Energy myth-busting: Is it cheaper to have heating on all day?

BikingBud

Petriix said:

BikingBud said:

Petriix said:

No, it doesn't depend... More heat = more heat loss. It's objective fact. There are many variables which can make it more difficult for people to observe the basic physics at play, but the fundamental principle is unquestionable.

Yes it depends upon the case in question which is entirely driven by the 7 or so variables. But most people on here don't want to or cannot understand the basic physics! For instance your comment more heat means nothing! 

Greater heat difference (dt) however does mean something. Energy levels will try to equalise and you either want to sustain a higher temp inside relative to outside, (+ve dt) heating, or sustain lower temp relative to outside, (-ve dt) air con.

All systems have observables and controllables. Older heating systems are very limited in what you can observe and control eg, higher or lower room temp, hotter water or cooler water. Interestingly energy consumption is not often observed and yet this is the real measure you are after tracking as this is your cost.

You can add discrete sensors to observe more state conditions but if you wish to control the system via those observations they need to be applied in a methodical manner to remove and correct the effect of other variables or invest in a more complex system such as external temp compensation.

Most people do not look at their poorly designed system critically and do not invest in understanding the difference between their own system and how it operates and how a well designed scientific test of a modern condensing boiler with defined and repeatable parameters to provide empirical evidence to support the result may offer an accurate result.

Comparing one day where the heating was left on or the temps were adjusted to the next where they turned it off misses so many potential variables and as the well-presented answer from heat geek advises you need to understand your system and what benefits you want and how much you are willing to pay for it, (the usage case).

I have lived in a four storey house with wet UFH, high thermal mass, all concrete, that took 3 days to heat up, with winter temps touching -20 deg C fairly common. To ensure it was running most efficiently it had external temp compensation and night set back, this was 20 years ago!

Hence my comment about it depends and everybody being correct and incorrect.

Simple answer and only one which fits all test cases, most efficient = turn heating off!

But we want the comfort of heating homes and that as with most of this discussion is subjective.

You've used a lot of words to say very little, and especially little about the actual question.

Unless you're talking about using an expensive form of immediate heating to make up for your cheaper heating method which you've switched off, the end result is fairly simple...

The less time your heating is on for (at the same thermostat temperature) the less energy you will use. There are no circumstances under which leaving the heating on all day while you're out will use less energy than leaving it off all day; as long as you are otherwise using the same settings while you're home. 

But it is not just about using less energy though is it? As I said simple answer and only one which fits all test cases, most efficient = turn heating off! 

However, in as few words as possible.

Heating:

• is there for a purpose
• to be fit for purpose
• it burns energy
• that costs money
• different systems have different operating characteristics
• understanding and exploiting those working characteristics efficiently will 
• give the optimum balance between cost and comfort

A use study ------ In the circumstance of the wet UFH with extremely low winter temps and outside temp compensation trying to run it as we generally operate UK household gas fired boilers for space heating would have used less energy but would not have provided the required comfort.

Spies

Today has been an exceptionally low day for gas usage, that's with putting my setback to 16c.

Outside temperature has been between 6.6c and 10.5c over the last 24 hours, setback applies between 9pm and 5:30pm, then 19c from 5:30pm to 9pm, boiler flow is 55c.

Will run it without changing anything this week and see if it stays consistent. The only other spanner in the works is my Honeywell Cm727 has optimum start built in, so it may adjust over time to fire the boiler earlier given the flow temperature is lower and the room takes longer to get up to temperature, the idea of optimisation is that the setpoint is reached at the time set on the thermostat vs it coming on at that time and then trying to get to the setpoint. 

orrery

Spies said:

...the idea of optimisation is that the setpoint is reached at the time set on the thermostat vs it coming on at that time and then trying to get to the setpoint. 

I find that this doesn't work as it should. If I get out of bed at 7am then I don't need the temperature up at 21C (my normal room temperature). If the house is at 19.5 or 20C and the radiators are blazing hot, then the room will feel perfectly warm. Just as a room that is cooling, with radiators stone cold, and the thermostat at 19.5 or 20 feels cold. The proximity of the thermostat to walls that may be hotter or colder than the air in the room distorts the picture.

Spies

I'll try turning it off and see what affect it has

Astria

The less time your heating is on for (at the same thermostat temperature) the less energy you will use. There are no circumstances under which leaving the heating on all day while you're out will use less energy than leaving it off all day; as long as you are otherwise using the same settings while you're home. 

Correct, however, the difference between the two can be the pretty much identical in some circumstances. The longer the heating is off for, the colder the house will get, the more heating is required to bring it back up to temperature, until you get to the point of equilibrium. In our house that temperature appears to be about 14c, or approximately 8 hours at the moment, depending on outside temperature of course.

Petriix

Astria said:

The less time your heating is on for (at the same thermostat temperature) the less energy you will use. There are no circumstances under which leaving the heating on all day while you're out will use less energy than leaving it off all day; as long as you are otherwise using the same settings while you're home. 

Correct, however, the difference between the two can be the pretty much identical in some circumstances. The longer the heating is off for, the colder the house will get, the more heating is required to bring it back up to temperature, until you get to the point of equilibrium. In our house that temperature appears to be about 14c, or approximately 8 hours at the moment, depending on outside temperature of course.

The longer the heating is off, the further from 'pretty much identical' the total energy consumption becomes. The rate of energy loss decreases as the temperature approaches that equilibrium so the lower the differential between internal and external temperature and the more time that passes, the less energy will be lost.

BikingBud said:

Petriix said:

BikingBud said:

Petriix said:

No, it doesn't depend... More heat = more heat loss. It's objective fact. There are many variables which can make it more difficult for people to observe the basic physics at play, but the fundamental principle is unquestionable.

Yes it depends upon the case in question which is entirely driven by the 7 or so variables. But most people on here don't want to or cannot understand the basic physics! For instance your comment more heat means nothing! 

Greater heat difference (dt) however does mean something. Energy levels will try to equalise and you either want to sustain a higher temp inside relative to outside, (+ve dt) heating, or sustain lower temp relative to outside, (-ve dt) air con.

All systems have observables and controllables. Older heating systems are very limited in what you can observe and control eg, higher or lower room temp, hotter water or cooler water. Interestingly energy consumption is not often observed and yet this is the real measure you are after tracking as this is your cost.

You can add discrete sensors to observe more state conditions but if you wish to control the system via those observations they need to be applied in a methodical manner to remove and correct the effect of other variables or invest in a more complex system such as external temp compensation.

Most people do not look at their poorly designed system critically and do not invest in understanding the difference between their own system and how it operates and how a well designed scientific test of a modern condensing boiler with defined and repeatable parameters to provide empirical evidence to support the result may offer an accurate result.

Comparing one day where the heating was left on or the temps were adjusted to the next where they turned it off misses so many potential variables and as the well-presented answer from heat geek advises you need to understand your system and what benefits you want and how much you are willing to pay for it, (the usage case).

I have lived in a four storey house with wet UFH, high thermal mass, all concrete, that took 3 days to heat up, with winter temps touching -20 deg C fairly common. To ensure it was running most efficiently it had external temp compensation and night set back, this was 20 years ago!

Hence my comment about it depends and everybody being correct and incorrect.

Simple answer and only one which fits all test cases, most efficient = turn heating off!

But we want the comfort of heating homes and that as with most of this discussion is subjective.

You've used a lot of words to say very little, and especially little about the actual question.

Unless you're talking about using an expensive form of immediate heating to make up for your cheaper heating method which you've switched off, the end result is fairly simple...

The less time your heating is on for (at the same thermostat temperature) the less energy you will use. There are no circumstances under which leaving the heating on all day while you're out will use less energy than leaving it off all day; as long as you are otherwise using the same settings while you're home. 

But it is not just about using less energy though is it? As I said simple answer and only one which fits all test cases, most efficient = turn heating off! 

However, in as few words as possible.

Heating:

• is there for a purpose
• to be fit for purpose
• it burns energy
• that costs money
• different systems have different operating characteristics
• understanding and exploiting those working characteristics efficiently will 
• give the optimum balance between cost and comfort

A use study ------ In the circumstance of the wet UFH with extremely low winter temps and outside temp compensation trying to run it as we generally operate UK household gas fired boilers for space heating would have used less energy but would not have provided the required comfort.

You're still answering a different question from that posed in the title of the thread. And this is a great demonstration of why wet underfloor heating is so horribly inefficient: it's far too slow to adjust the air temperature so it demands that you keep a higher than necessary temperature at times when it's not needed.

So, you're right that there are circumstances where you will get more comfort from using (wasting) more energy. But, with a sensible heating system, you can optimise both. My thermostat uses a predictive algorithm and the weather forecast to ensure that the desired set point is reached at the desired time. If I go out, I set my return time and it ensures that the minimum energy is wasted in achieving my required comfort level.

There are further optimisations I could make if I installed smart TRVs throughout the house or adjusted the boiler, but none of those things negate the fundamental truth that the less time the boiler is running, the less energy it uses; and the more time the room thermostat spends set at a lower temperature, the less time the boiler will run for. 

Spies

But a boiler isn't simply on or off, it modulates its flame.

Petriix

Spies said:

But a boiler isn't simply on or off, it modulates its flame.

That doesn't change the fundamental principle. It's not possible for a boiler to use less energy to maintain a higher room temperature for longer. 

Astria

Petriix said:

Spies said:

But a boiler isn't simply on or off, it modulates its flame.

That doesn't change the fundamental principle. It's not possible for a boiler to use less energy to maintain a higher room temperature for longer. 

Exactly, so to answer this thread in one simple paragraph to save everyone from reading the last 139 pages...

No, it's not cheaper to leave the heating on all day for any type of property. In some circumstances it may not be much difference to leave the heating on compared to turning it off depending on several factors such as outside temperature, but it's definitely not going to be cheaper to leave it on.

BikingBud

Petriix said:

You're still answering a different question from that posed in the title of the thread. And this is a great demonstration of why wet underfloor heating is so horribly inefficient: it's far too slow to adjust the air temperature so it demands that you keep a higher than necessary temperature at times when it's not needed.

So, you're right that there are circumstances where you will get more comfort from using (wasting) more energy. But, with a sensible heating system, you can optimise both. My thermostat uses a predictive algorithm and the weather forecast to ensure that the desired set point is reached at the desired time. If I go out, I set my return time and it ensures that the minimum energy is wasted in achieving my required comfort level.

There are further optimisations I could make if I installed smart TRVs throughout the house or adjusted the boiler, but none of those things negate the fundamental truth that the less time the boiler is running, the less energy it uses; and the more time the room thermostat spends set at a lower temperature, the less time the boiler will run for. 

Because the question in real life is not that simple.

You're comfortable or you're not!
 
How do you know what my comfort temp is?

Or my use case?

Have you lived with UFH?

Why would you need to adjust the temp rapidly? You miss the signifiant benefit of well designed and correctly set up UFH.

The same predictive algorithm can work for whichever system you want to install it's only a control system. How accurate are the weather predictions?

But like I said with an internet discussion like this everyone is correct and everyone else is wrong because their own interpretation and understanding and system demands are different.

You'll be telling me I make coffee the wrong way next