Tips & FAQs
Heat pump performance
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 uninsulated 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. f 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 our Insulation report for tips on how to cut down on heat 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 (see “What’s a heat pump”). 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.
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