Should I or shouldn’t I? As we go to bed on these chill Winter nights that’s the burning question in the heads of many thousands of Tasmanian households who warm their homes with heat pumps.
The short answer is turn it off. For most people in most homes the best thing to do is to use the heater’s timer to start up the heat pump about half an hour before you arise. That way you will save a heap of energy.
The reason why there is such confusion over this question is that nearly every heat pump installer tells the purchaser to leave it on all the time for best efficiency. If you doubt the veracity of that message then please read on…
How do heat pumps work?
Imagine taking the keys to your new high efficiency Prius hybrid car then being told by the sales agent that it’s best to leave the engine ticking over all night to keep the battery topped up. Well that’s rather like the perplexing message that’s normally given to heat pump owners.
To get to the heart of the matter let’s have a quick look at how heat pumps work, because understanding this will help to understand the best way to manage them.
Heat pumps are quite an amazing technology, being 300 percent more efficient that a traditional electric heater (i.e. electric radiant heater or fan heaters). Those heaters heat the room by agitating air molecules and speeding them up. A lot of energy is used to do that. By contrast, a heat pump works by extracting warmth from the outside air and transferring that heat to your living space. Much less energy is used because the technology does not heat air it simply pumps existing heat from one place to another.
(Amazing as it may seem, a heat pump even extracts warmth from outside air that’s close to freezing. If this sounds like magic and you want to know how it manages to do that then here is a site that explains simply how the technology works. But you don’t really need to understand the mechanics. It’s a bit like the workings of your fridge, but in reverse.)
So we can see from this that although a heat pump is a very efficient device for heating your home, it will have to work much harder to extract heat from really cold air than from warmer air. In fact, when the outside temperature drops below three degrees then it will start to struggle and run much less efficiently. The compressor unit (outside) will tend to freeze up and the unit has to then use extra energy to stop that from happening.
What actually happens if I leave it on all night?
[Owners are often advised to drop the thermostat setting to a lower level – say 14 degrees – and leave it on all night, and even all day].
If your home is well insulated: the room temperature won’t drop to that lower level for several hours, and in the meantime you will hear the compressor unit whirring away into the night, waiting for the room temperature to drop to that setting. (If you have neighbours close by then your neighbours will be hearing this too. Heat pump noise is a common source of annoyance for light sleepers and bad harmony between neighbours.) No heat is delivered until the room drops to that lower level, so the unit may as well be turned off for most of the night anyway.
If your home is poorly insulated: Your living room temperature will quickly drop to the lower setting then the heat pump will be supplying energy into your home all night long, and most of that heat will be escaping into the night sky… all night long. Far less wasteful to turn it off.
In either case, if you like to arise to a warm room, just learn how to use the timer.
If the night time temperature is over 4 degrees: the heat pump will easily warm your living area within half an hour.
If the night time temperature is likely to be very cold, under 4 degrees: then it may take the unit a little longer to do it’s job, give the timer an hour to do it.
If the household is typical of many and the occupants get out of bed, gobble down their breakfast and are then off to school and work – the home being empty all day – then leaving the heater on all night to keep the living area hot, just to cater for that breakfast half hour, is very wasteful practice.
(On the other hand, if the home is occupied 24 hours per day by retirees or parents with small children then keeping the home warm 24 / 7 may be desirable in the interests of health and comfort.)
So, why do the installers tell you to leave them on all the time?
A technical reason given by some electric utilities to keep your heat pump (or air conditioner) going all the time is that it is is more efficient to use off peak power than at peak times when (thermal) power stations are struggling to deliver maximum load. So even though leaving your heat pump on permanently may use more energy it is more efficient to use power in quiet periods than at peak times. (It is estimated that every air conditioner or heat pump that is installed costs the power utility $4,500 for the provision of high-cost peaking capacity.)
This is partly relevant in places where electricity is provided by coal, however, 6am is not a peak time and in most circumstances (when using the timer) the unit will bring your living space up to temperature early enough to avoid the peak. Also, in Tasmania, where hydro-electric turbines readily ramp up and down at no extra energy cost, this is barely an issue at all.
What about the mechanical demand on the heat pump?
It is sometimes argued that making a heat pump work really hard on a cold frosty morning puts an extra demand on it and that this can reduce its working life.
If you live in a place like Liaweenee or Chicago – that frequently has sub zero night time temperatures – then this may be true, but then if you live in a place like that a heat pump is not a good choice of heating. Even in the middle of Winter, Hobart has no sub zero nights and very few below 3 degrees minimum. Above that and this is not an issue. The problem is that the suppliers tend to apply the same information for all circumstances.
But perhaps the real reason this advice is given is that heat pumps don’t deliver radiant heat – that comfortable bone penetrating heat that, say, a wood fire gives out. They work by just gradually warming the air to a comfortable level. When a heat pump is first installed householders often report that they miss that radiant heat effect, though they do enjoy having a warm house to walk into any time day or night.
If this is their big advantage then, the argument goes, leave them on permanently.
And this brings us to what is called the ‘Rebound Effect’, or what scientists refer to as Jevon’s Paradox.
You can read about that here, but basically this says that whenever an efficient technology is discovered the theoretical energy savings that can be gained are rarely achieved because humans tend to use that efficiency to increase the use of that technology rather than to save energy. In the case of heat pumps we may pride ourselves in investing in an efficient technology but if we then choose to use that extra efficiency to increase our energy usage then arguably nothing is gained. (Heat Pumps allow us to use even more energy than ever by enabling them to be used for cooling in Summer as well!)
In a similar way, research has shown that when people buy a super-efficient car they tend to justify doing more trips, and so the efficiency advantage is (at least partly) lost. In the case of heat pumps, there is a tendency (and a temptation) for householders to use that improved efficiency to make their homes toasty warm 24 hours per day rather than to waste less energy.
What about using the heat pump during the day?
Note again that a heat pump works much more efficiently when it is warm outside than when it is cold. You can sometimes use this feature to good effect on late afternoons when the outside temperature may not have chilled down yet. By turning on the heat pump before dusk you can help to warm your home a bit before the sun goes down. This is most effective in those heavy federation style homes that have a high level of thermal mass that will keep the heat once heated.
And what about hot water systems that use heat pump technology?
It is strongly advised that if you have a heat pump hot water system, turn it off altogether at night. It is just plain silly extracting heat out of cold night air when you can do it much more efficiently during the day.
Again, using a timer is very useful in getting the best efficiency from these units – ask an electrician to install a timer. Set it to go on between 1pm and 3pm for maximum efficiency (when the daytime heat is at its highest).
Need to know more?
New Zealand seems to be way ahead of Australia with consumer advice on these subjects. Here is a link to an excellent guide on the most efficient use of heat pumps, produced by their national energy authority. You can download their estimated overnight running costs by clicking on the graphic at right.
Sustainable Living Tasmania is a dynamic community organisation providing a wide array of information to householders on how to reduce their energy usage and energy bills.
[Note that in mainland states of Australia heat pumps are usually called reverse cycle air conditioners.]
Author of above article, Chris Harries, is a qualified home energy assessor and is a member of the Tasmanian government’s Climate Action Council.