Heat pump hot water: the three things that decide whether one suits your house
The efficiency claim is real, and it is not the interesting part. What sinks a heat pump installation is almost always physical — where the compressor noise lands, how much air it can draw, and what happens on a 2 °C morning.
A heat pump hot water system is an air conditioner running in reverse: instead of burning energy to make heat, it moves heat that is already in the outdoor air into a tank of water. That is why it uses roughly a third of the electricity of a conventional element for the same hot water. The efficiency is not in dispute. What decides whether one belongs at your house is entirely physical, and it comes down to three questions: where does the compressor noise go, can it draw enough air, and how cold does it actually get here in winter? Get one of them wrong and you have an expensive appliance that annoys the neighbours, or that quietly runs on its backup element all winter.
What is a heat pump hot water system, exactly?
It is a storage tank with a small refrigeration unit attached. A fan pulls outdoor air across an evaporator, a refrigerant picks up that low-grade heat, a compressor raises its temperature, and the heat goes into the water. The tank itself is ordinary — same cylinder, same relief valve, same sacrificial anode as a resistance unit — so the maintenance story is the same as any other storage system. The two physical arrangements matter more than the brochures suggest:
| Arrangement | What it looks like | Where it suits | Where it causes trouble |
|---|---|---|---|
| Integrated | Compressor sits on top of the tank as one unit | A single outdoor spot with clear air and a drain nearby. Simplest install. | The noise is wherever the tank is — which is often the side path next to a bedroom. |
| Split | Compressor is a separate box, tank sits elsewhere, refrigerant lines join them | Houses where the only sensible tank spot is a bad noise spot. Lets you move the noise away. | More install work, refrigerant line runs are length-limited, and more joins to go wrong. |
How much noise does a heat pump make, and where should it go?
This is the complaint that comes up again and again, and it is entirely avoidable at the planning stage. A heat pump has a fan and a compressor, so it makes a low mechanical hum whenever it is heating — comparable to an outdoor air conditioning unit, and in the same low-frequency range that carries through walls rather than bouncing off them. Two practical rules:
- Ask for the sound power level in dB(A) from the manufacturer's specification sheet, not the salesperson's description of it. Units vary considerably and the figure is published.
- Do not site it under a bedroom window — yours or next door's. The narrow side path is the default spot for a hot water unit in most Australian houses, and very often exactly the wrong spot for a heat pump. Noise rules for residential plant apply, and they vary by state and council.
Most units also let you set a timer window. Running it in the middle of the day suits anyone with rooftop solar and moves the noise out of the hours when it will irritate someone; running it overnight on a cheap tariff is the opposite trade, and the one people regret.
What clearances and airflow does one need?
A heat pump is only efficient if it has air to work with. Box it into a tight alcove and it recirculates its own chilled exhaust, the efficiency collapses, and on a cold day it falls back on the electric element — which is the outcome you paid to avoid. Every model publishes minimum clearances in its installation manual: get them in writing on the quote and check them against the spot you have in mind before anyone turns up. Three things to confirm:
- Clear air in and clear air out, with nothing that lets the cold exhaust loop back around to the intake.
- Condensate drainage. A heat pump pulls moisture out of the air, so it dribbles condensate whenever it runs. That water needs a legal discharge point — not a path where it grows algae, and not the footings.
- A base that will not move: vibration through a slab or a timber deck turns a modest hum into a genuine nuisance.
Do heat pumps work in cold climates?
Yes, but with a caveat that catches people inland and at altitude. A heat pump's efficiency depends on how much heat is in the outside air, so its coefficient of performance falls as the temperature drops, and every unit has a minimum operating temperature below which it stops using the refrigeration cycle and switches to a plain electric element. If your winter mornings regularly sit near or below about 5 °C, ask two specific questions:
- Is this model rated for cold climates, and what is its minimum operating temperature? Some are engineered for it and some are not, and the datasheet says which.
- What refrigerant does it use? It changes cold-weather behaviour, and it changes what servicing looks like years from now. It is a fair question and a competent installer will answer it without flinching.
Frost is the other cold-climate detail. In humid cold the evaporator ices up and the unit runs a defrost cycle, costing it time and efficiency — normal behaviour, not a fault, but it does mean a unit in a cold inland town will not perform the way the coastal brochure figures suggest.
Sizing: heat pumps recover slowly, so volume matters more
A heat pump delivers heat gently. It cannot slam a tank back up to temperature the way a gas burner or a large element can, so its recovery rate is slower by design, and stored volume does more work for you here than on other system types. Size it on how many hot outlets run in the same twenty minutes, not on household headcount, and ask for the volume and the recovery rate.
What to confirm on the quote
Beyond the tempering valve, relief valve discharge, isolation valve and compliance certificate that belong in any hot water installation, a heat pump adds two of its own: the manufacturer's clearances, met and documented, and condensate drainage to a legal discharge point. Both are cheap to specify up front and awkward to fix once the unit is bolted down.
How far can you take this yourself?
You can do the thinking, and the thinking is most of it: measure the spot, check the clearances against the manual, work out where the noise lands, read the datasheet for the minimum operating temperature, and get the questions above written onto the quote. None of that needs a licence.
This far, and no further. Installing or replacing a hot water system is licensed plumbing work everywhere in Australia, the electrical connection is licensed electrical work, and the refrigerant circuit needs the relevant refrigerant handling authorisation. Do not open the electrical enclosure to look at the element or thermostat, even with the power off at the switchboard. Licensing and compliance paperwork are set by each state and territory, so check the requirements with the plumbing regulator where you live before you accept a quote — and ask the installer for their licence number.
Frequently asked questions
Is a heat pump hot water system worth it?
Energetically, yes — it uses roughly a third of the electricity of a conventional electric element for the same hot water. The practical caveats are all physical: somewhere to put it with clear airflow, a spot where compressor noise is not under a bedroom window, condensate drainage, and a model rated for cold climates if your winter mornings drop near or below about 5 °C.
How noisy is a heat pump hot water system?
It makes a low mechanical hum from the fan and compressor whenever it is heating, comparable to an outdoor air conditioning unit and in the low-frequency range that carries through walls. Ask for the published sound power level in dB(A) from the manufacturer's specification sheet before you commit, and do not site it under a bedroom window — yours or a neighbour's.
Do heat pump hot water systems work in cold weather?
They work, but efficiency falls as the outside temperature drops, and every unit has a minimum operating temperature below which it switches to a plain electric element. If winter mornings regularly sit near or below about 5 °C, ask specifically whether the model is cold-climate rated and what its minimum operating temperature is.
Why is my heat pump hot water system dripping water?
A steady dribble of condensate while the unit is running is normal — it pulls moisture out of the air the same way an air conditioner does, and that water has to drain somewhere. Water coming from the relief valve, or pooling under the tank rather than at the condensate outlet, is a different problem and needs looking at.
What size heat pump hot water system do I need?
Size on how many hot outlets run in the same twenty minutes, not on household headcount. Because a heat pump recovers more slowly than a gas or high-output electric system, stored volume does more of the work — ask for both the tank volume and the recovery rate.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Australian Government, energy.gov.au. Hot water systems — types, installation and energy performance
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)