Keep the heating in a house all day on?

[Picuino]

There are several oppinions about how to manage the house heater.

1. Some think it is cheaper to keep the heater all day on.
2. Others think it is better to switch off the heater and heat the cold house every time you arrive.

In my opinion is always better the option 2. You can waste a lot of energy heating the cold house, but it's less than maintaining all the time the house hot.

What are your arguments about the question?

[Kleinstein]

It depends:

If the house is very well insulated, there is not much difference, as even if one would turn of the heat, it would not cool down very much.
For the temperature felt, the surface temperatures are also part of the temperaure perception. So when the surfaces are still cold one may need a high air temperature to the same subjective feeling.
I may need less energy to get it warm only ehen you really need it, but it is also more complicated to regulate an one may overshoot to more than actually needed.

It also depends on the type of heating system. Some (like a wood oven) work better when at a relatively high power and than off for longer. Other work better at partial power. This can especially be the case for a heat pump / air conditioner in heating mode. Here the outside temperature also make a difference.

[CatalinaWOW]

I have temperature logging on my house, and examining the slopes of the heating and cooling curves the results show that if the heat is off for roughly twice the normal thermal cycle of the heater, energy is saved.  My house is well insulated and have a modest output fuel burning heater which results in a roughly two hour thermal cycle (temperature varies by about 1.5 deg C).  I find these results are in line with Kleinstein's observations and confirm the choice of option 2 if you are gone from the house for a normal workday.    In my situation it makes no sense to turn the heater off for short times away from the house.  Your situation may be different.

You also have to understand your needs.  In my case the house is a few degrees below comfort temperature for a sizeable part of an hour upon return from a long off cycle (also in the morning after leaving off at night).  You can usually conquer this by appropriate use of a timer, but not everyone will find this approach adequate.

[Alti]

There are several oppinions about how to manage the house heater.
1. (..) heater all day on.
2. (..) heat the cold house every time you arrive.
In my opinion is always better (..)

I think your question is poorly posed.
You compare two different situations with vastly different comfort levels.
With first scenario you do not need to bother about planning or changing settings, every time you want to enter the room/house, in every imaginable scenario, it is already cosy, no compromises. With second scenario it is all about planning and additionally it comes with the penalty when you sit and wait in cold house till it gets to temperature. You trade comfort for kWh in your question, IMHO.

Or did you intend to ask in #2: "(..)heat house up before you arrive."?

[Siwastaja]

Insulate your house. Then you can utilize the thermal capacity of the structures to your benefit. I live in a 1952 house with some insulation added in 1982 and I can cut all heating for half a day with little effect in room temperature.

When this is the case, i.e., the RC time constant is long enough, you can't save anything by cutting power for half a day; the "RC circuit" smoothes the "voltage" and same amount of power is delivered into the system in either case. I.e., you need more power to catch up with the small temperature drop.

But for reducing temperature during long enough, so that the duration of reduction last for longer than a few RC time constants, then sure it helps. For a well insulated house with some thermal capacity, that could be the "holiday temperature reduction". For a poorly insulated house (high "conductance" or low thermal resistance of materials), or one with very lightweight materials (low "capacitance" I mean thermal capacity), it could make sense to drop the temperature for just a few hours.

Given linear heating cost (like with gas, oil, resistive electicity), basically the average temperature directly determines the heating cost. In a well insulated house, you can't change the average fast. In poorly insulated, you can and should.

This is also why such trickery is not used in really cold climates; we have to insulate our houses and then the trick doesn't work. Except for those longer "holiday reductions".

[Picuino]

Or did you intend to ask in #2: "(..)heat house up before you arrive."?

Ok, you can change "(...) heat house up before you arrive"
For my it doesn't matter.
When you switch off the heater and the house lowers the temperature, you save energy always from the first minute. it doesn't matter if the house has to be reheated later.

[Siwastaja]

Yes, you are right. The saving starts immediately.

You seemingly "lose" part of the savings if you need higher power to heat up back later. But you did save more than that.

Only the average power matters in the bill, and it depends only on the average temperature difference across the envelope of the house, and by reducing average indoor temperature, you are reducing that average power.

With heat pumps, cost of heating is not linear so such trickery can be used in our advantage, i.e., heating when COP is better and not heating when it's worse. But there is usually not much to be gained.

[Neper]

For those who want to save without returning into a cold house:

We've fitted our radiators with radio-controlled thermostats and when we leave for a whole day we lower the temperature and then crank it up again via a mobile internet access an hour before coming home.

[Alti]

When this is the case, i.e., the RC time constant is long enough, you can't save anything by cutting power for half a day(..)

Shorter RC constant allows deeper setback temperatures and thus lower average temperature#. This can only be improved by either decreasing the Cp*mass of the house, or by increasing available hating and cooling powers. Both require different paradigm in design where the standard is quite opposite: high Cp*mass, lowest heating power (heat pump) and lowest cooling power (good insulation). Does not make much sense to me but somehow this solution is preferred, over a short RC house, at least in colder part of Europe.


#I assume we are talking about working mammals in this topic, those who sleep 1/3 of the time and spend significant part of the remaining time outside. If someone lives ones life running around the house 24/7 then whole setback saving won't work, irrespective of RC.

[elekorsi]

We have it ON all the time, with the temperature setpoint reduction during the night. Water temperature to the radiators is regulated by outside temperature and fine correction is done via current room temperature... Nice and comfortable all the time, higher setpoint kicks in about an hour before we wake up, so it is already warm.
We have a wood gasification boiler from Herz and a 2000 Liter buffer, at current weather (0°C - 10°C) we load it and fire it up every 2 days, in the winter once a day. It burns for around 6-7h, everything that is not consumed by the house goes to the buffer, which is then the source of the energy until next boiler loading...

I attached the screenshot of the current operation (just fired it up about an hour ago), if anyone is interested. It's from my little supervisory system I can access it from anywhere via web browser.

[richard.cs]

Only the average power matters in the bill, and it depends only on the average temperature difference across the envelope of the house, and by reducing average indoor temperature, you are reducing that average power.

In a pure physics sense, this totally correct, assuming no cycle-related inefficiencies in the heat source which is probably close enough. However Kleinstein's point about temperature perception and surface temperatures is very valid. If you come into a recently-cold house and all the surface you touch are cold, you will feel cold and likely want a higher air temperature to compensate. Human physiology and psychology comes into this because it is a question of achieving a particular comfort level rather than a particular temperature.

For the temperature felt, the surface temperatures are also part of the temperaure perception. So when the surfaces are still cold one may need a high air temperature to the same subjective feeling.

[armandine2]

.... whatever the physics, I think the psycho-economics will trump this for most of us. I lived next door to a tenant who had a debilitating condition which caused him to stay in all day with the heating on (quite high I thought) doing yoga or reading books. As I returned to my cold flat I often wondered who was worse off!

[EEVblog]

Depends on your thermal mass, insulation, and cost of available energy source.
e.g. if you have excess solar during the day then you would absolutely heat during the day and rely on residual heat at night.

[Siwastaja]

Shorter RC constant allows deeper setback temperatures and thus lower average temperature#. This can only be improved by either decreasing the Cp*mass of the house, or by increasing available hating and cooling powers. Both require different paradigm in design where the standard is quite opposite: high Cp*mass, lowest heating power (heat pump) and lowest cooling power (good insulation). Does not make much sense to me but somehow this solution is preferred, over a short RC house, at least in colder part of Europe.

Short RC indeed improves controllability, but for a high cost: small R means high loss of energy. Insulation slows down adjustments but quite obviously reduces the consumption. While it prevents such tricks, it also makes them unnecessary. So if low RC comes with low R, it has no upsides.

Now if we choose large R (good insulation) which is obviously always a good thing, this still lets us to adjust C. Choosing as small C as possible would bring us some of the controllability, not as short RC as in a poorly insulated house, but still something. This allows the trick discussed here (savings by dynamically adjusting the indoor temperature according real needs). However, high value of C allows different saving tricks:
* No need to switch between heating and cooling (or suffering overheating) when the average (24h) outdoor temperature is close (or tad below) to desired indoor temperature. Low-capacity houses suffer from the fact you need to heat the house during night time and then at daytime, when sun is shining (even worse if windows are large), the house overheats (you lose the free energy by opening windows and ventilating) or even worse, you need to start cooling the house.
* You can heat/cool whenever the energy cost is lower; for example, with heatpumps, heat when the temperature difference is smaller, cut down heating power during coldest time. Or, follow the hourly energy SPOT prices. Or, follow your own solar production.

I think these "tricks" are WAY more valuable than the quick controllability of low-C houses.

This is clearly visible here, old-style massive brick houses here, those that survived WW2, are fine without heating or cooling from late spring, through summer, to early fall. They consume less energy in reality than on paper (when simply calculated from thermal resistance of materials, obtaining U value). Modern low-capacity well insulated houses require on/off heating and cooling around the whole year. They consume more in reality than on paper.

But you can have both if you want to: build with low-C materials but add a water storage tank (at least some 2000-3000 liters). Then you have the storage capacity but it's under separate control so you can choose when to charge, when to discharge it by adjusting valve positions.

[Siwastaja]

In a pure physics sense, this totally correct, assuming no cycle-related inefficiencies in the heat source which is probably close enough. However Kleinstein's point about temperature perception and surface temperatures is very valid. If you come into a recently-cold house and all the surface you touch are cold, you will feel cold and likely want a higher air temperature to compensate. Human physiology and psychology comes into this because it is a question of achieving a particular comfort level rather than a particular temperature.

Yes you are totally right, we can't ignore the temperature perception. Yet it's still the secondary point. It's most important to get the base physics right first, then finetune. For example, as discussed in another thread, in-floor heating and ceiling heating both have the same surface area, resulting in the same theoretical heating water distribution temperature for the same power output, but in-floor heating would allow you to use a bit lower temperature because your feet will feel warm; especially if you are one of those who suffer from cold feet and crank up the temperature to compensate. This could translate into some 5-10% savings. It's important, but still a finetune compared to the base physics behind all this.

[Siwastaja]

Depends on your thermal mass, insulation, and cost of available energy source.
e.g. if you have excess solar during the day then you would absolutely heat during the day and rely on residual heat at night.

This is what I'm doing, I have a 3kWp system so during a sunny day run the air-to-water heatpump to store the energy into the 1200 l buffer tank, which then cools down during night back to the starting point. The larger the tank, the lower dT is required to store the same energy. The problem with my 1200l tank is, I need to bring it to some 15degC higher, which already significantly reduces the COP of the heatpump. But it's still way better than selling the power to grid, then buy it back for 2.4x the price. A larger tank would help, but I'm also letting the thermal capacity of the house participate, i.e., the heating water temperature could start dropping already at say 4 a.m., reducing the output power into radiators, but the inside temperature won't fluctuate much if heating is restarted around 10 a.m.

[Zero999]

It depends on what temperature you heat the house to, compared to the outside.

I like my house to be on the cool side. I'd rather wear an extra layer, if needs be, so I set the thermostat to 15°C. The timer turns the heating on, just before I get up in the morning and again, when I get home from work. I very much doubt it would be more efficient for it to run 24/7.

I can't remember how much gas and how often my heating kicks in. It hasn't been on since April. It will probably turn on next week, as a cold snap is forecast, which could bring the first frost of the season.

[Marco]

Homes built with below slab insulation and minimal above slab insulation will have too much thermal mass to get to a comfortable temperature from cold in a reasonable time frame.

Don't build homes like that.

[Siwastaja]

I like my house to be on the cool side. I'd rather wear an extra layer, if needs be, so I set the thermostat to 15°C.

Sounds very British!

What I mean is not an offence, but a cultural observation that the colder climate you get into, the higher the indoor temperatures. I'm like you and like my house to be on the cool side, also to save energy, to the point many would say I'm almost an extremist...

... but it's not 15degC - it's 20.5degC!

I'd hazard a guess the average here would be 23degC and it's not uncommon at all to set thermostats to 25degC. And if the indoor temperature fluctuates even by +/- 1degC, we instantly complain*. If it ever gets below 20 degC in a rented apartment, we ask for the authorities to perform official measurements and file complaints, or seek criminal charges against the owner; it's considered a health safety issue. At very least, your kids would be taken into custody if somebody heard you live at +15 degC.

*) necessitating a carefully tuned feedforward+P control system as described above by elekorsi

We also are almost hysteric when it comes to mold, and even the idea of dynamic temperature adjustments, or going below 20degC triggers irrational fears that it somehow automatically turns the house into mold nest in no time. This has a grain of truth in it, obviously; some temperature difference between indoors and outdoors is needed to prevent condensation.

[Zero999]

I like my house to be on the cool side. I'd rather wear an extra layer, if needs be, so I set the thermostat to 15°C.

Sounds very British!

What I mean is not an offence, but a cultural observation that the colder climate you get into, the higher the indoor temperatures. I'm like you and like my house to be on the cool side, also to save energy, to the point many would say I'm almost an extremist...

... but it's not 15degC - it's 20.5degC!

I'd hazard a guess the average here would be 23degC and it's not uncommon at all to set thermostats to 25degC. And if the indoor temperature fluctuates even by +/- 1degC, we instantly complain*. If it ever gets below 20 degC in a rented apartment, we ask for the authorities to perform official measurements and file complaints, or seek criminal charges against the owner; it's considered a health safety issue. At very least, your kids would be taken into custody if somebody heard you live at +15 degC.

*) necessitating a carefully tuned feedforward+P control system as described above by elekorsi

We also are almost hysteric when it comes to mold, and even the idea of dynamic temperature adjustments, or going below 20degC triggers irrational fears that it somehow automatically turns the house into mold nest in no time. This has a grain of truth in it, obviously; some temperature difference between indoors and outdoors is needed to prevent condensation.

I don't see how anyone could consider 15°C to be cold, unless they live in the tropics. Admittedly it's a little cool to be scantly dressed but perfectly fine with a jumper on. Heck it's 17°C outside today. I walked to the shop this morning wearing a jumper and by the time I got home, I was too warm and had to take it off.

I find it difficult to sleep in 25°C. When it's that hot, I just wear a pair of boxer shorts in bed: no sheets, blankets, or duvet. If I rented a place which didn't allow me to control the thermostat and it was that hot, I'd open all the windows and not pay the rent, until the landlord sorted it out.

I don't get much mold in my house. Perhaps a little bit around my bedroom window and bathtub, but it's easy to clean off. It's caused by humidity, not cold. I imagine increasing the temperature in my house would make it worse, since warm air holds more moisture, which would condense more on the colder walls and windows.

[tszaboo]

Depends on the weather more than you think. I have Nest thermostat at home, which has away assist, so it turns down the heating when I'm not at home. I checked the usage for last week, and during the day it was off, whether I was at home or not, because the sunshine, it was only going during the night.
But otherwise during the winter it does make a difference, which I can clearly see on the usage. And I can start the heating away from home early, so I arrive into a warm home.
So if you wonder, invest into a smart thermostat. I wouldn't actually recommend the Nest, as it measures the temperature wrong (wifi chip heats up the measurement, and I'm not even kidding here), had to RMA one, and it's not possible to extend it to multiroom, because they don't sell the same hardware in europe. So get some other type. It pays for itself, if you forget it on during one 1 week long winter vacation. But I would argue that it will pay for itself just with regular usage within a year or two.

[themadhippy]

If it ever gets below 20 degC in a rented apartment, we ask for the authorities to perform official measurements and file complaints, or seek criminal charges against the owner; it's considered a health safety issue. At very least, your kids would be taken into custody if somebody heard you live at +15 degC

Talk about nanny state,if  that was the case when  i was a kid half the population would be in care,waking up to frost on the inside of the windows  was accepted,you just got dressed and downstairs  as quick as possible, hoping your mum had got the fire going.
Maybe that's why i can sit here comfortable in a t shirt and shorts with the thermometer reading 18.4c

[Siwastaja]

I don't get much mold in my house. Perhaps a little bit around my bedroom window and bathtub, but it's easy to clean off. It's caused by humidity, not cold. I imagine increasing the temperature in my house would make it worse, since warm air holds more moisture, which would condense more on the colder walls and windows.

Increasing temperature means air can hold more moisture, it doesn't mean extra moisture is generated, so increasing temperature by heating (by a heat source which doesn't put water into the air, so a boiling tea kettle does not work) reduces RH, absolute humidity being what it is (as defined by how much moisture you breath out, wash dishes, bath, and so on). Now the surface mold you wipe off is caused by said surfaces being cold, below the dew point (which is an alternative way to describe absolute humidity). Absolute humidity is almost always higher indoors than outdoors which is why you will get condensate on the coldest surfaces if the heating or insulation is lacking. But increasing temperature of all surfaces removes this condensation because dew point is not reached, in other words, 100% RH is never reached.

This is also why we almost never ever see surface mold here anymore and if we do, many get totally hysterical seeing it (not understanding surface mold is not a big deal, you can indeed just wipe it off). Instead, the big issue is having leaks in vapor barrier so that hot and humid air goes inside structures where it cools down, hits the dew point, and condensates inside the insulation. If the insulation is otherwise OK, surfaces are nice and warm and always stay dry, but the problem is under the surface.

But totally cut the heating and buildings will slowly develop mold due to temperature and humidity variations causing conditions where RH hits 100% on a regular basis. Just a tiny bit of heating is enough to prevent this if there are no sources of added humidity (i.e., absolute humidity). The more there is added humidity such as bathing and breathing, the higher the temperature of surfaces must be to keep them below RH100%.

And to be clear about what I said earlier, I would enjoy living in 15degC if I had to, this like frozen windows was a normal living standard in 1950's, and I wouldn't want to control others about this choice but the boomers decided no one is allowed to experience what they did when they were kids.

So just describing the attitudes as they are here. Combining our standards of insulation (new buildings are required with equivalent of 250mm of top-class mineral wool in the walls, and 400-500mm(!) in the attic), heatpumps, and the British standard of acceptable room temperatures, we could cut CO2 down much more than we currently can due to people wanting to live at +25degC for reasons I really don't understand. Putting clothes on works for me.

[emece67]

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[tunk]

Around here much of the heating is done with electricity.
And the price varies during the day - I think it's lowest
during the night, and highest around breakfast and dinner
time. So you can save a bit if you switch off heating during
the high price periods.

More info here: https://www.nordpoolgroup.com/Market-data1/Dayahead/Area-Prices/NO/Hourly/?view=chart