Appliances
Heat pumps
Introduction
The right information is essential when choosing a heat pump. Our report brings together everything you need to make the decision easy.
Our guide to choosing a heat pump takes you through the steps from finding the right type and size to considering reliability and installation.
Then use our database of over 80 models to find one with the right price, specifications and features for your needs.
What is a heat pump?
Heat pumps are basically space heaters. They provide convenient, efficient, thermostatically-controlled heating that can be set to come on and off automatically at different times of the day.
The simplest versions are designed for a single room; the most complex, for a whole house. It takes 20 to 40 minutes to bring a room up to temperature, after which the level will be maintained within one or two degrees.
How heat pumps work
A heat pump works by extracting heat from the air outside your house and bringing it indoors. It's like a refrigerator in reverse. By trying to cool the world it can extract heat, or vice versa.
Use an old-style bicycle pump for a while and it will get hot. That's because gas (air) is being compressed. Spray an aerosol can and the valve area will become cold. That's because the compressed gas in the aerosol can is expanding.
Heat pumps (like refrigerators) have a system of pipes containing gas (refrigerant) that is continuously expanding in one part of the system and compressing in another. When the gas is being compressed, it gets hot. A heat pump's exterior unit compresses the gas, then pumps it to the interior unit where the gas runs over a series of finned coils, giving off its heat.
The gas is then returned to the outside unit, where it expands and runs through another set of finned coils, which become cold. The cold gas is then recompressed and the cycle continues. For summer cooling, the refrigerant flow is reversed, so the interior unit becomes cool, while the exterior cold.
Heat pumps shift more heat than the electrical energy consumed in compressing the refrigerant and running the fans, making them highly-efficient methods of heating - up to three times as much in the right conditions.
Is a heat pump the same as a ventilation system?
No. A heat pump uses refrigeration principles to shift relatively large amounts of heat in or out of your home to warm or cool it. A domestic ventilation system shifts drier air from the ceiling space into the living space, and is designed to reduce condensation.
Will a heat pump suit me?
Before you begin choosing a size, style or model of heat pump, you need to consider their pros, cons and cost-effectiveness.
Positives
Warm, dry and comfortable
Heat pumps can provide a level of all-round comfort not easily obtained by plug-in electric heaters. They can quickly bring a room up to temperature and then maintain it.
Lower heating costs
If you install a heat pump and keep your home about as warm as you do now, you could save a considerable amount in heating costs. But some of our subscribers with heat pumps tell us they use their units to keep their homes warmer than before, so their heating bills haven't dropped by much.
No gas charge
If you install a gas heater, you'll have to pay a gas connection charge (often around $30 per month) all year round, for a heating appliance you use for a few months.
Cooling
A reverse-cycle heat pump is the only type of home heating system that can both heat and cool a room.
Dehumidifying
If you switch a heat pump into cooling mode, it will also dehumidify the air in your house. In heating mode, heat pumps warm the air but don't dehumidify. Some units have a special dehumidifying mode, but this is designed for humid tropical climates and is not suitable for New Zealand winter conditions.
Air filtering
Many modern heat pumps incorporate a washable filter unit that removes dust and particles from the air. This could be an important feature for people with asthma and allergies. The filters need regular cleaning to keep the unit working at maximum efficiency. Some have a deodorising function as well.
House value
A heat pump installation may also add to your home's resale value.
Negatives
Noise
Whirring fans can be very annoying. Fans run in both the interior and exterior units all the time they are switched on. The fan in the inside unit of a heat pump should produce little more than a low hum in low-speed mode, but the compressor plus fan of the outside unit can be quite noisy. Check the manufacturer's specifications. Also, consider the impact on neighbours if the outdoor unit must be mounted near their sleeping areas.
Our 2009 member survey found noise was more likely to be an issue with older heat pumps - 15 percent of those bought before 2004 made enough noise to be "mildly disturbing". This fell to 7 percent for models less than two years old.
Not so good in low temperatures
Extracting heat from outdoor air gets more difficult as the temperature drops. Sometimes, especially on frosty nights, exterior heat pump units freeze-up and have to stop working for several minutes while they defrost. If you live in a frosty area see What are your needs for more about this problem.
Fifteen percent of owners in our reliability survey said their heat pump performed poorly in very cold weather. This was more likely to be a problem with older models that weren't inverter heat pumps - 25 percent didn't cope well with very low temperatures.
What type of system explains more about types of heat pump.
Draughts
Circulating air can cause draughts - which means you need to think about where to place the unit. You don't want one on the wall just above your favourite armchair.
How much can you save on winter heating costs?
The answer to our question is not as simple as you might think. Modern inverter-style heat pumps can adjust their power output to suit the heating requirements of the moment and are most efficient when working at part load.
Just where this efficiency "sweet spot" is to be found is difficult for us to test. Our testing procedure pushes the units on reverse-cycle heating mode towards their maximum output, where they are less efficient.
So if your installer makes sure the unit is large enough (or even a little too large) for your needs, you should get more heat per dollar of electricity than our test results suggest.
But - even if they're not always running at an optimum setting - all the models in our test will give three or more times the heat value of every dollar you spend on electricity.
How to choose a heat pump
If you've decided a heat pump is suitable for your needs, there are a number of things to think about before you choose a model.
Following our step-by-step guide will help you through the process:
- What type of system do you need? - choose a type of heat pump that suits your house or room layout.
- What is your main need? - is heating or cooling more important? If you live in a frost-prone area there are essential things you need to know. What features will be useful?
- Find the right size - make sure the cooling and heating capacity is right for the size of your room. Our interactive calculator will work out what you need.
- Efficiency ratings explained - understanding the ratings can save you money. Learn about Energy Star, Star ratings, COP, MEPS and more.
- Choose a reliable brand - the brand is one of the most important choices you can make. Our reliability survey will put you on the right track.
- The importance of installation - the right installer can make a big difference.
- Select your model - prices, specifications, features and recommendations for over 80 heat pumps.
- Check our test results for medium and large heat pumps - if it's a model we've tested you can see how it performed.
What are your needs?
If you're thinking about buying a heat pump, you need to consider the climate you live in and the features you require.
Climate
In areas with hot humid summers, good cooling performance may be important. If you live in a colder area, you'll want a model that has good heating performance. Look for a model that claims to be able to operate at temperatures below the worst you'd expect.
When the outside unit of a heat pump detects ice, it will automatically de-ice and stop producing heat. This is most likely to occur as the air temperature approaches freezing (at below-zero temperatures all the water in the air will have frozen and formed frost or snow, so the unit should no longer ice up). This can happen to all heat pumps but some do a better job of cold-weather performance than others.
H2 output capacity
This shows the heat output capacity of the heat pump when the air temperature is 2°C. The H2 output capacity really matters if you live in a colder area, especially where the night temperatures go below 5°C but don't often dip below zero. If this is your climate, insist on being told what the H2 output capacity is.
The bigger the H2 output capacity the better. It's optional to have H2 output capacity on energy labels, but we hope makers will adopt it. Where it's available, we include it in our database of specifications.
If the H2 information can't be supplied make sure your contract with the supplier says that you'll get adequate heating during cold nights.
Features
Think about the features you particularly want in your heat pump. These may include:
- Automatic de-icing is vital if you live in a cold area - otherwise, in winter, the pump will stop providing heat because of frost build-up on the outdoor heat-exchanger coils. This is a standard feature on newer inverter models.
- A timer lets you switch the heat pump on and/or off automatically at certain times. However, there are big differences. A clock-based timer allows you to programme an actual "on" and "off" time, and the times you set remain active until they're cancelled. A 7 (or more) day timer usually allows multiple on and off times.
- Sleep mode adjusts the temperature in several steps (up when cooling, down when heating) so that the system works less hard and more quietly when you're sleeping. You can programme how long you want the sleep mode to operate.
- Airflow-control settings provide reduced airflow for quiet operation and/or extra-high airflow (may be called fast or jet operation). Ideally, you want your heat-pump/air-conditioner to have a big range of airflow settings. A high airflow will help distribute the air in a room more quickly - but the higher the airflow, the noisier and draughtier it is. So you want a low fan-setting that circulates the air but does so quietly, especially if you're using the inside unit in your bedroom.
- Oscillating louvres allow the air to be distributed more evenly.
- Adjustable louvres can be pointed up for cool air and down for warm. Left and right adjustability helps direct air where it's needed.
- Fan-only mode blows air without heating, cooling, or drying. This can provide adequate cooling at some times of the year, without the cost of running the heat pump.
- Restart delay is a protective feature that prevents the heat pump from starting up again too soon after being switched off.
Heat pump FAQs
Is it more efficient to leave heat pumps on 24/7 using the “away from home” settings or to turn them on when heat is required?
We think the answer for most households is that it’s more efficient and less costly to turn them on as required - but it depends on how well your house is insulated. Or perhaps on how much of your heat is lost almost as soon as it reaches your room.
If you have a near new house that is air-tight (has no gaps around doors and windows, so no draughts), and has insulation that far exceeds the building code requirements, has double glazing, and thermal drapes, and a true heat exchanger ventilation system so you never need to open windows, it could be more efficient to leave the heat pump on all day every day.
But in an old draughty un-insulated Victorian villa, you'll be wasting a lot of energy moving heat into your house then leaking it out the walls and windows and gaps to the outside air, where the heat pump outside unit will take it back out of the air and pump it in again.
If you live in a house like this we advise running the heat pump in much the same way as you ran your old heating system. Use the timer to start the heat pump an hour or so before you normally would. If the heat pump replaced electric heaters, you’ll at least be able to get the same amount of heat for a lot lower cost.
We know that far too many NZ houses are poorly insulated, have “natural” ventilation from gaps and cracks, have poor moisture control and so on, which leads us to believe running a heat pump 24/7 in many cases will be wasteful.
Your house could be anywhere between the two extremes and we'll have to leave it up to you to decide what is the best way of running the pump. Check out “Saving Energy” and “Insulation” for tips on how to cut down on loss.
It’s essential you stop as much of the heat-loss as you can before you run your heat pump constantly.
Why does my heat pump stop heating every now and then when it’s frosty?
The short answer is that it’s defrosting. The reason why is quite complicated.
First, it’s important to know that your heat pump gets heat out of the air by trying to cool the whole planet. It’s like a fridge running in reverse. The outside unit of the heat pump has a panel like the inside cooling plate in a fridge. You’ll often see ice form on the evaporator panel in your fridge – because it’s a very cold panel in a cool “climate”. Something very similar happens outside on your heat pump external unit.
On a cold night when temperatures are heading towards a frost, as the air cools it loses its ability to hold water. Relative humidity rises as the temperature drops. Eventually the air temperature falls to the dew point, which is where relative humidity has risen to 100 percent. A relative humidity of 101 percent is against the laws of physics, so below the dew point excess water can’t stay in the air any longer. This excess water separates out of the air as dew, condensing onto everything that’s not under shelter. If the air cools further, to zero or below, frost forms.
Your heat pump’s outside evaporator panel is out there in that dewy air, being kept very cold so that it can pull heat out of the air. Ice is inevitably going to form on the panel at low temperatures and high humidity. When the ice gets thicker, it will act as an insulator, and limit the amount of heat that can be extracted. The heat pump senses this and changes to a defrost cycle. While defrosting it won’t deliver heat.
The worst conditions for ice formation are when the dew point is close to, but still above, zero. On a still day the actual dew point temperature in the evening depends on the relative humidity at the highest air temperature during the day.
So if you have a relatively humid climate where the overnight temperature regularly drops to the dew point, but not quite to freezing, your heat pump is going to suffer from severe icing. And it will have to stop heating fairly often to defrost itself. It’s the Achilles heel of the heat pump.
All of this is why we think all heat pumps sold in New Zealand should be tested at 2°C in air that is at the dew point. We also say it should be mandatory to have the tested heat output at 2°C on the energy rating label.
If my heat pump does not deliver full heat at 2°C in the Waikato, how can it work at all in Otago?
It’s very simple. It’s colder in Otago. Heat pumps work just fine in Canada where it’s colder still. They get to minus 20°C and more over there, but it’s no problem. OK, OK, you’re not convinced! Read the FAQ above, and we’ll carry on down below the frost point.
What has just happened? Frost has formed because it’s below freezing and the dew has settled out of the air that could no longer hold all the water it did earlier in the day, at say 12°C. Below zero, and below the dew point, there’s almost no water left in the air and your heat pump should not have to defrost at all.
But - it’s freezing - there’s no heat left in the air either. Not at all. There’s plenty. There’s still heat in air at minus 270°C. Not much mind you, as that’s just 3° above absolute zero. There is no heat left at minus 273°C. Obviously there’s less heat at minus 10° C compared to 10°C. But it’s not a lot less.
Heat pumps don’t have a problem getting heat out of the air if it’s just below freezing, as long as they’ve been designed to work down to say minus 10°C or so. Which is plenty for most parts of NZ.
So, at 6am, when it’s minus 3°C in Queenstown and plus 3°C in Hamilton, who has the defrosting problems? Not the Southerners.
Can the heat pump be made to perform better in frosting conditions?
Maybe. There are a few things worth trying.
Part of the problem could be that it’s trying too hard to heat up your room. This could be because it’s a little too small for the job. If it was recently installed, and it’s the size recommended by the installer, discuss it with them. Ask if you can have it replaced with one that has a little more heating capacity at 2°C installed. (For more information see What size heat pump and What are your needs.)
If it was installed to heat one room and you leave the doors open to other rooms – shut the doors. It’s almost certainly too small to heat your whole house.
You may have turned the thermostat up to try and get more heat out of it. Sorry – that’s wrong. You are forcing the heat pump to work harder, making the outside evaporator plate even colder, and ice form faster. The harder it works, the more ice. It’s also less efficient at maximum output, so your running costs can increase.
Try setting the thermostat lower, to around 18°C, so the unit doesn’t have to work so hard to get up to temperature. 18°C is much warmer in the morning rush than 10°C!
Another way of easing the load is to set the timer’s starting point a bit earlier, so it has more time to achieve the desired temperature.
Part of the problem could also be restricted air flow through the outside unit. Check that the fins are not blocked, and the intake and fans etc are not covered in debris, dust or cobwebs. It should work better if there is a bit of a breeze. Or it may be that you’ve got it alongside the coldest wall of your house. It will work better if moved to a warmer spot. The north side of the house is usually warmest.
And finally, if you only have occasional problems because it’s only rarely frosty, keep a cheap 2000 watt electric fan heater for backup. Run the heater on full to ease the heat pump load until the room warms up to the point where the heat pump can maintain an acceptable temperature. If it takes 30 minutes to do this, you’ve only used one unit of electricity or less than 30c worth.
Should I install a DVS/HRV system or a heat pump? Or both?
First we’ll deal with a misconception. The heavily advertised HRV systems are not heaters. They, and other similar products, are positive pressure ventilation (PPV) systems. In ideal circumstances, like sunny days if you have a corrugated iron roof, they can provide warm drier air sourced from the roof space which is effective at reducing condensation.
If you want cheaper heating when the sun is not shining, install a heat pump first, not a PPV system. Increasing the temperature in your house will help fight condensation as relative humidity drops when the temperature rises.
PPV systems are not an ideal match to heat pumps, because they work a little against a heat pump by pushing air into your house to force out the warm damp air which is the cause of condensation. This means pushing out some of that warm air your heat pump has worked so hard to warm up.
A true heat-exchanger (or balanced pressure) system will work with a heat pump, by extracting heat from warm damp air it extracts from kitchens and bathrooms and transferring that heat into incoming cold drier, fresh air. A balanced pressure (BP) system can also act as a heat transfer system by delivering the warm dry air to colder rooms not serviced by the heat pump, but your house must be reasonably air-tight.
Though BP systems are expensive, nowadays they can be competitive with multi-vent, electronically controlled PPV systems, which can cost $5000 and more. DVS and other PPV suppliers now offer BP systems as an alternative to their traditional range.