Heat pumps: the emerging technology that quietly arrived

If you had asked an energy futurist in 2007 to name the technology that would most change New Zealand homes over the next twenty years, few would have said "the air conditioner". Yet the heat pump, a reverse-cycle air conditioner by another name, has done more to warm New Zealand houses than almost anything else in that time. It arrived without fanfare, one outside unit at a time.

How a heat pump works

A heat pump moves heat rather than making it. A refrigerant circulates between an outdoor unit and an indoor unit. Outside, the refrigerant absorbs heat from the air, even on a cold day. A compressor raises its pressure and temperature, and indoors it gives that heat up to the room. Reverse the cycle and it cools the house instead.

Because it is moving heat rather than generating it, a heat pump can deliver several units of heat for every unit of electricity it uses. In typical New Zealand winter conditions, a good modern unit delivers around three units of heat or more for each unit of electricity. A plug-in fan heater or oil column heater delivers one. That difference is why heat pumps are usually the cheapest way to heat a room with electricity.

Efficiency does fall as the outdoor temperature drops, which matters in inland and southern areas with hard frosts. Models rated for low-temperature performance are worth looking for there.

Why they spread so fast here

Several things came together:

• Cheap, mostly renewable electricity compared with many countries, and many homes with no reticulated gas.
• Cold, damp houses. A lot of New Zealand housing was built with little or no insulation, and open fires and portable heaters were common.
• Grants and rules. Government insulation and heating programmes have helped many lower-income households, and the healthy homes standards set minimum heating for rentals.
• Summer cooling. Warmer summers made the cooling function a bonus.

The healthy homes heating standard

For rental homes, the healthy homes standards require one or more fixed heaters that can directly heat the main living room, the largest room used for everyday living. The heating must meet a required capacity calculated for that room, using the official heating assessment tool, the formula in the regulations, or a professional assessment.

The heaters must be fixed, not portable, and at least 1.5 kW. Heat pumps and electric heaters must have a thermostat. Open fires and unflued gas heaters don't count. And if the room needs more than 2.4 kW of heating, electric heaters other than heat pumps can't be used to meet the standard, except to top up qualifying heating that was already installed before 1 July 2019.

In practice, that means most average and larger living rooms in rentals end up with a heat pump or an efficient wood or pellet burner. Tenancy Services has the full details and the assessment tool.

Getting the size right

Too small, and the heat pump runs flat out and still leaves the room cold. Too big, and you pay more up front than you need to, and it may cycle on and off inefficiently.

Size depends on the room's floor area, ceiling height, insulation, glazing, and how cold your area gets. As a very rough guide, the Level website from BRANZ suggests heating capacities in the range of 2 to 3 kW for rooms under 20 square metres, rising to 5 to 6 kW for rooms over 40 square metres, with more needed for high ceilings or lots of glass. A reputable installer should calculate the size for your actual room rather than guessing.

Getting the most from one

• Set a sensible temperature. Around 18 to 21 °C is comfortable for most people. Every degree higher costs more.
• Use heat mode, not auto. Auto can switch to cooling unexpectedly.
• Keep it steady. Heat pumps are most efficient holding a temperature, not blasting from cold. Use the timer to warm the room before you need it.
• Clean the filters. Dusty filters reduce airflow and efficiency. Many manufacturers suggest checking them every few weeks during heavy use.
• Keep the outdoor unit clear. Leaves, plants and stored junk around the outdoor unit restrict airflow.
• Close doors and curtains. Heat the room you are in, and keep the heat there.
• Insulate first. A heat pump in an uninsulated room is heating the street. Ceiling and underfloor insulation are usually the first things to do.

Installation involves refrigerant handling and electrical work, so it is a job for qualified installers and a licensed electrician.

The quiet lesson

The heat pump never had a launch event or a visionary founder. It was an incremental improvement on a mature technology, made cheaper and better by mass production, and it solved a real problem for ordinary households. That is often how the future actually arrives.

The next step for many homes is heat-pump hot water, which we will cover in a later note. Gen Less has independent guidance on choosing and using a heat pump.