Heat pumps, filters, sizing and service

For many New Zealand homes the heat pump is the main heater, and heating makes up about a third of the average household's energy bill. The care is simple: clean filters, a clear outdoor unit, sensible settings, a service when it is due, and a unit that was the right size to begin with.

Clean the filters monthly through winter

Behind the front panel of the indoor unit sit one or two mesh filters. They catch dust, pet hair and cooking grease. Once they clog, the unit moves less air, works harder and costs more to run. EECA's advice is to clean them with a vacuum cleaner or warm water once a month through winter, and more often if they are dirty. If you use it for cooling in summer, keep going.

Turn the unit off, lift the front panel and slide the filters out. Let them dry fully before they go back in. If you find mould on the filters or on the fins behind them, the inside needs a professional clean.

This is not optional. Manufacturers' warranties commonly list filter cleaning as the owner's job, along with keeping the drain clear and the outdoor unit unobstructed.

Settings that save power

• Between 18 and 21°C. EECA's guidance: 18 or above to keep damp and mould at bay, below 21 to save power.
• Timer, not all day. Warm the room before you wake or get home, and turn it off when no one is there. Running a heat pump around the clock uses more energy, not less.
• Fan on auto. Low fan is quieter but less efficient.
• Cooling on cool, not auto. On auto it can switch itself to heating.

Close the doors to rooms you aren't using. If damp is the problem rather than cold, start with condensation and mould in winter.

Why sizing matters

A heat pump is sized to the room or area it heats, not to the house. EECA puts it plainly: too small and it will struggle, too big and you will pay more. An undersized unit runs flat out on a cold night, never quite reaches the set temperature, and spends more of its time defrosting.

The output on the label is measured at 7°C outside, and it falls as the temperature drops. Mitsubishi Electric's own example: a unit rated at 5.6 kW is likely to give about 4.76 kW at 0°C. Ask whether the unit has an H2 figure, its output at 2°C under a voluntary rating EECA introduced. That is closer to a cold New Zealand night.

A good supplier comes to the house, looks at the size of the room, the windows and your climate, and recommends a size from that. Ask for the sizing in writing. One major brand's New Zealand warranty states that the heat load calculation is the installer's responsibility, not the manufacturer's, so if a unit turns out too small, that conversation is with whoever chose it.

Keep the outdoor unit breathing

The outdoor unit needs air moving freely through it. Keep plants, firewood, bins and fences clear of its sides and front, and clear leaves from around it each autumn. A unit boxed in under a deck or behind a tight screen struggles in cold weather.

Water on the ground beside it in winter is normal. In heating mode the outdoor coil gathers frost, and the unit pauses now and then to melt it off. Water dripping inside the house is not normal.

Near the coast, salt attacks the coil and the casing. Rinse the outdoor unit gently with fresh water from a hose from time to time, never with a water blaster, which flattens the fins. If you are buying for a coastal property, ask about corrosion protection before it goes in. At least one major brand's warranty does not cover corrosion where protection suited to the site was not specified.

The service

Advice on frequency differs. Manufacturers such as Rinnai and Hitachi recommend a professional service every year for a unit in regular use, ideally before winter. Consumer NZ's view is that if you keep up the cleaning yourself, a technician every few years is enough unless something goes wrong. Your warranty settles it for you. Rinnai's, for example, says that missing the annual service during the warranty period may void it. Read yours, and if it asks for a yearly service, book it every autumn.

A service should cover cleaning the indoor coil and fan, clearing the condensate drain, cleaning the outdoor coil, checking the electrical connections, and running the unit in heating and cooling to confirm it performs as it should.

Call a technician sooner for new noises, water dripping from the indoor unit, or poor heating that cleaning the filters hasn't fixed.

What the technician should leave you with

Ask for a written service report before they leave. It should name the model and serial numbers of the indoor and outdoor units, the date, what was checked and cleaned, any readings taken, any fault found, any part replaced, and when the next service is due.

Keep it with the rest of the unit's paperwork:

1. The installation invoice, showing the installation date.
2. The electrical Certificate of Compliance for the installation.
3. The warranty, and confirmation it was registered if the manufacturer asks for that.
4. The manual.
5. Every service report.

A warranty claim starts with proof of purchase and the installation date, and some manufacturers may ask to see the Certificate of Compliance before they start. These documents are what a heat pump's history looks like, and they are what gets lost. Keep them in your record, so the next technician starts with the history rather than a guess. More on that in keeping warranties alive.

When to call a professional

• Mould on the filters or fins, or a musty smell when it starts: a professional clean.
• Water dripping inside, new noises, or poor heating after a filter clean: a heat pump technician.
• A room that never gets warm on a cold night: ask an installer to check the sizing against the room.
• A new or replacement unit: a licensed installer, with a licensed electrician for the electrical connection.