A buyer's guide to heat pumps

Heat pumps are able to heat your home in winter and cool it in summer, so the one device does the work of both a heater and air conditioner. They perform this clever trick by having two units – one on the outside of your house, and one on the inside – which are connected by pipes. The pipes are filled with a refrigerant gas, and by circulating this gas between the units the pump can cool the inside of your home by dumping heat outside – or work in reverse in winter to warm your home. Even when the outside air temperature is below zero, there’s still warmth out there that a heat pump can scavenge and transfer into your house.

The real genius of heat pumps is they work with remarkable efficiency, and this is what makes them the greenest home heating option in the long run. Heat pumps save hugely on electricity, which is both a personal saving and a plus for the environment, since in New Zealand 23 per cent of our electricity is still generated from fossil fuels. Reducing the demand for electricity also means we don’t have to construct more power stations or mar more of the landscape with expensive wind turbines and transmission lines.

Choose the right heat pump

To unlock the greatest energy savings it’s important to choose a heat pump that scores well on efficiency. According to the government’s Energy Efficiency and Conservation Authority (EECA) there can be quite a big difference between various models, with the best units saving you $150 each year in electricity costs compared to less efficient models producing the same amount of heat.

In Table 1, we list our selection of some of the most efficient heat pumps in different sizes, while the EECA website (which is a must-see if you are buying a heat pump) lists 146 Energy Star-rated models.

It’s also important to choose the right size machine for the job. Like cars, heat pumps come in many sizes and several configurations, such as wall, ceiling, or floor mounted. Small models produce 2.5kW of heat and the largest 16kW. Most are between 3kW and 10kW. So how do you decide what size you need?

EECA provides an online calculator (see ‘handy URLs’ below) which tells you which size heat pump (measured in kWs) will suit you based on where you live, the size of the area to be heated and how well insulated it is. However, like cars, heat pumps work most economically when they are cruising rather than working hard, so it’s better to opt for a slightly larger capacity model than go for one that will barely do the job.

For those in rural areas it’s also worth knowing that performance is often quoted based on an outdoor temperature of 7°C, but the countryside is typically 2–3°C cooler than cities so it’s wise when buying to ask about performance at 2°C. Also, EECA’s Energy Star ratings consider both heating and cooling, but if you’re mainly interested in heating, that can alter the rankings. Choose a model from a major manufacturer.

What are the downsides?

Are there disadvantages to heat pumps? Apart from the high initial cost (see Table 1), they don’t operate especially well at temperatures just above freezing. This is because dew will condense on the outdoor metal radiator fins, and since these will be chilled by the cold internal refrigerant, that dew will become ice which cuts airflow and acts as an insulator, affecting the efficiency of the whole system. Being somewhat smart, the heat pump de-ices itself by going into air-conditioning mode – meaning it warms the outside cooling fins for a time but also pumps cold air into the house! Strangely, icing is less of a problem in colder parts of the country. Once air is below freezing, it holds no moisture so there’s no dew and no ice!

Heat pumps can also generate a modest amount of noise and produce draughts, considerations when deciding where to site them. The outside part of the unit is best installed somewhere not too cold, and since the cheapest installations have inside and outside parts of the pump separated only by the wall of your house (saving on the cost of long hoses, etc) that too affects where you should best place the unit.

It’s also important not to get carried away running your heat pump at all hours, or you risk getting a shock on your power bill. Use a timer to have your heat pump switch on before the household gets up and again before people arrive home from work, and use the thermostat – don’t crank the device up to its maximum manually when you want to warm the house. Instead, set the temperature to the level you want and let the device do its thing.

It’s worthwhile having a reputable installer set up your heat pump, not least to advise on siting. Salt air and geothermal fumes have the potential to cause corrosion that may not be covered by manufacturers’ warranties. A competent licensed installer reduces the possibility of these problems. A properly installed unit will carry a warranty of up to six years and should last 10–20 years. Most consumers report few problems. Installation costs for a simple system are $500–$900 and a government subsidy may be available in areas with air pollution problems (see URLs box). Larger units (above 5–6kW) require dedicated wiring from a control board, increasing installation costs.

Heat pumps come with many possible options in terms of timing modes, automatic operation, air filtration, dehumidifying, airflow control and complexities of installation, so table 1 is only a rudimentary guide to a selection of the most energy-efficient models. Prices can vary substantially (up to 40 per cent) between retailers so shop around or use www.priceme.co.nz or similar.

Table 1: A selection of the most efficient models across the range of sizes

COP stands for coefficient of performance, which measures heating efficiency in a ratio of heat produced to power consumed. EER stands for energy efficiency ratio and measures cooling performance. Higher numbers point to greater efficiency. Mit Heavy Ind stands for Mitsubishi Heavy Industries, which is different from Mitsubishi. These units are ceiling-mounted.

Table 2: Home heating costs compared

Figures in Table 2 are based on a heat requirement of 30,000 kWh, being 5kW needed to heat a ~30 square metre lounge, for 4 hours a night, for 100 nights a year for 15 years. Fuel costs at the top of the indicated range have been used as prices are unlikely to diminish over 15 years. If you get free or cheap firewood, that option is probably least expensive.

Verdict

For most people, heat pumps will be the best home heating option. To minimise heating costs, ensure your house is well insulated, install thermal drapes, blinds or consider retrofitting extra glass to some windows to give double glazing. A light-coloured roof and exterior walls will reduce the need for summer cooling but will not heat up much in winter sun.

Handy links

EECA list of Energy Star-rated heat pumps
Online calculator for choosing the right sized heat pump
Government funding for heat pumps