Should I leave my heat pump on all night?

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.

“Yes” say some experts. “No” say others. So, what’s the sensible thing to do?

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.

Jevons Paradox

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.


11 comments ↓

#1 John on 07.25.12 at 5:17 pm

This is good, sound advice. Following these suggestions will significantly reduce your electricty bills.

#2 Corey Peterson on 07.26.12 at 6:55 am

Excellent write-up as always Chris and thanks for helping inform Tasmanians about technology use in Tasmanian conditions. Also appreciate the plug for Sustainable Living Tasmania. Cheers, Corey SLT President

#3 chris-195 on 07.26.12 at 7:30 am

Thanks Corey,

The article was long enough not to add any more, but this issue is so similar to the longstanding and widely held myth that it was more efficient to leave fluorescent tubes on then to keep turning them off. Some people still believe this to this day – despite that myth being debunked by science a long time ago.

That old myth was so widely accepted that entire cities of skyscrapers were left lit up permanently, and for no reason.

The Australian climate office and all similar agencies now give the same advice… if you aren’t using lighting then turn it off.

It is now verified with ample testing that the short burst of energy used to turn on a fluorescent tube is recovered within seconds. Very frequent turning on or off of fluorescent tubes can shorten the tube’s lifespan, so they are not generally used in situations like private toilets where lighting is only needed for a minute or two. Other than that it’s a non issue.

#4 Julie on 07.26.12 at 4:59 pm

Thank you Chris. At last an authoritative piece that explains everything. We are constantly coming up against the same arguments on our round of home assessments and it is hard work getting people to accept the logic of turning off the heat pump overnight when they were told otherwise by “experts”!

#5 Lisa on 07.27.12 at 8:44 pm

What about the risk of the water freezing outside in the pipes? That’s the reason I’ve heard for leaving heat pumps on in winter.

#6 chris-195 on 07.28.12 at 9:11 am

Hi Lisa,

I don’t know where you live, but if you live in Antarctic-like conditions such issues can come to the fore. That’s not the case for most places in Tasmania.

Note that heat pumps are not connected to your water supply. When freezing happens it’s to do with water that condenses from the air not in pipes. Heat pumps operate with a closed loop and the pipes conjoining the compressor and the inside heater unit are always well lagged.

If, in your location, you experience winter overnight temperatures well below zero then there is a case for leaving the unit ticking over all night, but keep that option for really extreme conditions. In Tasmania that may mean whilst we have a really cold snowy front blowing over. Most of our winter nights are not in that category. In Hobart I run a heat pump all winter without any overnight use and it easily warms up the living area when we set the timer to come on, even on cold nights.

#7 Ted Reid on 11.29.12 at 6:36 am

Great article with a lot of information but a few more pictures of how a heat pump actually works would be great. A great site I found on the topic was http://www.heatpumpguide.com for anyone else looking for more information, maybe you can add it to your article chris?

#8 evaporative air conditioning perth on 02.13.14 at 7:26 pm

Hey there! Someone in my Myspace group shared this
site with us so I came to look it over. I’m definitely loving
the information. I’m book-marking and will be tweeting this to my followers!
Great blog and excellent design.

#9 Doug on 02.23.14 at 2:20 am

A heatpump compressor will not be running if there is no demand when you turn the temp down at night. That’s what the thermostat is all about.

If you turn the heatpump down at night, the next a.m. it will cycle “on” for a long time to warm the house back up. This long heating cycle will cause the internal resistive heater strips to turn on, thus quadrupling the cost of the heat (heater strips are the most inefficient means to heat). (You could disconnect your heater stips – perhaps you don’t even have them in your system as they can be optional?) Either way, you are putting a large mechanical strain on the unit when it is forced to operated for hours, rather than short cycle to maintain temp.

You may want to discuss the subject with a few professionals and publish an updated article rather than cost others money.

Just some thoughts from a thermal engineer that has done studies in common residential applications years ago.

#10 chris-195 on 02.23.14 at 8:53 am

Hi Doug,

Judging whether or not to leave the machine on all night depends on local climate conditions. In very extreme sub-zero environments doing this would put less demand on the machine than trying to warm up a freezing home in a short period.

In most situations (for instance here in Hobart) where night time temperatures rarely drop below 3 degrees the machine will provide heat to your living area within half an hour without the resistance strip being activated.

If I lived in Liawenee (where most Winter nights drop) below zero it would be more sensible to leave it on all night at a low setting. However, in really cold environments a heat pumps are not the ideal heating source.

I should add that this issue also revolves around expected ‘comfort’ levels. If the resident demands a very warm house (say 25 degrees) in cold climate conditions then the demand on the heater to bring the space temperature up rapidly would place a demand on the machine.

On warmish nights I will sensibly use the timer to click the machine on half an hour before rising, on a cold night give it an hour. Again, if you expect the machine to take the chill off the air initially, it will do it will no fuss. If you demand that it takes the temperature rapidly through 15 degrees it will struggle.

For homes that have sick or aged people in them then maintaining a constant heat is an imperative. The article’s prescription is about managing your heat pump appropriately, knowing its performance limits.

#11 chris-195 on 02.23.14 at 9:41 am

Some extra thoughts Doug,

You are correct to point out that it is not sensible to blindly run your heat pump without thought, or without knowing how it works.

For readers: some factors that determine efficient heat pump operation include:

1. Weather conditions (as mentioned above). Householder would be wise to note forecast for the night and do what’s appropriate.

2. Condition of house: keeping a totally uninsulated home warm right through the night (apart rom being energy wasteful) puts very high demand on the machine.

3. Characteristics of the heat pump: Some heat pumps have no provision for low overnight temperature setting, meaning that if left on the space needs to be over-heated.

4. Quality of the heat pump: Many of the cheaper brands are glorified air conditioners and these do not have the capacity to heat efficiently in low temperature conditions. Buying a good machine in the fist place is the most important advice to give.

5. Capacity of the heat pump: if the size of the heat pump is so small as to barely able to heat the living space, then greater demand is put on it to heat up space when the air cools down. If the heating space is really well insulated then bringing up the temperature will put little demand on the machine.

5. Expectations and needs of the residents: if saving energy is not a priority to the household (for either running cost or environmental reasons) then the question of keeping the home warm 24 hours per day is not an issue.

7. Demands on the machine: I know the performance of my family’s domestic heat pump and know that it will easily warm up our living area within an hour after a cold night, within half an hour on a less cold night. It’s a good idea for every owner to understand the limits of their heat pump and its characteristics. Learning this doesn’t take long.

For people who have bought second rate heat pumps, they’ve blown their money and have to live with a liability, most unfortunately.

Thanks Doug for your extra feedback.

Leave a Comment