Wood Fireplace Installation in Mulgrave: Flue Through Existing Chimney
Wood fireplace installation Mulgrave case study — new wood heater into existing chimney, flue liner, hearth pad, compliance. Prime Plumbing…
How Does a Heat Pump Hot Water System Work? A Plumber's Explanation
How Does a Heat Pump Hot Water System Work? A Plumber's Explanation, a Prime Plumbing & Gasfitting plain-English guide to how does a heat pump hot water system work for Melbourne homeowners. Below we cover what works, what doesn't, and when to call a licensed plumber.
Quick answer: A heat pump hot water system works on the refrigeration cycle: the same physics as your fridge or air conditioner, run in reverse to heat water rather than cool air. A small compressor circulates refrigerant through a closed loop. Outside, an evaporator coil absorbs heat from the surrounding air; inside the unit, a condenser coil releases that heat into water in an insulated storage tank. The compressor uses electricity, but the heat moved into the water comes mostly from the ambient air outside, which is why heat pumps are 3-4× more efficient than direct electric water heating. Modern Melbourne-suitable units operate efficiently down to about -5 °C ambient and use about 1 kWh of electricity to deliver 3-4 kWh of heat. The trade-offs are: higher upfront cost (offset by VEU + STC rebates), the unit needs outdoor space with airflow, and there's compressor noise (~45-55 dB at the unit).
The basic physics, heat pumps move heat, they don't make it
The thing that confuses people about heat pumps is the name. They don't pump heat in the way a water pump pumps water. They use a small amount of energy (electricity) to move heat from one place to another, from a cooler place (the air outside) to a hotter place (the water inside the tank).
This works because of a fundamental physics principle: a fluid that changes state (liquid to gas, gas to liquid) absorbs or releases heat as it does so. The refrigerant inside a heat pump cycles between liquid and gas, picking up heat in one place and dropping it off in another.
Compare to direct electric resistance heating: 1 kWh of electricity gives you exactly 1 kWh of heat. No more.
In a heat pump, 1 kWh of electricity drives a process that moves 3-4 kWh of heat from outside to your water. The heat itself wasn't created, it was already there in the ambient air outside, and the heat pump just relocated it.
This is what people mean when they say heat pumps are "300-400% efficient." They're not breaking thermodynamics, they're just measuring a useful ratio (heat delivered ÷ electricity used) rather than the underlying energy transfer.
The four components of the refrigeration cycle
A heat pump hot water system has four main components, all working together in a closed loop:
1. Compressor
The compressor is the only part that uses serious electricity. It takes refrigerant gas at low pressure and squeezes it to high pressure. Compressing a gas heats it up, same reason a bike pump gets warm in use, or a hand pump on a tyre warms the inlet end.
By the time the refrigerant exits the compressor, it's a hot, high-pressure gas, significantly hotter than your target hot water temperature. This is the working fluid that's about to release its heat into your tank.
Compressors in modern heat pumps are typically scroll or rotary types, quieter and more reliable than the reciprocating compressors used in older designs.
2. Condenser
The hot, high-pressure refrigerant gas flows through a condenser: a coil of pipe in close thermal contact with the water in your storage tank.
As the refrigerant releases heat into the cooler tank water, it condenses from gas back to liquid. The water in the tank gets warmer; the refrigerant gets cooler.
By the time the refrigerant exits the condenser, it's a warm liquid at high pressure. The water in the tank has been heated by a few degrees per pass through this stage.
3. Expansion valve
The high-pressure liquid refrigerant flows through an expansion valve (sometimes called a thermal expansion valve or TXV). This is essentially a precise restriction, a small orifice that drops the refrigerant pressure dramatically as it passes through.
Dropping pressure on a liquid causes it to flash partially to gas. This phase change is endothermic, it absorbs heat from itself, dropping the refrigerant temperature dramatically. By the time it exits the expansion valve, the refrigerant is a cold mixture of liquid and gas at low pressure.
4. Evaporator
The cold refrigerant flows through the evaporator: typically the outdoor coil with the fan blowing across it.
The refrigerant is now colder than the surrounding outdoor air. Heat naturally flows from warm to cold, so heat moves from the outdoor air into the refrigerant. As the refrigerant absorbs heat, it evaporates completely from liquid to gas.
By the time the refrigerant exits the evaporator, it's a low-pressure cool gas, and ready to be sucked back into the compressor for another loop around the cycle.
The fan blowing air across the outdoor coil is what makes heat pumps audible. It's also why airflow around the unit matters, if the airflow is restricted, the evaporator can't pull enough heat from the air, and efficiency drops.
Putting it together: the loop in operation
In normal operation, the four components work together continuously:
- Compressor pressurises and heats refrigerant gas
- Condenser releases heat to water tank, refrigerant condenses to liquid
- Expansion valve drops pressure, refrigerant cools dramatically
- Evaporator absorbs heat from outdoor air, refrigerant evaporates back to gas
- Cycle repeats
For every 1 kWh of electricity the compressor uses, typically 3-4 kWh of heat ends up in your water tank, depending on outdoor temperature, target water temperature, and unit design.
The performance metric for this is COP (Coefficient of Performance). A heat pump operating at COP 3 delivers 3 kWh of heat per 1 kWh of electricity used. A heat pump operating at COP 4 delivers 4 kWh per 1 kWh.
Modern Melbourne-suitable heat pumps run at COP 3-5 in normal conditions, dropping toward COP 2-3 on the coldest winter nights when there's less heat available in the outdoor air. Even at the cold-night minimum, they remain substantially more efficient than direct electric heating.
What's inside the storage tank
The water side of the system is essentially a normal insulated hot water tank, similar to what you'd see on an electric or gas storage unit. Typical residential sizes:
- 170 L: small household, 1-2 people
- 270 L: typical mid-range residential, 2-4 people
- 315 L: larger household, 4-5 people
- 400+ L: large household, 5+ people, or twin-bathroom heavy use
The tank holds water at a target temperature (typically 60-65 °C in the tank, mixed down to 50 °C at the outlet via a tempering valve under AS/NZS 3500.4). A thermostat senses tank temperature and switches the compressor on when temperature drops below setpoint.
Most heat pumps include an electric resistance backup element: a heating element similar to what you'd see in a regular electric storage unit. This kicks in only when the heat pump can't keep up (very cold day, very high demand, or compressor fault). Day-to-day, you don't want the backup running because it negates the heat pump's efficiency advantage. Properly sized units rarely need backup in Melbourne conditions.
Efficiency considerations day-to-day
Several factors affect how efficient your heat pump is in actual operation:
Time of day
Running the heat pump during the warmer middle of the day is more efficient than running it overnight when ambient temperatures are lower. Many modern units have programmable schedulers that let you align hot water production with daytime hours.
Solar PV alignment
If you have solar PV, scheduling the heat pump to run during peak solar production (10am-3pm typically) means most of the running cost is offset by self-consumption of solar electricity. This is the configuration where heat pump operating cost approaches zero.
Tank temperature setpoint
Higher setpoint = more energy per litre of hot water + less heat available to mix down at the outlet. Most installs run at 60-65 °C tank temp; pushing higher costs more energy.
Tank insulation
Modern heat pump tanks are well-insulated, standby heat loss is typically less than 1 kWh/day. But the tank still loses heat over time, so sizing the tank to your actual usage matters. Oversized tanks waste energy keeping water you're not using hot.
Hot water usage pattern
Households that use most of their hot water in concentrated bursts (morning shower rush) put more strain on the heat pump's recovery rate than households that spread usage. Properly sized units handle either pattern, but the sizing calculation should reflect your usage.
Service area
Prime Plumbing & Gasfitting installs and services heat pumps across Melbourne's eastern, south-eastern, inner-east, and bayside suburbs. See all suburbs we service
Book a heat pump consultation
Call 0475 407 670 or send through the contact form. Tell us your existing hot water type, household size, outdoor space available, and whether you have solar PV, we'll quote with rebates calculated and explain the technology choice for your specific situation.
- BPC #103414: Plumbing Industry Commission licensed
- Type A Gasfitter: registered with Energy Safe Victoria
- VEU and STC accredited: rebates applied at quote time
- 4.8 stars on Google
Integrated unit vs split system
Heat pumps come in two main configurations:
Integrated (compact / monoblock)
The compressor, evaporator, condenser, and tank are all in one unit, typically installed outside or in a covered patio area. All-in-one means simpler install, just water and electrical connections, no refrigerant lines to run.
Trade-off: the entire tank is exposed to outdoor temperatures, which can slightly reduce efficiency in winter.
Split system
The compressor and evaporator are in an outdoor unit (similar to an air conditioner outdoor unit). The condenser is wrapped around the tank, which sits indoors or in a sheltered location. Refrigerant lines connect the two.
Trade-off: more complex install (refrigerant work needs licensed refrigeration mechanic), but better efficiency in extreme conditions because the tank is sheltered.
For most Melbourne residential installs, integrated units are the more common choice, simpler, cheaper, and adequate for the climate. Split systems are sometimes preferred for larger systems or cooler-climate sites.
Why it works in Melbourne specifically
Heat pumps are most efficient in moderate climates, and Melbourne is squarely in heat-pump territory.
The relevant numbers:
- Melbourne average winter overnight low: typically 5-8 °C in the metro area, occasionally dropping to 0-3 °C
- Melbourne average daytime temperature: typically 12-18 °C in winter, 20-30 °C in summer
- Modern heat pump operating range: efficient operation down to -5 °C, useful operation down to -10 °C for cold-climate units
So most of the year, a Melbourne heat pump operates in temperatures where it's running at 3-4× efficiency. On the coldest nights, efficiency drops but the unit keeps producing hot water. There's no scenario in Melbourne where a heat pump simply stops working.
For comparison, heat pumps designed for sub-Arctic climates exist (Reclaim, Sanden CO2 heat pumps, etc.) and operate efficiently down to much colder temperatures. They cost more upfront but are unnecessary for most Melbourne installs.
When to call a licensed plumber for a heat pump
The Victorian Building Authority maintains a plumbing licence search for verifying any installer.
Frequently asked questions
No, the compressor cycles on and off based on tank temperature. After a hot water draw, the compressor runs until the tank reaches setpoint, then switches off until the water cools (or another draw triggers reheat). Most heat pumps run a total of 4-8 hours per day in residential use.
The heat pump stops working. The hot water already in the tank stays usable for the duration of the outage (insulated tanks hold temperature for hours to days depending on draw). When power returns, the compressor restarts and reheats the tank. There's no "manual restart" needed.
Annual or biennial check is recommended, clear debris from the outdoor coil, check tempering valve operation, check the sacrificial anode (extends tank life), confirm electrical connections are sound. Refrigerant typically lasts the life of the unit; the refrigerant circuit is sealed and shouldn't lose charge unless there's a fault.
Most current Melbourne residential heat pumps use R134a, R290 (propane-based, very low global warming potential), or R744 (CO2). The trend is toward lower-GWP refrigerants. The refrigerant choice doesn't materially affect day-to-day operation but does affect environmental footprint and (in some cases) cold-climate efficiency.
The output is then mixed down to 50 °C at the outlet via a tempering valve (mandatory under AS/NZS 3500.4). Same arrangement as gas or electric storage units.
It increases your electricity use modestly (typically $250-$500/year for a typical household) but eliminates gas use for hot water (typically $700-$1,100/year saved). Net effect: lower overall energy cost. With solar PV, the increase in electricity use is largely self-consumed solar, so the net cost reduction is bigger.
The footprint is similar; the visual bulk comes from the outdoor compressor unit, which is a separate component a gas system doesn't have. The tank itself is roughly the same size as a comparable gas storage tank. For installs where space is tight, smaller integrated units exist (170-200L) at the cost of less stored hot water.
Before You Book
A quick checklist to share with your plumber when you book:
- When did the issue start?
- Is it isolated to one fixture or multiple areas?
- Are there any visible leaks, smells or unusual sounds?
- Have you turned off the relevant isolation valve?
- Heat pump hot water installation Melbourne: supply and install with VEU and STC rebates
- Hot water systems Melbourne: full hot water service: gas, electric, heat pump
- Hot water replacement Melbourne: like-for-like and upgrade replacements
- See also: Heat Pump Hot Water Rebate Victoria 2026: VEU and STC rebate explainer
- See also: Heat Pump vs Gas Hot Water in Melbourne: running cost and reliability comparison
