Energy

There are various configuration options for heat pumps.

On this page:

• Configuration options
• Outdoor unit
• Indoor unit

Configuration options

Air-to-air heat pump design options include:

• split – the outdoor unit supplies a separate indoor unit located in the space being heated or cooled
• multi-split – one outdoor unit supplies a number of indoor units
• ducted – one indoor unit located in a building void space (such as above the ceiling) has many supply ducts to a number of rooms, separately controlled by air flow rate.

Heat pumps may operate as:

• an inverter system – where a variable speed compressor motor maintains a constant temperature setting with small fluctuations
• a fixed speed system – where a single speed compressor in the outdoor unit operates at a constant speed and is either on or off.

Inverter systems are more commonly installed as they are approximately 30% more efficient than fixed speed systems. They achieve the set temperature more quickly than fixed speed systems, and the speed control gives a quieter operation.

Other features to consider include:

• demand defrost control – this will minimise the defrost cycle, reducing heat pump energy use and supplementary energy use
• programmable thermostat – allowing the home occupier to preset operating times, which reduces the temptation to oversize equipment to get rapid warming or cooling
• variable speed handler – providing better operational performance, particularly in hot climates
• corrosion-resistant coils – especially useful in coastal areas and areas with a high sulphur level, because coils are more prone to corrosion.

Outdoor unit

Heat pumps should be installed by experienced professionals.

To maximise heat transfer and heat pump capacity:

• the outdoor unit’s air inlet path needs to be unobstructed
• the outlet air needs to be directed away from the coil and the air inlet.

It is essential to have unimpeded air flow around the coil. The gap from the outdoor unit to any obstruction needs to be at least 500 mm on the air inlet and outlet faces, and 100 mm on any other face. 

Consider the heat pump’s primary use when siting the outdoor unit. For best performance for heating, locate the outdoor unit in the warmest location, e.g. on a north or west facing wall (and vice versa for cooling). Heat pumps are most efficient when the temperature differential between outdoor and indoor temperatures is at its lowest.

The unit’s compressor and fans will create noise, so position the unit to minimise inconvenience to the occupants and to neighbours:

• Locate it away from quiet areas like bedrooms.
• Mount the heat pump chassis on neoprene isolation mounts or pads (to absorb any vibration).
• Avoid wall-mounted outside fans as these are likely to transmit vibrations.
• Maintain the unit regularly to ensure worn bearings and other parts do not become noisy.
• Use a fence or other barrier to block noise from neighbours – the most effective is a block mass in line of sight of the unit.

Particular care is required in retrospective installations, to ensure sound and vibration insulation is effective.

For efficiency, locate the outside and inside units as close as practically possible. If a longer pipe run is necessary, a professional installer may add another pump to the line. Make sure that the pipe manufacturer’s recommended run lengths aren’t exceeded.

Finally, BRANZ investigations of installed heat pumps have found some common errors to be avoided. In particular:

• Ensure that outdoor units are fixed with at least the recommended 100 mm ground clearance.
• Ensure the units are stable. Where the unit is connected to brackets to keep it off the ground, the brackets themselves should be firmly connected to something solid.
• Seal the gaps around the pipes coming out of the ducting.
• Make sure hoses draining condensation from the indoor unit are directed into drains or gardens and not onto paths.

When specifying a heat pump for a coastal or geothermal area, ensure there are additional anti-corrosive coatings applied to the outdoor unit.

Indoor unit

The indoor unit heats/cools, dries and circulates the room air and normally contains a coil, fan, air filter, air vanes and condensate pipe. The indoor unit can be:

• wall or high wall – mounted high on the wall, this brings air in from above or below and directs air downwards or horizontally into a room
• under ceiling – mounted on the underside of the ceiling, near to a wall, this brings air in from below or the wall side and directs air across the room at ceiling level
• ceiling cassette – mounted within the ceiling panels, this brings air in from below at its centre and directs air out from each of the four edges at ceiling level
• floor – mounted on or just above the floor and against a wall, this brings air in from below or from floor level and directs air up the wall.

For air to freely circulate, there should be no obstruction around the unit. The best location for a reverse cycle heat pump is near ceiling level. The best location for a heating-only heat pump is at floor level. (Note that these units are not interchangeable – those designed for use high on a wall should not be used close to floor level.) For a ducted system, the best location is in the ceiling space, with the supply and return air ducts located as far apart as possible.

Effective air flow is important:

• If mounted too close to the ceiling, a unit may short cycle and shut down prematurely.
• If the unit is located in a corner, the room may be only partially heated or cooled.
• If the location is subject to draughts, the unit’s performance may be affected.

Careful thought should therefore be given to the precise location of the indoor unit. In a BRANZ study of households with heat pumps, over a fifth of the households said that their heat pump was not installed in the optimum location – a very poor outcome. Location should consider room entry and exit points and likely furniture layout so occupants are not made uncomfortable by airflow draughts.

Householders should also be told that good performance of a heat pump depends partly on the filter in the indoor unit being cleaned to remove dust. With heat pumps used frequently, this should be done several times a year.

More information

• www.smarterhomes.org.nz – heating
• Neighbourly installation of heat pumps (Dunedin City Council)

Update: 12 August 2021