Do heat pump hot water systems work in cold climates?

"Do heat pumps work in winter?" is one of the most common questions about them, and the honest answer is that it depends on the model more than the place. The ACT Government, writing for Canberra, the coldest capital, says certain models still work efficiently in sub-zero temperatures. The ones that cause trouble are units not designed for the cold, or units quietly running on their electric element.

Why do heat pumps struggle in the cold?

Because they pull heat from the air, and cold air holds less. Your Home says efficiency "decreases as the refrigerant is less able to absorb heat" in colder climates, while noting that "many heat pumps are designed to operate efficiently in cold climates and in frequent freezing or cold and dry conditions" (Your Home).

The federal review of heat pump regulation explains the two problems: the colder the air, the harder it is to extract energy and "the more likely that frosting occurs (around zero), reducing capacity and efficiency", and at low temperatures units can take "significantly longer to reheat" (Decision RIS, April 2026).

Cold air still has plenty of heat in it, though. The ACT Government points out that "air at 0°C still has over 90% of the heat energy of air at 40°C" (ACT Government).

How cold is too cold?

energy.gov.au warns: "Not all models are designed for cold locations where it regularly drops below 5°C in winter" (energy.gov.au). Here is how the capitals compare, using Bureau of Meteorology long-term averages for the coldest month:

Sources: BoM climate statistics for Canberra, Hobart, Melbourne, Adelaide, Sydney, Perth, Brisbane and Darwin; zones from the CER postcode zones. A monthly mean minimum hides the coldest nights, which are lower.

Canberra and Hobart sit at or below the 5°C line on average. Melbourne and Adelaide are close to it. Suburbs can also differ from the CBD: the CER puts some outer Melbourne postcodes in zone 5 rather than zone 4.

What should I look for in a cold-climate heat pump?

• A low operating temperature on the heat pump alone. The ACT Government says to check "that the system can operate on the heat pump alone down to -5°C". Some units hand over to the element below a set point, which is much less efficient. For example, Thermann says its integrated unit runs from -6°C and the element activates outside that range, while Stiebel Eltron notes its SHP-O Plus uses the element "in colder climates" (see the brand overview).
• A zone 5 rating and good savings. The ACT advises models "rated for zone 5 climates with an energy saving of at least 65%".
• A high STC figure for your zone. Your Home suggests the number of certificates on the Clean Energy Regulator register is a fair way to compare efficiency for your climate zone. See STCs explained.
• The efficiency curve. Renew suggests checking the efficiency curve in the datasheet, and says many units suited to sub-zero climates use CO2 as the refrigerant (Renew).
• Frost protection and insulated pipes. Victoria's buyers guide says all water pipes should be insulated to reduce heat loss and frost damage, and that some systems include frost protection (VEU buyers guide).

Does the backup element defeat the purpose?

If it runs a lot, yes. Solar Victoria says some heat pumps "struggle in air temperatures approaching freezing and may use a resistive electric element as a booster. This can end up using quite a bit of extra energy" (Solar Victoria). Victoria's buyers guide suggests researching products "that can operate in colder conditions without the use of a booster element". A cold-climate unit that runs on the heat pump through a frosty night is the goal.

On efficiency in the cold, the ACT Government says certain models suited to Canberra's winter "can still achieve a CoP of at least 2 in sub-zero temperatures", against over 4 for most systems in summer. Even a COP of 2 uses half the electricity of an element.

Does it matter when the heat pump runs?

A lot, in a cold climate. Your Home notes that timing water heating to the warmer afternoon improves efficiency (Your Home). A unit set to heat overnight on an off-peak tariff is working in the coldest air of the day. The federal review also warns that some models need a continuous power supply to defrost and protect components in cold weather, so they "may not be able to make use of off-peak tariffs". Check the manual before wiring a cold-climate heat pump to a restricted-hours circuit.

City guides for cold climates

• Canberra: the coldest capital, with ACT loans and concession rebates.
• Hobart and Tasmania: zone 5, with no current state program.
• Melbourne: zone 4 in the CBD, with some outer postcodes in zone 5.