Replacing a dinosaur gas central heater in Victoria, the ACT, or Southern New South Wales usually comes down to one critical decision: getting the unit capacity right. If you undersize the unit, your family wakes up to cold rooms on frosty mornings while the fan runs nonstop. If you oversize it, the burner short-cycles on and off, burning excess gas, warping the heat exchanger, and dumping noisy bursts of hot air through your floor registers.
In Australia, gas ducted heaters are marketed and rated by gas consumption in Megajoules per hour (MJ/hr), but the warmth that actually enters your rooms is measured in Kilowatts (kW) of delivered heat. This guide explains how to convert between the two, how to choose the right unit tier under AS 4556, and what pipe and outlet limits you must meet under AS/NZS 5601.1.
Comparing gas ducted heating to electric heat pumps? If you are planning a whole-home electric conversion or sizing a ducted reverse cycle system, check our Residential Heat Load Calculator. For individual bedroom split systems, use the Air Conditioner Size Calculator.
Gas Ducted Heating Sizing Chart
An average 3-bedroom Australian home (130m² to 170m²) in Melbourne or regional Victoria requires an 80 to 95 MJ/hr gas ducted heater (delivering 20 to 24 kW of thermal heat). Compact units require 60 MJ/hr, while large two-storey homes or Highland alpine properties require 120 to 140 MJ/hr systems.
| Floor Area (m²) | Typical Layout | Outlets Count | Gas Rating (MJ/hr) | Thermal Heat (kW) | Benchmark Models (Braemar / Brivis) |
|---|---|---|---|---|---|
| 70 – 110 m² | 2-Bed Unit / Villa | 4 – 5 vents | 60 MJ/hr | 15.0 kW | Braemar TQ315 / Brivis Buffalo BX315 |
| 120 – 170 m² | 3-Bed Single Storey | 6 – 8 vents | 80 – 95 MJ/hr | 20.0 – 24.0 kW | Braemar TQ420, TQS623 / Brivis SP420, SP623 |
| 180 – 240 m² | 4-Bed Family Home | 8 – 11 vents | 105 – 120 MJ/hr | 26.0 – 30.0 kW | Braemar TQS630 / Brivis StarPro SP630 |
| 250 – 320 m² | Large / 2-Storey Home | 11 – 15+ vents | 130 – 140 MJ/hr | 33.0 – 35.0 kW | Braemar TQS735 / Brivis StarPro SP635 |
How to Convert Megajoules (MJ) to kW
To convert gas ducted heating Megajoules (MJ/hr) to delivered Kilowatts (kW), multiply the MJ/hr input by the heater’s thermal star rating efficiency, then divide by 3.6. One kilowatt of thermal energy equals exactly 3.6 Megajoules per hour.
Because gas heaters burn natural gas or LPG to produce flame, not all energy enters your living room. The difference between gas input and heat output depends entirely on the star rating:
- Standard 3-Star Unit (66% Efficiency): A 100 MJ/hr unit delivers only 18.3 kW of warmth. The remaining 34% of gas purchased goes straight up the metal flue into the sky.
- High-Efficiency 6-Star Unit (91% Efficiency): The same 100 MJ/hr input delivers 25.2 kW of usable room warmth because a secondary stainless steel condensing heat exchanger captures heat from the exhaust vapour.
Star Rating Efficiency & Running Costs
Upgrading from an old 3-star gas central heater to a modern 6-star or 7-star condensing heater cuts household gas heating consumption by 25% to 30%. In cold southern states with high winter heating demand, this represents an annual bill reduction of $350 to $550 per winter.
| Star Rating Tier | Thermal Efficiency | Flue Type | Est. Daily Cost (6 hrs run) | Winter Bill (90 Days) |
|---|---|---|---|---|
| 3-Star (Standard) | ~65% – 68% | Twin-wall metal flue | $16.80 / day | ~$1,510 / season |
| 4-Star (Mid-Tier) | ~75% – 78% | Twin-wall metal flue | $14.70 / day | ~$1,320 / season |
| 5-Star (High Non-Condensing) | ~83% – 86% | Twin-wall metal flue | $13.40 / day | ~$1,200 / season |
| 6-Star (Condensing) | ~90% – 92% | Room-sealed PVC Power Flue | $12.50 / day | ~$1,120 / season |
| 7-Star (Ultra-Condensing) | ~95% – 97% | Room-sealed PVC Power Flue | $11.90 / day | ~$1,070 / season |
Condensing units run their exhaust gases through a secondary heat exchanger until water vapour condenses into liquid, releasing latent heat. Because the exhaust exits at low temperatures (around 45°C to 55°C), condensing heaters do not use hot metal flues. Instead, they vent through neat PVC or polypropylene power flues through external walls, with a drain line connected for acidic condensate disposal.
Minimum Outlets & Duct Airflow Rules
Every gas ducted heater requires a minimum number of permanently open outlets to maintain safe airflow across the heat exchanger. Running a high-capacity heater with too few vents open triggers the internal high-limit safety thermostat, causing the burner to shut down to prevent cracked exchangers.
- Airflow Rule of Thumb: Central gas ducted heating systems require approximately 30 to 35 Litres per second (L/s) of supply air per kW of heat output to deliver comfortable 45°C discharge air at the floor or ceiling registers.
- Minimum Open Vents: A 60 MJ unit requires at least 4 open outlets; an 80–95 MJ unit requires at least 6 to 7 open outlets; and a 120–140 MJ unit requires at least 9 to 11 open outlets.
- Zoning Precautions: If your home has electronic duct zone dampers (e.g. living zone vs bedroom zone), ensure that each individual zone contains enough outlets to meet the heater manufacturer's minimum operating airflow limit, or install an automatic spill damper.
- Return Air Grille Sizing: Undersized return air grilles create loud rushing air noise in hallways. Allow at least 0.25m² of free face area for every 1,000 L/s of fan airflow.
Gas Pipe Sizing to AS/NZS 5601
Under Australian Standard AS/NZS 5601.1:2022 (Gas Installations), the gas piping network must supply the full rated Megajoule demand of all appliances running simultaneously with a maximum pressure drop of no more than 0.12 kPa from the gas meter regulator to the appliance valve.
Because gas ducted heaters are the single largest gas consumer in an Australian home (often drawing 80 to 140 MJ/hr compared to just 36 MJ/hr for a 4-burner cooktop), undersized pipe causes burner starvation:
- Pipe Diameter Limits: A dedicated DN20 (3/4") copper line is generally sufficient for runs up to 12 to 15 metres on an 80 MJ unit. For larger 120 MJ or 140 MJ units, or pipe runs exceeding 15 metres, a licensed gasfitter must run DN25 (1") copper pipe.
- Checking Existing Lines: When replacing an old 15 kW heater with a modern high-output 30 kW unit, never assume the existing copper feed line is large enough. Use our Natural Gas Pipe Sizing Calculator to verify flow capacity and pressure drop before connecting.
- Certificate of Compliance: In Australia, replacing or installing a gas ducted heater is legally prescribed plumbing work. Your licensed gasfitter must provide a signed Victorian Building Authority (VBA) or state regulatory Compliance Certificate confirming flue clearances, pressure testing, and carbon monoxide safety.