blows out a split-second later?

If the fuel gas keeps flowing without a flame, the unburnt gas mixes with air and pools inside the appliance casing. If a spark occurs a few seconds later, you don't get a burner flame; you get a deflagration. A pressure wave from a delayed ignition can blow the doors off an oven, destroy property, and fatally injure personnel.

Step 1: The Pre-Purge (Clearing the Ghosts)

Before we even calculate the start gas, we have to guarantee the chamber is empty. If there is leftover gas from a previous failed attempt, our calculations are useless.

• The Rule: AS 3814 Clause 2.20 and AS 1375 Clause 3.8 dictate a strict pre-purge. The fan must force a volume of air equal to at least 5 times the total internal volume of the appliance.
• The Rate: The purge airflow must be at least 25% of the maximum combustion air rate.
• Hazard to Watch: Complex oven geometries often create "dead zones" with restricted airflow. If your purge velocity isn't high enough, heavier gases like Propane (LPG) will easily pool at the bottom of these stagnant areas.

Step 2: The Flame Establishment Period

The "Flame Establishment Period" is the exact window of time the gas valves are open, feeding gas, before the Flame Safeguard system steps in and says, "I see a flame, we are good to go," or "No flame detected, shut it down."

• Forced Draught Burners: AS 3814 Clause 3.2.2.3 strictly limits this to 5 seconds.
• Atmospheric Burners: Usually limited to 15 seconds (for small pilots).

During those 5 seconds, raw gas is entering the combustion chamber. Our job as gasfitters is to prove that the amount of gas entering during this time won't turn the appliance into a bomb.

Step 3: The Three Methods of Compliance (AS 3814 Clause 3.2.3)

To legally commission a burner, you must mathematically prove your start gas rate complies with one of three methods outlined in AS 3814 Clause 3.2.3:

Method A: The "Dilution" Method

This method relies on high airflow to immediately dilute the gas so it never reaches its Lower Explosive Limit (LEL).

Under AS 3814 Clause 3.2.3(a), the gas concentration must stay safely below specific LEL percentages. For example, Natural gas has an LEL of 5.0%. Under Method A, airflow must dilute the gas so the global mixture never exceeds 50% of the LEL (a maximum of 2.5% gas-in-air by volume). Because propane and butane are heavier than air and require a lower concentration to ignite, they present a higher pooling risk and are capped at a stricter 25% of the LEL (Table 3.1).

Best for: Systems with massive fans running during ignition.

Method B: The "Energy Release Limit" Method

If you don't have enough fan air to dilute the gas, we use Method B (AS 3814 Clause 3.2.3(b)). This method assumes the worst: the gas pools and it does explode. However, we limit the volume of gas so heavily that the resulting "bang" is too weak to break the steel casing.

The Background: Old UK Gas Council research found that 75 kJ/m³ generates about 14 kPag of pressure (enough to rattle but not necessarily rupture standard industrial metal). AS 3814 plays it extremely safe and cuts that limit in half to 35 kJ/m³.

Method C: The "Critical Time" Method

If Method A and B fail, we pull out the heavy maths from AS 1375:2023 Appendix D.

We calculate the Critical Time—the exact number of seconds it takes for the gas/air mix to reach an explosive limit capable of breaking the appliance. AS 3814 Clause 3.2.2.3 allows you to extend the flame establishment period, but only if you can mathematically prove your ignition time is less than 70% of the appliance's calculated Critical Time.

Start Gas is Heavily Regulated

You cannot just guess a start gas rate. State regulators demand absolute, mathematical proof that your Type B appliance won't fail catastrophically:

• Victoria (VIC): Under the Gas Safety (Gas Installation) Regulations 2018, Schedule 9 legally requires you to submit full purge time and start gas rate calculations to Energy Safe Victoria (ESV) before they will even look at your acceptance application (Regulation 26).
• Western Australia (WA): The Gas Standards (Gasfitting and Consumer Gas Installations) Regulations 1999 (Regulation 22 & 32) prohibit leaving a Type B appliance permanently connected without an inspector verifying your compliance against AS 3814.
• South Australia (SA): Gas Regulations 2012 (Regulation 43) mandates that all commissioning and mathematical examinations strictly follow AS 3814.

Conclusion: Stop Fumbling with Paper Math

Between calculating the volumetric energy release, working out the stoichiometric ratios, accounting for Flame Failure Response Times (FFRT), and adjusting for the new Hydrogen-blending LEL shifts hitting our networks, the math is getting harder and the regulatory scrutiny is getting tighter. A simple mathematical error on a Schedule 9 submission will cost you days of delays with the regulator. Worse, getting it wrong in the field puts lives at risk.

That is why we built the GasCalc suite of calculators. GasCalc takes the heavy lifting out of AS 3814 and AS 1375 compliance. Simply input your combustion chamber volume, gas type, and airflow, and GasCalc will automatically process Method A, Method B, and Method C critical time equations. It generates the exact, standard-compliant mathematical proofs you need to hand to the inspector or regulator, guaranteeing a smooth certification process.