Turning heat off over night or lowering it?

[Alfred001]

Is anyone aware of any studies (or maybe you know from your own experience) that would settle the question of whether it is cheaper to turn your heat off over night or just lower it?

[swansont]

From an energy standpoint, turning it off will use less energy (or equal, in the extreme case) than lowering the thermostat.

 

I remember TA-ing a lab in college where the students tested this concept with a beaker of water and an immersion heater.

[CharonY]

That's a bit vague, isn't it? I mean, it would depend a lot on what the parameters are. Of course turning off will lower cost, but I assume that how much is neede to heat up during the day? That would obviously depend on how well insulated your home is, as well as the outside temperature differential.If the home is in freezing conditions, turning it off can lead to pipes freezing, for example. It also depends how efficient your system is.

 

For example, if in your house with your weather condition the HVAC runs for a total of 30min throughout the night and then an additional 5 mins to reach day temparature, then turning it off will save you money if it requires less than 35 mins to reach the desired temp in the morning. This is under the assumption that the HVAC (as most do) run cycles between fully on or off.

[swansont]

That's a bit vague, isn't it? I mean, it would depend a lot on what the parameters are. Of course turning off will lower cost, but I assume that how much is neede to heat up during the day? That would obviously depend on how well insulated your home is, as well as the outside temperature differential.If the home is in freezing conditions, turning it off can lead to pipes freezing, for example. It also depends how efficient your system is.

 

For example, if in your house with your weather condition the HVAC runs for a total of 30min throughout the night and then an additional 5 mins to reach day temparature, then turning it off will save you money if it requires less than 35 mins to reach the desired temp in the morning. This is under the assumption that the HVAC (as most do) run cycles between fully on or off.

 

 

It will take less energy to reheat than to maintain some temperature above ambient. The amount of insulation changes the magnitude of the effect, but not the effect itself. The best-case scenario is you're so well-insulated that you never drop to the target temperature, in which case the two scenarios are equal. But if the heat has to kick in at all to maintain the temperature, it will use more energy than being off.

[studiot]

Is anyone aware of any studies (or maybe you know from your own experience) that would settle the question of whether it is cheaper to turn your heat off over night or just lower it?

 

I'm with CharonY here.

 

The answer depends upon circumstances.

You need to supply a great deal more information to answer this.

 

@swansont

 

The question was about saving money not energy.

[swansont]

I'm with CharonY here.

 

The answer depends upon circumstances.

You need to supply a great deal more information to answer this.

 

@swansont

 

The question was about saving money not energy.

Energy costs money, and the answer given was in terms of time the system is running, which means it's in terms of energy.

 

You can look at whether turning the heat on and off is different from being one for a long stretch, but I don't think that tips in favor of leaving it running. If there are inefficiencies in starting up, then running for a long stretch is more efficient than several shorter stretches of equal time. The only issue that goes the other way is if starting up after ~8 hours off costs more than starting up while maintaining temperature. (But that issue was not brought up before)

[studiot]

Energy costs money, and the answer given was in terms of time the system is running, which means it's in terms of energy.

 

You can look at whether turning the heat on and off is different from being one for a long stretch, but I don't think that tips in favor of leaving it running. If there are inefficiencies in starting up, then running for a long stretch is more efficient than several shorter stretches of equal time. The only issue that goes the other way is if starting up after ~8 hours off costs more than starting up while maintaining temperature. (But that issue was not brought up before)

 

Like I said, it depends upon circumstances.

 

Economics is not always about pure physics or engineering.

 

I recently changed my heating from a gas fired boiler to an electric heat pump feeding a heat store.

 

My electricity supplier offers a tariff (for politico/economic reasons) where the night time rate is less than half the day time rate.

 

The economics of the heat pump itself are heavily influenced by politics from both the EU and UK governments and the so called RHI initiative.

The RHI brought down the Northern Ireland government last week)

[swansont]

My electricity supplier offers a tariff (for politico/economic reasons) where the night time rate is less than half the day time rate.

 

 

Does not paying for electricity at a lower rate cost less than paying for it? Does that change if the rate is higher?

 

Like I said about insulation, this changes the magnitude of the effect, but not the effect itself.

[CharonY]

 

 

It will take less energy to reheat than to maintain some temperature above ambient. The amount of insulation changes the magnitude of the effect, but not the effect itself. The best-case scenario is you're so well-insulated that you never drop to the target temperature, in which case the two scenarios are equal. But if the heat has to kick in at all to maintain the temperature, it will use more energy than being off.

 

That makes sense.

[studiot]

 

 

Does not paying for electricity at a lower rate cost less than paying for it? Does that change if the rate is higher?

 

Like I said about insulation, this changes the magnitude of the effect, but not the effect itself.

 

Sorry, but I am not sure what you mean.

 

There are also good engineering circumstances when leaving a tightly controlled system permanently on can save money over cruder on/off systems.

 

The 'heating experience' will be different when the systems are compared, but again that depends on circumstances.

[swansont]

 

Sorry, but I am not sure what you mean.

 

There are also good engineering circumstances when leaving a tightly controlled system permanently on can save money over cruder on/off systems.

 

The 'heating experience' will be different when the systems are compared, but again that depends on circumstances.

 

 

But you're comparing two different systems, not the same one under different conditions.

[studiot]

 

 

But you're comparing two different systems, not the same one under different conditions.

 

So?

 

We don't know which sort of system Alfred has or if he is operating it correctly.

 

That is why I asked for more information.

 

 

Further, if his system is on a low night rate tariff, it makes sense to operate it as much as practicable at night and store the heat somehow.

[swansont]

 

So?

 

We don't know which sort of system Alfred has or if he is operating it correctly.

 

 

 

But we can be sure it's the same system.

[Sensei]

 

Does not paying for electricity at a lower rate cost less than paying for it? Does that change if the rate is higher?

 

Like I said about insulation, this changes the magnitude of the effect, but not the effect itself.

 

Studiot wanted to say that he can purchase cheap energy at night, heat whatever he wants to be heated,

and then shutdown/decrease during more expensive day rate..

 

Which obviously would be cheaper than heating during higher day price rate, and then shutting down during lower night price rate..

 

(energy usage in the both above cases is pretty much the same, while money not)

[swansont]

 

Studiot wanted to say that he can purchase cheap energy at night, heat whatever he wants to be heated,

and then shutdown/decrease during more expensive day rate..

 

Which obviously would be cheaper than heating during higher day price rate, and then shutting down during lower night price rate..

 

(energy usage in the both above cases is pretty much the same, while money not)

 

 

That's a fair point. If you reheat after the rates change, then that brings rate into the problem.

[studiot]

Is anyone aware of any studies (or maybe you know from your own experience) that would settle the question of whether it is cheaper to turn your heat off over night or just lower it?

 

Returning to the OP

 

What you consider a minimum night time temperature below which you would want to reheat anyway will also affect this.

 

Where I live the weather is not particularly severe so an outside temperature of -5^oC will lead to an indoor temperature of around 6 to10^oC by morning.

 

So some reduced running during some nights may be desirable, unless you want to sit and shiver.

 

 

 

Of course with my heat pump, the efficiency is lower at the colder night temperatures so extracting energy during the day may be more attractive.

[Delta1212]

I wrote out a very, very long post that I realized was getting more complex than I intended and that was resulting in a breakdown of how I was explaining things such that even I was having trouble following it, so I'm going to start over from scratch and with a better idea of what I'm trying to communicate:

 

I'm with swansont on this.

 

Let's say that a system loses heat at a rate of x. The amount of heat energy lost during time t is then xt.

 

Now, as a system's temperature drops toward ambient, the rate at which it loses heat decreases. Given the above, a system where the heater is turned off and then later back on consists of three stages: cooling toward ambient, at ambient, and heating back up to target. A system that is dropped to a lower maintained temperature also has three stages: cooling to the lower temperature, remaining at the lower temperature, and heating back up to target.

 

The first and last period of an unheated system will always take longer than the corresponding periods of a system where temperature is being maintained.

 

So, t₁ is the time it takes for the low temperature system to cool to the maintained temperature. t₂ is the additional time it takes for the unheated system to reach ambient temperature. t₃ is the time the unheated system remains at ambient temperature. t₄ is the time it takes the unheated system to heat back up to the maintained temperature of the low temp system. t₅ is the time it takes to heat back up from the maintained lower temperature to the normal target temperature.

 

So the whole period of time under discussion for both system is made up of t₁+t₂+t₃+t₄+t₅

 

Now, for t₁, x is the same for both systems because both are cooling from the target temperature for the same amount of time, and other than what we are doing as far as turning the heat down or off, the systems are identical. So we'll call this initial rate x₁.

 

For t₂, the the rate of loss is going to be lower for the unheated system, because it's temperature drops below that of the maintained system. So we're going to call the unheated system's rate of loss during this period x_2a and the maintained system's x_2b.

 

For t₃, the unheated system has reached ambient temperature and is no longer losing heat. The maintained system is still at the maintained temperature and is therefore still losing heat at a rate of x_2b.

 

For t₄, the unheated system's temperature is raising. Since it is being raised to the maintained temperature during this period, it's average temperature is lower than the maintained system's and therefore so is its rate of loss. We'll call this rate x₃ which is less than x_2b.

 

For t₅, both systems are being raised from the maintained temperature to the target temperature, so their temperatures are the same and so is their corresponding heat loss.

 

So, now obviously in the above situation, the amount of heat that is lost by the unheated system during each period of time is less than or equal to the amount of heat lost by the maintained system, which on the face of it seems like it means you're going to be paying more since you're paying to generate all of that lost heat energy, but let's look at what the actual difference is.

 

No, the only times that the heater is actually on for the unheated system are t₄ and t₅. Meanwhile, the heater is on for t₂, t₃, t₄ and t₅.

 

However, we can't just add up xt for each of those times because we don't just need to maintain the current temperature at each of those times, but raise the temperature back up to target, which requires adding energy back in. But how much.

 

For the unheated system, you need to maintain temperature during t₄ and t₅ plus add back in all of the energy lost during t₁ and t₂.

 

For the maintained system, you need to maintain the system during t₃, t₄ and t₅ plus add back in the energy lost during t₁.

 

So, the unheated system's total energy expenditure during time t is equal to:

 

x₁t₁+x_2at₂+x₃t₄+x₄t₅

 

The maintained system's is given by:

 

x₁t₁+x_2bt₂+x_2bt₃+x_2bt₄+x₄t₅

 

If you subtract those out, you get a difference of:

 

(x_2bt₂-x_2at₂) +x_2bt₃+(x_2bt₄-x₃t₄)

 

And since, per above, x_2b is always greater than both x_2a

and also x₃, this expression always yields a positive number, which means that the maintained system always uses more energy than an identical system that simply has the heat turned off.

 

You can vary the amount by changing conditions such as the temperature and insulation and overall duration of time t, but as swansont said, this will simply change the magnitude of the effect, it will never render the above a negative number and as such will never result in the same system using more energy with the heating turned off completely than with the thermostat simply set to a lower temperature.

 

The only possible scenario where this is not the case is if you add an energy cost to turn on the system that does not exist for raising the temperature. If that cost is large enough, it could push the number negative over short enough durations, but this is rather unlikely to be more than a negligible effect in any realistic set up, and the best action in a case where it isn't negligible would then still be to turn the thermostat down to the lowest possible temperature that does not involve shutting the heating off completely.

Edited January 23, 2017 by Delta1212

[Alfred001]

 

So?

 

We don't know which sort of system Alfred has or if he is operating it correctly.

 

That is why I asked for more information.

 

 

Further, if his system is on a low night rate tariff, it makes sense to operate it as much as practicable at night and store the heat somehow.

 

Storing the heat is not an option. I'm u using central heating which runs water through radiators, the water is heated by gas. I can either turn it off over night, lower the temperature or keep the temp same as during day.

 

There's no difference in gas price between day and night.

 

For what its worth, its winter and my house is sandwiched in between two houses, so it shares walls on either side of it with another house.

 

 

Someone mentioned the possibility of pipes freezing if the heat is turned off. Can that really happen over night with cetnral heating?

[Alfred001]

So... fellas, does anyone have the answer to the pipes freezing bit? Is there a risk of that if I turn the heating off over night?

I imagine the answer is no, because then that would be happening with my faucets, but I want to be sure.

 

The consensus on the matter of heat off vs. heat low over night seems to be in favor of the former, but I'd like to be sure on the pipes freezing business.

[Delta1212]

How cold does it get at night where you are?

[Alfred001]

I think -24 Celsius is the lowest recorded (-11.2 fahrenheit)