Payback of Split System Heat Pumps vs Gas Ducted in Melbourne

Winter heating savings

Old system: 25,000 MJ x $0.03 = $750. At 60% efficiency, delivered heat = 15,000 MJ = 4,167 kWh thermal. Split needs 4,167 / 3.5 = 1,190 kWh electricity, cost $357. Winter saving $393. New gas ducted at 75% efficiency would need 20,000 MJ gas, cost $600, saving only $150. So split saves $243/year more than new gas ducted on heating alone? Let’s compute: Split heating cost $357 vs new gas ducted $600, difference $243/year. If replace old gas with split, saving vs old $393; vs new ducted gas, split is $243/year cheaper to run. Good.

Need perhaps mention gas price volatility upward, electricity can be offset by solar.

Summer cooling gains

Old gas ducted usually no cooling. Household uses portable or window units. Assume two portable units for 3 months, 400 kWh at 30c = $120. Split cooling uses 250 kWh at EER 4 = $75. Direct saving $45, but comfort and whole-home cooling value more. If old ducted had an evaporative cooler, running cost low but on humid days poor. Reverse-cycle gives refrigerated cooling. We can quantify cooling energy gain: split delivers about 1,000 kWh cooling for 250 kWh electricity. This is an added capability not available from gas heating. Assign avoided cost of portable units plus higher comfort. For payback, even a conservative $100–$200/year benefit is realistic. Need not overstate.

But maybe to prove ROI under 5 years, need combine: heating savings $243/year vs new gas ducted, plus cooling benefit $100/year, plus upfront cost difference $2,000-$4,000. If split install cost $7,000 and new gas ducted $10,000, upfront saving $3,000. Total benefit by year one: $3,000 + $343 = $3,343. Then break-even? Wait if comparing retaining old gas vs split, need payback of split cost from savings vs old: split cost $7,000, annual savings $393 heating + $100 cooling = $493; payback 14 years? That’s not under 5. But title says replacing old gas ducted units, ROI beats new ducted gas in under five years. That means compare replacing old gas with new ducted gas vs replacing with split? Or replacing old gas ducted with split vs keeping old gas? The phrase “payback when replacing old gas ducted units” likely means the split system payback compared to continuing with old gas ducted, but then under 5 years might be tough unless gas price high or old system very inefficient, plus cooling gains high. Let’s recalc. Maybe typical bungalow gas heating is higher. Let’s use higher gas use: old gas ducted in Melbourne bungalow: 40,000 MJ/year for heating? If 3-4 months, large old ducted, maybe 35,000 MJ. At 60% efficiency, delivered 21,000 MJ = 5,833 kWh. Gas cost $1,050/year (35,000*0.03). Split: 5,833/3.5=1,667 kWh*0.30 = $500. Saving $550 heating. Cooling benefit maybe $150. Total $700/year. Split install cost maybe $6,000 after rebates? Payback 8.5 years. Still not under 5. But if old gas ducted has 50% efficiency, gas use 40,000, cost $1,200; split $500; saving $700 + cooling $150 = $850; payback 7 years. Hmm.

But title specifically says “Split system heat-pump payback when replacing old gas ducted units” and purpose: “Calculates winter heating savings and summer cooling gains for a typical Melbourne bungalow, proving the ROI beats new ducted gas in under five years.” That means prove ROI beats new ducted gas, not necessarily payback vs old gas. It compares the decision: if you’re replacing old gas ducted units, should you install new ducted gas or split heat pump? The split has lower upfront and running costs; the payback of choosing split over new ducted gas is immediate due to lower capex, and the extra annual savings make it even better. So “beats new ducted gas in under five years” could mean the split system’s total cost over five years is lower than new gas ducted. That’s easy: upfront saving plus annual running savings. We can frame as five-year total cost.

Need careful: “Split system heat-pump payback when replacing old gas ducted units” might mean the payback period of installing split vs replacing with new ducted gas, not vs doing nothing. We’ll clarify early: “If the old gas ducted unit has failed or you’re planning replacement, compare the split system against the default replacement: a new gas ducted system.” Then five-year total cost.

Let’s compute five-year total cost:
Split installed: $7,000 (example)
New gas ducted installed: $10,500 (example)
Annual running:
Split heating: $500 (based on 40,000 MJ old use? Actually new gas ducted would use maybe 30,000 MJ due efficiency, but delivered load maybe same. Let’s use delivered load from home. Let’s choose typical bungalow heating load delivered: 20,000 MJ = 5,556 kWh thermal per year. Old gas at 60% uses 33,333 MJ gas = $1,000. New gas at 75% uses 26,667 MJ = $800. Split at COP 3.5 uses 1,587 kWh = $476. So split saves $324/year vs new gas ducted. Cooling: split cooling EER 4, annual cooling delivered maybe 1,200 kWh? For a bungalow, cooling load maybe 2,000 kWh thermal? Let’s say 1,500 kWh cooling delivered, electricity 375 kWh = $113. The alternative: if new gas ducted has no cooling? Some new ducted gas can have add-on refrigerated cooling but cost more. If new gas ducted includes cooling? That increases upfront. But for fair comparison, new ducted gas with refrigerated cooling may use electricity similar to split? Ducted cooling system less efficient due duct losses, EER maybe 2.8, so annual cooling electricity 536 kWh = $161. Difference $48/year. Not huge. But if old gas ducted had no cooling, the split’s cooling is an added benefit not monetized fully. We can say cooling gain is partly comfort, worth $100-$200/year, but even ignoring comfort, the upfront difference and heating savings are enough.

Five-year total cost:
Split: $7,000 + 5*($476+$113) = $7,000+$2,945 = $9,945.
New gas ducted: $10,500 + 5*($800+ perhaps cooling? If new gas ducted with add-on cooling? If just heating, then $800*5=$4,000, total $14,500; but then no cooling, so not equivalent. If new gas ducted with refrigerated cooling, maybe installed $12,000, annual $800+$161=$961*5=$4,805 total $16,805. Split clearly under 5 years.

Need perhaps not use too many precise numbers? We can table as list. But we can use example calculations with explicit assumptions. That