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.


20 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 24 hour plumbing service on 07.09.14 at 7:30 pm

We’re a group of volunteers and starting a new scheme
in our community. Your web site offered us with valuable information to work on. You’ve done an impressive job
and our entire community will be grateful to you.

#9 Robert on 12.07.14 at 5:10 am

i am in Canada and I have a heat pump it works good thanks for advisce

#10 David on 12.18.14 at 2:11 pm

I am a bit confused. If the temperature is above the heatpump setting, the compressor shuts off correct? Like and AC in the summer. Leaving your AC on at a temp of 72, when the house gets to 71 or so, the outside AC shuts off. My heatpump works the same. If I have heat set at 72, and temp gets to 73, the heat pump / compressor shuts off. Do some units run 24×7 unless you shut them to off position? I thought that is what the thermostat does.

#11 chris-195 on 12.18.14 at 2:30 pm

Hi David,

You are partly correct. The compressor will, in time shut down, but it still needs to remain active and stop and start depending on room temperature.

what I did with this article is to get people to use their heat pump intelligently, rather than take the suppliers direction to just leave it on permanently. This is ok for convenience but not for optimisation of energy saving.

As the article describes, how you manage your heat pump depends partly on the level of insulation of your home, its thermal mass and other factors.

Another area (not explored fully in the article) is the potential to optimise heat transfer when outside temperature is fairly high (mainly in afternoons) but getting best from the unit in these circumstances can only really be done well if the home is very well insulated and has significant thermal mass.

In accordance with this, a heat pump operated hot water system is best timed to only run during afternoons and off when ambient outside temperature hardly has any heat in it to extract.

#12 David on 12.19.14 at 12:55 am

Thank you for your comments Chris. I am a little unsure / confused yet :) If I use my programmable thermostat and at night the thermostat temp is set to 60, but the room temperature drops from 72 to 62, then the compressor would not be running during this time correct? The electricity use would be $0 during this time? Or does the compressor still use some electricity and runs during this “Off Cycle” mode?

I do have another question regarding my heatpump if you can give advice. When I had the units installed, I had a 92% efficient natural gas furnace and the high seer heatpump. At first the installer had just gas furnace and AC, I asked how much more for a heatpump, he said $400. But that I would be wasting my money. Who knows what natural gas prices were doing and at that time they were 3X what they are now. I pay about .47 per therm now, and I have been using heat in the EMER mode so it uses the natural gas furnace and not the heatpump. I have spray foam insulation which seems well. My gas bills for this furnace, my second old 80% unit, stove, grill, and water heater runs about $40 during a normal winter month, and $60 if really cold. LOL, I am in Atlanta, so it never gets really cold like in WI. I keep the house comfortable 72-74 during day, and about 65 at night. I will probably test again, but with gas prices low, I thought it is cheaper to run the EMER mode using gas rather than the heatpump. Elect rate is .078 per KWH. Any suggestions, or probably does not make much difference.

#13 David on 12.19.14 at 1:09 am

Hello Chris,
I was looking to get more info about your location then I realized we are on opposite sides of the world. I do not know much about Tasmania. My wife has been close, but we do hope to visit as soon as I am not required to do that working thing :)

Thanks again,
David

#14 GMS on 12.30.14 at 2:44 pm

Hello Chris,

I live in virginia. I have a gas furnace for first two floors and heat pump for next two floor. This is the combination. I though HP are supposed to reduced utility bill but something opposite happen. My electricity bill came around 2 times that of Gas bill. Gas is also used for heating water.

I was worried why this happened called the electrician to check the HP. He filled more freon gas as he said that was less and heating was not happening properly.

Query: You are saying not to run it all night and i was advised to keep it running all time. Eg: When i was left home to work,i kept it at 65 F, and when i returned from office i made it 70F, programmable thermostat. Since there is a diff of 5, the HP is kicking in the AUX heat, i,e it is running completely on electricity full throttle till it reaches 70F. Similarly at night i was keeping it at 65 and using a room heater and in morning temp to 70 F. Again the Aux Heat kicks in.

So this is perhaps leading to very high electricity bill…

Please advise your thoughts on this.

#15 Pam on 01.02.15 at 10:35 am

Is there a way of stopping my heat pump from sending out cool air once the room temperature is reached? It would feel so much better if it simply shut off instead and then came on again when the temperature drops. We have a Samsung Whisper. Your response will be greatly appreciated.

#16 chris-195 on 01.02.15 at 10:49 am

Hi Pam, I think there’s little control over that.

The higher quality heat pumps, such as Daikin, don’t do this.

There are many off-the-shelf cheaper brands on the market and many of those do not operate very well but once isntalled you are stuck with them. I would have thought that Samsung would be not at the bottom of the range though – most of their products are reasonable.

Generally the floor based heat pumps that are primarily designed as heat pumps are better engineered. Many air conditioners sold in Australia are primarily built for cooling, but will also go into reverse cycle when you need to heat.

#17 JB on 01.08.15 at 5:50 am

I think it is worth noting that this article may only be applicable to those that live down-under. In the US there appear to be some major differences, including that compressors usually turn off when the unit is not running (see paragraph titled “If your home is well insulated”).

In the United States, most heat pumps are installed with a backup electrical strip heater (http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/35547/How-NOT-to-Use-Your-Heat-Pump-Thermostat) which uses about 3 times as much energy as a heat pump when heating your home.

In the US, the reason installers tell folks to leave their heat pumps on all day and night in the winter is that if you have electrical strip heating as a backup to your heat pump, it will be used to heat your home (at 3 times the cost!) whenever your thermostat setting is more than 2 degrees above the current temperature inside the house. That is, when you bump your thermostat up 10 degrees in the morning or in the afternoon to heat your home, most thermostats in the US will activate the electrical strip heaters.

There are some programmable thermostats that will ‘learn’ your heat pump and will not use the strip heaters unless the heat pump cannot keep up, but most homes do not have this type of thermostat. My thermostat calls this “recovery mode”.

If you have a heat pump backed up by electrical strip heaters and you have a non-programmable thermostat, you can avoid extra costs by turning up your thermostat setting one degree at a time (wait for the unit to shut off and turn it up another degree).

Hope this helps some of the Yankees coming to this page!

#18 chris-195 on 01.08.15 at 9:04 am

Thanks JB,

It’s true there are few places in Australia that have as severe Winter nights as you do in the Unites States and Canada. The purpose of the above article was to try to get people to use their heat pump intelligently, rather than just switch it on and leave it. Convenience may be your only yardstick, in which case there’s no need to do anything. Where people like to save money, reduce wear and tear on the machine or wish to reduce wasted energy, then it pays to use the machine dynamically.

Okay, so what we tend to do is keep an eye on the weather forecast.

In our case we perhaps use the heat pump during 6 months of the year. For most of that period the night time temperature stays above the level where the heating strip (or reverse cycle) switches on to prevent the machine freezing. This would be the case in most locations in both Australia and the US. So running the machine all the time every day, to cater for those nights when temperatures run are close to zero is not the best way to manage the machine generally. We keep en eye on the forecast and opt to run the machine at a low temperature setting only during those clear sky very cold nights.

If you live in a truly freezing situation where your night time temperature is nearly always around freezing then it is arguable that an (outside compressor) heat pump is not the most efficient way to heat your home. If that’s what you are stuck with, then what you say may be true, it may be better to generally leave it on during the night on a low setting, then boost it up in the morning. The better brands will enable you to set it on, say, 12 degrees (Celsius) during the night, but some don’t have this facility. The rub is that keeping your home toasty warm all night loses unnecessary heat, so it’s still better to manage your heat pump.

I realize that many people have busy lives and don’t necessarily worry too much about power wastage, so leaving the machine on permanently is an easy solution. Meanwhile, the manufacturers find it too hard to educate their customers, aware that each home and its owners have their own characteristics, so the easy way out for them is to give out a one-size-fits-all message.

Many thanks JB for taking the time to relay your thoughts on this.

Chris Harries

#19 chris-195 on 01.16.15 at 10:05 am

Hi GMS (further above),

It’s hard to diagnose at a distance. Sounds like your system may be faulty. Your auxiliary heating system is kicking in rather than saving energy? It’s probably true that if you have your thermostat set and an auxiliary heating system automatically kicks in when the heat pump is off, then the above advice may not be relevant to your set up. The advice in this article is for stand-alone heat pumps.

#20 Rachel Heagney on 01.23.15 at 11:35 pm

you don’t need to leave your heat pump since it’s a waste of energy and your bills could raise up. Consider this site as a great reference when it comes to effective heating approach http://brivis.com.au/blog/whats-efficient-way-heat-home/

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