Heat pump energy sources

Most heat pumps in New Zealand are air-to-air units. The heat source is plentiful, but can be variable, which can affect efficiency. As temperatures fall, air-to-air heat pumps require defrosting and then are not providing heat. This makes them less effective in areas with extremely cold winters (particularly if the humidity is also high).

There are two other sources of energy used for heat pumps:

• ground-to-air heat pumps have the advantage of fairly constant ground temperatures year-round (and so fairly high efficiencies even on very cold winter nights) but they are considerably more expensive to set up and most parts of New Zealand do not have climate extremes, or heating demands, to justify the cost
• water-to-air – water is also an excellent source of low grade heat but is only suitable where buildings are located close to a water supply.

Energy efficiency

As heat pumps only move heat, and do not actually generate it, they have a very high ratio of heat output to energy input. This heating energy efficiency is expressed as a coefficient of performance (COP), while cooling energy efficiency is expressed as an energy efficiency ratio (EER). Typical domestic heat pumps have a COP of 3 to 4.5 – which means the heat pump produces about 3 to 4.5 times as much heat as the electricity it uses (under optimum conditions). The efficiency of a heat pump decreases as the temperature difference between source and supply increases – for instance, as outside temperatures drop, the heat pump’s energy efficiency reduces.

Heat pumps are currently the only form of heating where the COP is (usually) greater than 1, which makes them the most efficient form of purchased space heating available.

The cooling energy efficiency ratio (EER) is typically about 2.5 to 4.0 – which means the heat pump produces about 2.5 to 4 times as much cooling power as the electricity it uses.

Under the Energy Efficiency (Energy Using Products) Regulations 2002, heat pumps for sale in New Zealand must meet the Minimum Energy Performance Standard (MEPS) given in AS/NZS 3823.2.

Heat pump capacity

The heating capacity of an air-to-air heat pump (in the heating cycle) is typically between 2 and 10 kW for domestic systems. The cooling capacity (in the cooling cycle) is generally about 10–20% less than heating capacity.

To reflect a variation in capacity depending on outside temperatures, heat pumps can have three heat capacity ratings. These are based on standardised testing under laboratory conditions.

• H1 rates the unit’s heating output when the outside temperature is 7°C

• H2 rates the heating output at 2°C ambient temperature

• H3 rates the heating output at —7°C ambient temperature.

These ratings allow you to select the appropriate heat pump for the climate and household requirements (ie the design temperature and heating load) of individual situations.

It is important to note the advertised COP values are based on lab tests – in practice the overall efficiency of a heat pump is likely to be lower, eg one rated as COP of 3.0–4.5 may have an actual overall operating efficiency of around COP 2.5–3.0.

Heat pump size

Getting sizing right for heat pumps is crucial for maintaining their efficiency. 

If the heat pump is too big for the space requiring heating (or cooling):

• the purchase price will be unnecessarily high

• fixed speed systems will cycle on and off repeatedly (as the target temperature will be reached very quickly), which is inefficient and reduces the life expectancy of the unit

• ducted systems will have higher than necessary pressure, which can cause duct leakage.

If the heat pump is too small, the unit will run continuously as it attempts to reach the set point, which reduces the outside coil temperature and cause it to defrost regularly.

A general recommendation is to avoid using extremes of annual temperatures when selecting the ambient design condition, and similarly to avoid exaggerating the set points for indoor heating and cooling.

Heat pump sizing calculations commonly use the figure of 1 KW of output for each 10 square metres of floor area. However, this assumes all heating is provided by the heat pump and no allowance is made for passive solar heating.

Ducted heat pump systems require careful design. They can have heating energy loss of more than 30% if the ducts are overlong and/or have many bends, or travel through uninsulated space such as attics.

Typical heat pump configurations

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 either:

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

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

When selecting a heat pump, consider its primary use. If it will mostly be used for heating, select for a high COP; if mostly used for cooling, select for a high EER.

Other factors to consider:

• 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 pre-set 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.

Heat pump outdoor unit

To maximise heat transfer and heat pump capacity:

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

It is essential to have unimpeded airflow around the coil. Do not put the outdoor unit under the home or under decking.

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, eg 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 the household and to neighbours:

• locate it away from quiet areas like bedrooms

• mount the heat pump chassis on neoprene isolation mounts or pads (to absorb the 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.

Locate the outside and inside units as close as practically possible, as excessive pipe runs limit output by reducing compressor cooling, and can even cause compressor failure.

Heat pump 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. Indoor units can be:

• wall or high wall – mounted high on the wall, brings air in from above, and directs air downwards
• under ceiling – mounted on the underside of the ceiling, near to a wall, 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, 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, brings air in from below or from floor level, and directs air up the wall.

For air to freely enter the unit, there should be no obstruction around it. 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.

For your clients

To help your clients make decisions about space heating options, refer them to www.smarterhomes.org.nz/energy/heating.