Reverse-Cycle Marine Air Conditioning vs Diesel Heating
South of about Sydney the interesting question is not cooling at all. Almost nothing has been written about this comparison for Australian conditions, and the two are less interchangeable than they look.
The short answer: alongside with shore power, reverse cycle is cheaper to run and does both jobs. At anchor, in the cold, or once the water drops below about 7 °C, a diesel heater is the one that still works.
Plenty of southern boats end up with both, and that is not extravagance — they fail in different conditions.
Reverse-Cycle Marine Air Conditioning vs Diesel Heaters
Efficient, while the water is warm
The same machine that cools runs backwards and pulls heat out of the seawater. Around 3 kW of heat for 1 kW of electricity in 12 °C water — cheaper to run than anything else on the boat.
- One system, both jobs, no extra install
- Very cheap to run on shore power
- Dry heat with no combustion products or moisture
- Output falls away below about 7 °C seawater
- Needs mains power or a substantial bank — not an anchor-out solution
Right if you are on a berth, in Sydney, Melbourne or Hobart, and you want one system for the whole year.
Works anywhere, always
Burns diesel from the boat's own tank and blows hot air. Indifferent to how cold the water is, and needs no shore power.
- Full output at any water temperature
- Runs at anchor on a modest 12 V draw for the fan and pump
- Dries the boat out, which in a southern winter is half the point
- Heats only — no help at all in a Queensland summer
- Fuel, an exhaust through the hull, servicing, and a combustion appliance aboard
Right if you cruise in the cold, anchor out, or want heat that does not depend on a marina.
Boat Heating at a Berth, at Anchor and in Cold Water
| Situation | Reverse cycle | Diesel heater |
|---|---|---|
| Marina berth, Sydney winter | Ideal — cheap and effective | Works, but you are paying for diesel next to a power point |
| At anchor, Tasmania | Impractical — the bank will not carry it | The right tool |
| Seawater below 7 °C | Output falls sharply | Unaffected |
| Damp boat, condensation | Helps — dry heat | Helps more — high airflow, very dry |
| Brisbane, February | The whole reason to own it | No use whatever |
| Underway | Needs a generator or big inverter | Runs happily off the bank |
Choosing Boat Heating for Southern Australia
If the boat lives on a berth and you want one system, reverse cycle is the better buy — it covers the occasional hot week as well as the winter, and running cost is a fraction of diesel.
If you cruise Bass Strait, Tasmania or the southern coast and spend nights at anchor, fit the diesel heater first. Reverse cycle is a comfort at the dock; a heater that works regardless of water temperature and shore power is closer to equipment.