Reverse Cycle Heating Explained (for Canberra Homes)

Reverse cycle doesn't create heat — it moves heat from outside to inside. Even at 0°C outside, there's plenty of heat energy in the air; a heat pump concentrates it.

The same fridge-cycle refrigeration loop that keeps your food cold, run in reverse, warms your living room. It's the same physics — a compressor, an expansion valve, and two heat exchangers.

Moving heat is roughly 4x more efficient than creating heat. That's why a reverse cycle unit can deliver 3–5 kWh of heat for every 1 kWh of electricity it consumes.

A resistive heater — oil column, fan heater, ceramic panel — turns 1 kWh of electricity into exactly 1 kWh of heat. There is no thermodynamic path to do better with resistance heating.

In practice a modern reverse cycle unit's real-world COP at Canberra winter conditions is 3.0–3.8. Marketing sometimes quotes 5.0+ but those are 7°C outdoor test conditions, not 0°C at 6 am.

Below −5°C the physics gets tougher — moisture in the outdoor coil freezes and the unit has to periodically defrost. Modern Hyper Heating variants still deliver full capacity down to −15°C, but they cost more.

Standard units lose 30–40% of rated heating output at −5°C. That's why sizing for Canberra should be done off the heating load at design outdoor temperature (usually −3°C for Canberra CBD, −5°C for Tuggeranong), not the cooling load at 32°C.

Every 30–90 minutes in cold conditions, the outdoor unit briefly reverses to cooling mode to melt ice off its own coil. For 3–7 minutes the indoor air blows cool because the compressor is now dumping heat outside.

Well-designed units minimise defrost frequency and duration with variable-speed compressors and smart controllers. Cheap units run predictable timed defrosts even when they don't need to, wasting energy and creating noticeable cold drafts.

A rule of thumb: bedrooms 100 W/m² heating, living areas 130–170 W/m² heating, poorly insulated homes 200+ W/m². Cooling loads in Canberra are usually lower than heating loads for the same room, which is opposite to Sydney and Brisbane.

That's why 'oversize the cooling and heating will follow' — a common shortcut — doesn't work here. You size for heating and confirm the cooling is adequate, not the other way around.

Both work. Ducted spreads heat evenly through the home; split concentrates it where the head is mounted. If you only heat one or two rooms, splits are more efficient because you're not heating unused zones.

Whole-of-home heating is much more comfortable on ducted, especially in Canberra's coldest weeks when temperature stratification between rooms is more pronounced.

Gas ducted: comparable comfort, worse running cost at ACT gas prices, and end-of-life for most Canberra gas installations. Also emissions-heavy.

Wood fires: cheap fuel if you cut your own, but a nuisance, restricted by ACT air-quality days, and requires flue maintenance.

Hydronic (in-slab or radiator): beautiful heat quality, expensive to install and repair, and typically driven by gas or heat pump anyway.

Reverse cycle wins on cost, ease and installation flexibility. It only loses on aesthetics if you specifically want a visible heat source.

First: outdoor units mounted in south-facing shaded courtyards struggle in winter. Wherever possible mount them where they get morning sun — it cuts defrost frequency noticeably.

Second: don't mount the outdoor unit under a leaking eave. Ice can form on the fan blade overnight and imbalance the fan, ruining bearings in a season.

A refrigerant liquid evaporates when it absorbs heat, and condenses back to liquid when it releases heat. Repeat that cycle continuously and you have a heat pump.

In cool mode: refrigerant evaporates indoors (absorbing heat, cooling the room) and condenses outdoors (releasing that heat).

In heat mode: a reversing valve swaps the direction. Refrigerant evaporates outdoors (absorbing heat from outside air, even at −5°C) and condenses indoors (releasing heat into the room).

The compressor does the work of pumping refrigerant around the loop. Everything else is heat exchangers, valves and sensors.

Gas ducted heaters are usually rated on gross output. Reverse cycle is rated on capacity at test conditions (7°C outdoor). At −3°C Canberra design condition, deliverable capacity is 65–75% of the nameplate.

That's why a 14 kW reverse cycle system replaces a 20 kW gas ducted system on paper but delivers similar comfort in practice. The comparison isn't apples to apples.

Always size reverse cycle on actual delivered capacity at design conditions, not nameplate. A good installer does this for free at quote stage.

Timed defrost: predictable but wasteful — runs every 45 minutes regardless. Cheap units.

Demand defrost: sensors detect coil ice and trigger only when needed. Mid-range and premium units.

Hyper-heat/Nordic variants: enlarged outdoor coil and additional refrigerant charge to reduce defrost frequency altogether. Premium and cold-climate specialised units.

In Canberra winters, demand-defrost or hyper-heat variants noticeably outperform timed-defrost systems on real-world comfort.

Mount outdoor units on brackets or plinths at least 200 mm off the ground — snow is rare in Canberra but ground frost isn't, and coil airflow suffers when it's blocked at the base.

Avoid south-facing courtyards for the outdoor unit if possible. Morning sun helps defrost performance.

Insulate refrigerant pipework to full manufacturer spec — uninsulated pipework loses efficiency in cold weather.

Add a drain-pan heater kit on outdoor units in frost-hollow suburbs (Hall, Wanniassa, Chapman, parts of Tuggeranong). Prevents ice build-up under the coil.

Comfort: reverse cycle wins on evenness. Wood wins on radiant warmth and ambience.

Running cost: reverse cycle is cheaper except for households cutting their own wood.

Emissions and air quality: wood loses badly. ACT no-burn days are becoming more common and are enforced.

Maintenance: wood needs an annual flue clean plus chimney sweep. Reverse cycle needs an annual service. Both are similar cost.

Hydronic (in-slab or radiator) delivers beautiful even radiant warmth and is silent. Install cost $30,000–$60,000+ for a whole home.

Reverse cycle ducted delivers forced-air comfort with slight airflow perception and mild noise. Install cost $12,000–$20,000 for a whole home.

The premium build market is increasingly combining both — hydronic in bathrooms and bedrooms, reverse cycle for living areas and cooling.

'Heat pumps don't work in cold weather' — they do, but standard units lose capacity below −5°C. Hyper-heat variants do not.

'Reverse cycle produces dry air' — it does slightly, which is a comfort win in humid climates and a mild negative in Canberra winters. A cheap room humidifier fixes it.

'The COP number is marketing' — it's a regulated efficiency measurement. Marketing tends to quote peak COP; real-world seasonal COP is lower but still 3–4x resistance.

Consider a 25 m² Canberra living room with west-facing double glazing, R4.0 ceiling insulation, R2.0 walls, and typical air changes.

Cooling load at 32°C outdoor: approximately 3.0 kW.

Heating load at −3°C outdoor: approximately 3.8 kW.

Nameplate capacity at 7°C outdoor: 5.0 kW is a common pick.

Delivered heating capacity at −3°C: roughly 3.5–4.0 kW on a standard unit, or 4.6–4.8 kW on a hyper-heat variant.

That's why a 5 kW nameplate is right for this room, not 3.5 kW. Sizing on the nameplate alone would under-heat on Canberra's coldest mornings.

Every 30–90 minutes in cold weather, the outdoor unit briefly reverses to cooling mode. For 3–7 minutes the indoor supply air blows cool (not cold — most modern units drop below room temp only briefly).

Premium models with variable-speed compressors handle this smoothly, and most household occupants don't notice.

Cheap or older units run timed defrost — every 45 minutes regardless — and the cold-draft moments are more noticeable, especially at night.

If defrost is frequent, longer than 8 minutes, or the indoor unit blows outright cold air during defrost, book a service. Something is off.

Standard reverse cycle: rated to keep working down to −10°C but loses 30–40% capacity below −5°C. Fine for most Canberra homes most winters.

Hyper-heat / Nordic / cold-climate variants: maintain full capacity down to −15°C or better. Cost 15–30% more up front.

Worth the premium if: you're in a frost-hollow suburb (Hall, Wanniassa, Chapman, parts of Tuggeranong), you have poor insulation, or you rely on reverse cycle as your sole heating source with no backup.

Not worth the premium if: you're in inner-city or well-insulated suburbs, you have solar-hours pre-heat, and you have a wood or gas backup for the coldest 5 mornings a year.

The ACT has committed to net-zero emissions and 100% renewable electricity. Reverse cycle heating aligns with that path — as the grid decarbonises, so does your heating.

Gas heating, by contrast, is being progressively phased out under ACT policy. New residential builds increasingly won't connect to gas at all.

Choosing reverse cycle in 2026 is choosing the direction the ACT is heading. Choosing gas is choosing a fuel with rising prices and shrinking infrastructure.