Sizing a gas line correctly is critical to making sure every connected appliance gets its full rated gas flow. An undersized line means peak simultaneous demand starves one or more appliances, your cooktop drops to low flame when the HWU fires up, or the pool heater struggles to reach temperature because the BBQ is also running. The fix is gas line upgrade, which is more expensive after the fact than getting the sizing right initially.
What "sizing" means
Sizing means selecting the pipe diameter that can carry the required gas flow at the required pressure over the required distance without too much pressure drop. The relevant variables are:
- Total simultaneous demand in MJ per hour
- Pipe run length from meter to the most distant appliance
- Pipe material copper, PEX, polyethylene each have different flow characteristics
- Pressure available at the meter usually 1.1 kPa standard residential, sometimes higher for commercial
- Acceptable pressure drop typically 0.075 kPa across the residential line
Typical gas appliance demand
Approximate consumption when running at full output:
- Gas cooktop (4-burner): 35-50 MJ/hr at full output (rarely sustained, mostly intermittent)
- Gas oven: 15-25 MJ/hr
- Gas continuous flow HWU (Rinnai Infinity 26): 200 MJ/hr at full output
- Gas storage HWU: 20-40 MJ/hr
- Gas BBQ: 30-60 MJ/hr
- Gas pool heater: 200-500 MJ/hr depending on size
- Gas indoor heater (medium room): 30-80 MJ/hr
- Outdoor fire pit / fireplace: 50-150 MJ/hr
The cooktop is rarely a sizing constraint because real-world use is intermittent (not all burners on full at once). The HWU and pool heater are the big simultaneous demands.
Sizing for typical Gold Coast scenarios
Scenario 1, standard family home
4-burner cooktop + oven + gas continuous flow HWU + BBQ point. Peak simultaneous demand approximately 280 MJ/hr (HWU 200 + cooktop 50 + BBQ 30). For a typical 15m run from meter to most distant appliance, 25mm copper or PEX handles this comfortably.
Scenario 2, premium canal-front home
Cooktop + oven + HWU + BBQ + pool heater (300,000 BTU = 320 MJ/hr) + indoor gas heater (60 MJ/hr). Peak simultaneous if everything runs at once is 660 MJ/hr. For typical 25m run, 32mm copper is needed.
Scenario 3, large hinterland LPG home
Cooktop + oven + LPG continuous flow HWU + indoor gas heater + outdoor BBQ + outdoor fire pit. Peak simultaneous approximately 400 MJ/hr. Combined with potential long runs from bottle cage to dwelling (40-100m), needs careful sizing with 32mm internal main and 25mm appliance branches.
Why undersizing is a real problem
An undersized gas line does not fail catastrophically, it just delivers less gas than the appliance needs at peak. The symptoms are:
- HWU cannot reach full heat output, hot water tepid at high simultaneous demand
- Cooktop flame drops when other appliances fire up
- Pool heater struggles to maintain temperature
- Appliances generate error codes
The fix is upgrading the gas line. Replacing a 20mm line with 25mm or 32mm is typically $400-1,200 depending on length and access. Doing it right at install time is much cheaper.
When sizing matters most
- Adding a high-demand appliance to an existing house, especially pool heater. Existing gas line was sized for the original appliance set, adding a 300+ MJ/hr pool heater can easily exceed line capacity.
- Switching from gas storage HWU to gas continuous flow. Gas storage uses 20-40 MJ/hr, continuous flow uses 200 MJ/hr. The line that was fine for storage may be undersized for continuous flow.
- New build with full premium gas scope. Multiple high-demand appliances, line must be sized from day one.
- Long internal runs (acreage and large lots). Pressure drop accumulates over distance, longer runs need larger pipe for the same flow.
The whole-home gas audit
If you are planning a major renovation that adds gas appliances, or you are seeing performance issues that suggest undersized line, we can do a whole-home gas audit. We measure existing line size and run lengths, sum your current and planned appliance demand, and tell you in writing whether the line is adequate or needs upgrade. $360-580 typical for the audit.
What the gas fitter actually does
- Measures the run length from meter to each appliance
- Sums the connected appliance load
- Looks up the AS5601 sizing tables for the pipe material and pressure conditions
- Selects the smallest pipe size that meets the demand with acceptable pressure drop
- Confirms with manometer pressure test after install
Sizing is a gas fitter job, not something the homeowner needs to calculate, but understanding the concept helps when you are discussing options at quote stage.
The AS5601-2022 update and what changed
AS5601 is the Australian gas installation standard and it was revised in 2022 with several changes that affect residential sizing. Key updates worth knowing:
- Updated PEX-AL-PEX flow tables: multi-layer composite pipe (Buteline, Auspex, Rehau) now has clearer sizing capacity tables. Slightly different flow ratings than rigid copper for the same nominal size.
- Pressure drop allowances revised: the maximum allowable pressure drop across a residential consumer piping system was tightened in some scenarios. Old installs sized to the previous edition sometimes need uprating when modifications happen.
- Diversity factors clarified: the standard now explicitly allows diversity (the recognition that not all appliances run at full demand simultaneously) on multi-appliance residential systems. A licensed gas fitter can apply diversity factors that reduce the required pipe size in some scenarios.
- Outdoor BBQ points: clearer rules on quick-connect outdoor bayonet points, the gas isolation requirements and proximity to combustibles.
If your house was built or last gas-modified before 2022, an audit against the current edition sometimes reveals undersized sections that need uprating before adding a high-demand appliance like a pool heater. The audit is also useful before sale of the property, a clean compliance report on the existing gas system gives a buyer confidence and we have seen them used to support asking price.
Pipe material choice and how it affects sizing
Three materials dominate residential gas work on the Gold Coast and each has different flow characteristics:
- Copper (Type B, gas grade): the traditional choice. Higher pressure capacity, slightly smaller internal diameter than nominal. Suits external runs, exposed runs, and runs requiring sharp bends. Lifespan 40+ years. Joins are silver-brazed.
- PEX-AL-PEX (Buteline, Auspex, Rehau gas-rated): faster to install, flexible (fewer fittings), continuous run through wall cavities. Slightly larger internal diameter than copper for the same nominal size, so it carries more gas per equivalent size. Push-fit or crimp joints. Increasingly the default for new builds.
- Polyethylene (PE100): for underground runs from meter to a detached structure (granny flat, shed, pool plant). Fusion-welded joints. Excellent corrosion resistance underground.
Mixing materials within one install is normal, PE for the buried main, copper or PEX-AL-PEX for the internal distribution. Some installers default to one material everywhere for simplicity, which is fine, but the choice should follow the run conditions not the installer preference.
Real Gold Coast scenarios we have sized this year
Three recent jobs that show how sizing decisions play out:
Burleigh canal-front, premium scope: existing house had a 20mm copper main feeding cooktop, gas storage HWU and BBQ. Owner adding a 320,000 BTU pool heater plus upgrading the HWU to gas continuous flow (Rinnai Infinity 26). Total peak demand jumped from 90 MJ/hr to 580 MJ/hr. The 20mm main was completely undersized. We replaced the run from the meter to the pool plant area with 32mm PEX-AL-PEX and stepped down to 25mm for the kitchen branch. Cost $1,600 for the line upgrade, paid for itself in avoided performance issues on the new heater.
Coomera new build, all-electric kitchen but gas HWU: owner wanted gas continuous flow HWU only, no cooktop or BBQ. 20mm copper from meter to HWU run of 12m. Peak demand 200 MJ/hr. Single appliance, no diversity needed. 20mm copper sufficient with margin. Simple install.
Tallai acreage, LPG twin 45kg cycle, full gas scope: cooktop, oven, LPG continuous flow HWU, indoor heater, BBQ point. Bottles 35m from house. We ran 32mm PE underground from bottle cage to dwelling, stepped to 25mm internal copper main, 20mm appliance branches. Peak demand 320 MJ/hr. The long underground run was the sizing constraint, not the appliance count. Total install $4,200.
Diversity factors, the legitimate way to size smaller
AS5601-2022 explicitly permits diversity factors on residential multi-appliance systems. The idea is that not every appliance runs at full demand simultaneously, the cooktop is mostly intermittent, the BBQ is occasional, the indoor heater is seasonal. Applied correctly, diversity factors let a smaller pipe carry the design load with acceptable pressure drop.
Common diversity factors we apply in residential sizing:
- Cooktop, 50-70% of nameplate (rarely all burners on high simultaneously)
- BBQ point, 20-40% of nameplate (occasional use, often offset from cooktop use)
- Indoor heater, 30-60% seasonal
- Pool heater and HWU, 100% (no diversity, treat as full simultaneous load)
The pool heater and HWU are the appliances we never apply diversity to because both run at full demand for extended periods. The sizing constraint on any premium-scope home is almost always these two appliances combined. Cooktops and BBQs are sizing rounding errors by comparison.
Common undersizing patterns we find on the Gold Coast
Houses where we get called for performance complaints usually share one of three patterns:
- Original 15-18mm main on a 1980s build: sized for the era when cooktops were 25 MJ/hr and HWUs were gas storage 30 MJ/hr. Adding a modern continuous flow HWU (200 MJ/hr) starves the kitchen.
- Long single-line run with stepped-down branches in the wrong place: 25mm main steps to 20mm at the first appliance, then continues 20mm to the rest of the house including the HWU. The HWU starves because the main was supposed to stay 25mm to the HWU branch.
- Pool heater added without main upgrade: very common pattern in 2010-2020 builds. Pool builder installs the heater on a branch teed into the existing main without checking total demand. Heater never reaches design output.
The whole-home gas audit, what you get
The audit takes 60-90 minutes on site. We measure every pipe section (internal diameter where accessible, nominal stamp where not), map the run length from meter to each appliance, record each appliance nameplate MJ/hr rating, and run a pressure drop calculation per AS5601-2022 for the current and proposed loads. You get a written report identifying any undersized sections, the recommended fix size, and a quote for the upgrade work. The audit fee ($360-580) is credited back if you proceed with the upgrade work within 60 days. Saves you from the much more expensive scenario of installing a new appliance and discovering the gas line cannot feed it.
The audit also covers regulator condition, isolation valve presence and operation, flexi connector ages, ventilation around appliances, and any non-compliances against the current AS5601 edition. Useful before any major renovation, pre-sale of a property, or after any unexplained gas appliance performance issue.