A hot water heat pump heats your water the way a fridge cools food, only backwards. Instead of pumping heat out of a box, it pulls heat out of the outside air and pushes it into a cylinder of water. It runs on electricity, but it uses a small amount of power to move a much larger amount of heat. That makes it roughly three times as efficient as a standard electric hot water cylinder.
It’s a different machine from the heat pump on your lounge wall. That one heats the air in a room. A hot water heat pump only heats water.
How does a hot water heat pump work?
- A fan draws outside air across a coil filled with refrigerant.
- The refrigerant soaks up heat from the air and turns into a gas.
- A compressor squeezes the gas, which makes it much hotter.
- That hot gas passes its heat to the water in the cylinder, turns back into a liquid and goes round again.
It keeps cycling until the water reaches the set temperature. Cloud, rain and the middle of winter don’t stop it, because it takes heat from the air, not from the sun. Plenty of people mix it up with solar water heating.
→ Related: Heat pumps and solar power in NZ
What are the main types of hot water heat pump?
All-in-one (integrated). The heat pump sits on top of, or alongside, the cylinder as one unit. It takes up less space and can be simpler to install. It’s heavier, and it goes outside.
Split system. A separate outdoor heat pump unit is piped to a cylinder that can be inside or outside. The cylinder can sit closer to the bathroom and kitchen, which suits homes with little outside space. Some split units connect by refrigerant pipe, and the installer then also needs a refrigerant handling certificate.
Who does a hot water heat pump suit?
A hot water heat pump costs more to buy and install than a standard cylinder, and you get that back through lower power use. So it pays off best for:
- Medium to large households with steady hot water use. A family with teenagers doing daily showers and washing will save far more than a couple will.
- Homes on LPG instantaneous hot water. LPG is relatively expensive to run, so switching to a heat pump water heater gives a big drop in yearly energy costs.
- Homes with solar panels, or a power plan with cheap off-peak hours. The cylinder stores the hot water, so the heat pump can run when power is cheapest.
- Anyone replacing a cylinder that’s failing anyway.
And it suits these less:
- One or two people who use little hot water. They’re unlikely to save enough to cover the extra cost.
- A cylinder that’s working fine. Replacing a perfectly good water heater just to switch type is unlikely to pay for itself.
- Homes with a ring main, the loop that keeps hot water circulating to the taps. It loses heat and drags down the heat pump’s efficiency, and most units aren’t made to work with one.
→ Related: How to choose a heat pump
What size hot water cylinder do I need?
Bedrooms are the usual starting point, because they’re a fair guide to how many people live there. As a rough guide to daily hot water use from the cylinder:
- 2 bedrooms: about 100 litres a day
- 3 bedrooms: about 150 litres
- 4 bedrooms: about 200 litres
- 5 bedrooms: about 250 litres
- 6 bedrooms: about 300 litres
The installer then adjusts for your household: a big bath, a washing machine on hot or a lot of visitors all push it up. Bigger isn’t automatically better here either. An oversized system costs more and loses more heat from the cylinder.
Where should a hot water heat pump go?
The outdoor unit needs good airflow and space around it. The cylinder should be fairly close to the bathroom, kitchen and laundry, because long pipe runs waste heat while the water travels.
Good placement also means:
- Away from bedrooms and the neighbours. Ideally not within 2 metres of a bedroom or 3 metres of a neighbour’s window.
- Out of heavy leaf fall, so the fan stays clear.
- With access, because the unit is heavy and needs a clear path for installing and servicing.
- Garages and carports with care. An all-in-one unit can sometimes go in a garage if there’s enough airflow, but it will make that space colder.
→ Related: Heat pump noise: what’s normal and what isn’t
Are hot water heat pumps noisy?
There’s a compressor and a fan in there, so it makes some noise, much like a fridge or a heat pump outdoor unit. Most run at around 40 to 55 decibels. That’s between a quiet library and moderate rain. Where it goes matters more than which model you buy, which is why those placement rules keep it away from bedrooms. Many units also have timers, so you can choose the hours it runs.
Do hot water heat pumps work in a cold climate?
A hot water heat pump gets less efficient as the air gets colder, and on frosty days it has to stop now and then to clear ice off its outdoor coil. While it defrosts, no heat goes into the water. A unit that’s too small, or not suited to the local climate, defrosts more often and can leave you short of hot water.
Most models sold in New Zealand are rated to work down to minus 5°C or lower, and some go further. Many also have an electric backup element for the coldest days, or when the house is full of guests. That element heats water the same way an ordinary electric cylinder does, with none of the heat pump’s efficiency, so you want it running as little as possible.
If you live somewhere with hard frosts, like Central Otago, the Mackenzie Basin or inland Canterbury, ask for a model with good low-temperature performance. A unit that suits Auckland may lean on its backup element far too often in Alexandra.
Salt air on the coast and sulphur in geothermal areas like Rotorua can also shorten the life of a unit. Ask the installer whether the model has a coating or treatment for those conditions.
→ Related: Best heat pumps for cold NZ climates
What’s involved in a hot water heat pump install?
Most installs need two trades: a licensed plumber and a registered electrician. A typical unit draws around 0.9 kW, and a backup element can draw another 1.5 to 3.5 kW, so the electrician needs to check your wiring can take it. Replacing an existing water heater is generally exempt from building consent, but the plumbing must still be done by an authorised person. See who can install a heat pump and heat pump building consent for the details.
Who installs hot water heat pumps near me?
Browse installers who fit heat pump hot water systems, or pick your town to see who works near you.