Buying a heat pump hot water system: the seven factors that actually matter
Monday's post covered what a heat pump hot water system is. Here is how to judge one, in the order the factors usually matter for a Melbourne home.
1. COP: the efficiency number
The coefficient of performance tells you how many units of heat you get per unit of electricity. Quality units deliver COP 3.5 to 4.5 or better; bargain units can sit closer to 3. Check the number is quoted at a realistic air temperature, not only the lab-friendly one: performance drops as air gets colder, and Melbourne winter mornings are the exact moment you want hot water.
2. Tank size
Roughly 50 litres per person is the standard rule, so a family of four wants around 200 to 300 litres. Too small means cold showers when guests stay; too big wastes energy keeping unused water hot. Heat pumps reheat more slowly than gas, so sizing matters more than it did with your old system.
3. Noise
The compressor is an outdoor unit like a small air conditioner, and it runs for hours at a time, often on a daytime timer. Quality units run at 37 to 45 decibels, budget units meaningfully louder. Check the dB rating on the datasheet and think about where it will sit relative to bedrooms, including your neighbour's. This is the number one regret in cheap installations.
4. Refrigerant
The current good answer is CO2 (R744) or propane (R290) refrigerants: they perform better in cold air and carry no phase-out risk. Older refrigerants still work but check cold-weather performance carefully. CO2 units, notably the Japanese brands, excel in cold climates at premium prices.
5. Cold-climate performance
Ask directly: what is the unit's minimum operating temperature, and does it have a backup element for frosty mornings? Melbourne rarely goes below minus 2, so almost anything copes, but the COP of a budget unit can sag badly in winter exactly when hot water use peaks.
6. Warranty, split honestly
Look for the warranty on the tank (5 to 10 years typical), the compressor and refrigeration circuit (often shorter, and the part that fails), and labour. A "7 year warranty" that covers the tank only is marketing. Get the split in writing.
7. The installer and the rebate stack
The three-way rebate stack (VEU, Solar Victoria, STCs) means a heavily discounted price is normal, not automatically suspicious. What separates good from bad is everything above, plus a licensed plumber and electrician doing the work and an itemised quote showing each discount. If the deal only works with a brand you cannot find independent reviews for, that is your answer.
The short version
Buy a quiet, well-warranted unit from an established brand, sized to your household, from an installer who itemises the rebates. Expect to pay something real out of pocket; free rarely survives seven Melbourne winters. Figures last checked 31 August 2026.