The age-old question: Should I leave my heating on all day or not? and even with heat pumps: On all day and night?
There are industry experts often saying yes, and websites like Money Saving Expert and even the Energy Saving Trust saying absolutely not! All with little explanation to back up what they say, just anecdotal comments.
We’re here to give you the exact variables and science behind it, and tell you they're all wrong. As with all of these things, the actual answer is… it depends.
There seems to be an issue in our industry (and perhaps worldwide) that the simplest explanation is the one that is most widely spread and accepted. Everyone wants a yes or no, they don't want the reality of grey which is the real world we live in.
To find what best suits your property and lifestyle, you should consider things like your appliance type, internal and external wall insulation levels, the thermal mass of the property, property size and layout, your emitter types, usage pattern i.e. high/low, regular or irregular, and occupancy (high or low).
Now one of these variables on their own won't tell you, it's the adding them up altogether that tells you.
Additionally, it's not whether to heat all the time or not, but how much in either direction you should heat and also the amount of ‘set back’ you should use. The answer is not the binary “yes” or “no” people so desperately want, more on that later.
First of all, I'd like to address that ‘leaving your heating on all the time’, often is a misunderstanding. It does not mean set your thermostat to say 21°C permanently. It means making use of ‘setback’ temperatures.
That is to say, rather than your heating being ‘off’, the target temperature is just lowered slightly. So at night, you might drop to 18 or 19°C, rather than being ‘off’.
Or, if you have advanced weather compensation, the flow temperature will be slightly lower to drip feed the heat in (we'll cover this later).
Now how often this temperature is changed and by how much is what we are really looking at. Almost no one wants their property at 21°C at night, but to turn it down to 16°C will essentially be turning the heating completely off as most homes don’t drop by that much overnight to reactivate the heating.
The second thing that must be understood is that the property heat loss is not the same as fuel consumption.
That is to say, if a property lost 40kW/h of heat through its walls throughout a 24 hour period, that typically doesn't mean you have used 40kW/h of gas from your boiler or 40kW/h of electricity from your heat pump.
If the boiler has consistently run at 85% efficiency you will use 47kW/h of gas. If you can get 93% efficiency from that same appliance you will only use 43kWh gas.
And therein lies the problem. Many arguments for turning the heating off are talking about saving heat loss, not saving primary fuel usage. The difference between the two relies heavily on the first variable we need to ask ourselves, what is our heat source?
If you have electric heating such as electric underfloor heating or infrared heating panels, these work at 100% efficiency, so a 40kW/h heat loss will equate to a 40kW/h fuel use.
Now the lower we can get the flow temperatures and in turn fume temperatures the more condensing and higher efficiency.
Lower temperatures, of course, means less responsiveness from the system as a trade-off though, so would lean toward longer running times being needed.
This is often worth it for the extra efficiency, depending on the other variables below. Read our condensing theory article for just how much efficiency this can add!
It's also worth noting that condensing oil boilers have half of the efficiency gains that gas condensing boilers do.
Running the unit as cool as possible year-round in a more ‘low and slow’ manner will result in an average of 500% efficiency. Clearly, even if you even had the same fuel bill anyone would rather heat the property to 1 sustained level during the day. Why not?
The important concept here is, the lower the flow temperature (or radiator) the higher efficiency you will have with these appliances. And the lower your flow temperature the low and slow your heating will have to be.
And this is our main aim with the ‘low and slow’ method, aka steady-state heating. To be much more gentle on the condensing boilers and heat pumps and give it a chance to continuously feed your properties ‘thermal mass’ (more on that later) with high-efficiency heat.
Rather than blasting high-temperature heat at the property when needed at much lower efficiencies. If you don't have a condensing appliance or a heat pump though, don't bother, just time it on and off all day, if you wish, but you may also find it less comfortable too.
It's a bit like saying, I'm going to drive 100mph to the shops as I'll only be driving for 30 seconds. Actually, if you drive at 5 mph you will take 3 mins, it may ‘take longer’ but it will be much more efficient with fuel.
As most people have condensing boilers or heat pumps, that's what we'll address here today.
But before we go into the variables of what uses the least fuel in your specific case, you should also be aware of the other benefits of a stable state, lower temperature heating.
So even if you do use a fraction more fuel by running all day, the small amount of extra fuel may be worth the lower maintenance costs and increased comfort.
Each of these points is covered in more detail in our article ‘the benefits of low-temperature systems.’
So outside of what your heat source is, here's your first variable. Is your usage pattern high or low? Regular or irregular?
If you are in your property from 8 at night, in bed by 10, and out of the house by 7 am, 5 days a week, as well as being busy out of the house on weekends, then clearly heating your house throughout the day would be very wasteful.
However, if your property is used more often, perhaps you are retired, then you could be experiencing higher levels of comfort throughout the day with all the benefits listed above.
If you or other occupants are in and out at random points for random amounts of time then it may not be worth trying turning your heating off, or perhaps if you do turn it ‘off’ you may want to use a slightly higher setback temperature.
Thermal mass is how much mass your building has to hold heat.
A 1930s building with a brick inner skim and brick internal wall will have a lot of thermal mass, a modern building with stud plasterboard internal walls will have a low thermal mass.
The benefit of thermal mass is that it will store heat then slowly release it into the property after the heating has been turned off.
Conversely though, it also means the heat source will take longer to heat the property, so high mass buildings typically have to turn on their heating earlier and off earlier and have wasted heat while waiting to warm up and wasted heat during cold down, so higher mass buildings will suit more steady state heating.
This is a quick overview, much more on our thermal mass article.
The next variable is what emitters you have.
Emitters are what puts the heat into the room like radiators or underfloor heating.
Now, these have different response rates. The underfloor has a much slower response to radiators as it has to heat the floor before the air.
This means that you would rather suit the emitter to the property type and usage pattern, but you have to work with what you have.
Anyone who has underfloor heating knows that it only works with stable state heating. It can take hours to heat and cool down due to the thermal mass in the floor. Radiators are obviously more suited to intermittent heating.
To show this further, if you have underfloor heating for example, you can feed that underfloor heating with blasts of higher temperature heating. The floor will store that heat, and slowly release it into the room.
You can achieve a pretty steady temperature in the room throughout the entire day with just two blasts of heat.
However, why would you want to blast the thermal mass of your floor for 2 hours in the morning and 2 hours in the evening at 88% efficiency? When you can put in 1/3 of the power for 12 hours at a lower temperature at 95% efficiency.
If you only have a system that can operate at a fixed temperature, and particularly at a high temperature, then the ability of actually getting these efficiencies are reduced.
Although it would perhaps pay to upgrade your system controls to ones that operate at lower temperatures as it will lower bills and increase comfort, whether you choose to use steady-state heating or more intermittent heating.
If you do though, you will want to choose between load compensation and weather compensation controls.
Make sure you check out our weather compensation vs room compensation article.
So as I say, in no way answering one of these variables would you decide on whether your property is best heated continually or not, but I think most people can guess knowing the above.
And remember, the answer to this question is not binary. It's not a simple answer of “yes” or “no” which is what everyone seems to give and yearn for.
The more suited to continuous heating your situation is, the closer your set back temperature should be to your comfort temperature, and the less changing of temperatures you should have.
The more suited to intermittent heating your property is the further your setback temperature and comfort temperatures should be and the more temperature changes you can and should program.
Lastly, people's idea of what continuous heating is, is changing as controls are improved. Older styles of controls that didn't use setbacks, but just timed the heating to be completely on or off are dying out, and the idea of heating for 2 hours in the morning and 2 hours at night is too.
Everyone can experiment here, but If you have a heat pump you will find a constant day temp and say 2 or 3 degree night-time setback is best for running costs.
If you're a bit geeky and question the maths or theory here please take a look at our why not to zone heating systems video to understand the concepts.
Don't forget to sign up for our newsletter for our latest articles!
Enjoyed this article? Want to know more about system design and how to become a top heating engineer?