How do I choose a hot water heat pump?

A hot water heat pump is one of the simplest ways to cut a power bill. An electric storage tank turns one unit of electricity into one unit of heat; a hot water heat pump gives three to four, and hot water is typically 23 to 25% of a home’s energy bill. A few years ago there were only a handful on the market, and now there are dozens with smart names and big claims. Phil sees a lot of people let down by the one they chose, so here’s what he checks.

What efficiency figure should I trust?

Every heat pump is sold on its COP, the units of heat it makes for each unit of electricity. A COP of 4 means 400%. The trick is in the test conditions. A heat pump finds it much easier to make lukewarm water from warm air, so some sellers quote their COP with the air at 25 or 30 degrees and the water at 45, and the number looks wonderful. Your tank has to hold water at 60 degrees, and the air on a Perth winter night is a lot colder than 25.

Ask for the figure at about 15 to 20 degree air and 60 degree water. At those conditions, Phil looks for a COP of about 3.8 to 4. They mostly use the same technology and the same physics, so the honest figures land close together. His view of the bigger claims: “if it’s something higher than four I don’t particularly believe them.”

What about the compressor and the noise?

The compressor is the engine of the heat pump, and some last far better than others. Many cheap units come out of the same couple of factories with different badges on them, so the name on the front tells you little. Ask what compressor is inside, and ask someone who repairs refrigeration for a living which ones they’d want in their own house.

For noise, look for a sound power level tested to a named standard, below 60 decibels and ideally 52 to 56. A good one sounds like a big fridge humming. You won’t notice it in the day, but you will at night if it’s under a bedroom window, so put it by the laundry or the bathroom. We go into noise properly in how loud a heat pump is and how to keep it quiet.

How do I keep the heat I’ve paid for?

A heat pump makes hot water in the day, often while your solar panels are producing, and the tank has to keep it hot until the morning showers. Most tanks have about 35 to 40 millimetres of insulation, which is fine. Some have 70, and those lose their heat far more slowly overnight. The pipes and valves coming off the tank get hot too, so they should all be insulated, with a lagging that stands up to the Australian sun.

Phil’s take on two popular features is short. A phone app: “I think it’s a complete gimmick. A good heat pump is efficient and you just let it do its own thing.” Timing it to your solar panels is a good idea if it’s easy and cheap to set up. If it isn’t, don’t worry, because a heat pump uses only a few kilowatt-hours a day and the difference on your bill is small.

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What maintenance does a hot water heat pump need?

Mostly one part: the anode. Most tanks have one, a metal rod that slowly corrodes so the tank doesn’t. On most units it’s a sacrificial anode that needs checking and replacing every few years, more or less often depending on your water. Forget it and the tank itself starts to go.

Some heat pumps use an electric anode instead, which protects the tank with a tiny current and needs no maintenance at all. The one condition is that the unit stays plugged in, because it can only be switched off for half a day or so before the protection stops. Phil prefers the electric kind, but either works as long as you know which one you have.

The other thing worth having is an electric backup element. Phil’s rule is that hot water should always have a backup, because you can live a few days without heating and nobody wants several days of cold showers waiting for a repair. The element switches on by itself if the heat pump can’t keep up or develops a fault.

What size do I need?

Smaller than most people think. The tank stores water at 60 degrees, and at the shower it’s mixed with cold to around 40, so a tank delivers a good deal more hot water than its label says. In Phil’s example, a tank of about 220 litres gives about 360 litres of 40 degree water once it’s mixed. His rule of thumb is that a tank of about that size suits a household of up to four, and a household of six wants one of about 300 litres. A tank that’s too big costs more and wastes heat keeping water hot that nobody uses.

On price, listed hot water heat pumps of up to 425 litres earn small-scale technology certificates under the federal scheme. The model has to be on the Clean Energy Regulator’s register, and most people sign the certificates over to the installer in exchange for a lower price. Check that the exact model you’re quoted is listed before you sign.

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Where to go from here

If you’re in Perth or the south-west, the 5-minute quote gives you your options in writing within 3 working days, each with its price and a modelled running cost. Mention your current hot water system and how many people live in the house, so it’s sized properly. If you’re replacing a gas unit, read should I replace my gas hot water with a heat pump first, and our heat pump page covers the hot water side in brief.