Elementary thermodynamics - all else being equal, the general model is that the heat exchange between a house or box or refrigerator or whatever, is proportional to the temperature difference. (All insulation does is slow the speed by which heat escapes - the number of calories/BTU per minute) So very simple - your house loses less calories to the outside when it is cooler than when it is warmer.
Does it take more energy to heat the house back up? No. If you keep the house at a constant temperature for 12 hours, you are simply compensating for heat (calories/BTU) lost to outside minute by minute. If you let the set point go down, your heat loss to outside is less during that time. Bringing the internal temperature back up again simply adds those calories/BTU that were lost to the outside (which is why the house cooled off) but the house being cooler, you lost less calories to outside during the low setpoint.
You use less energy, for example, boiling a kettle when you need it than keeping it boiling all the time.
I always thought a car was more efficient staying at speed than slowing down and speeding up again.
It is easier to maintain a speed than it is to change a speed (indeed, some modern cars will turn off pistons once at cruising speed to reduce fuel consumption). So, why doesn’t your room temp work like that (I get your furnace cannot turn off pistons)?
Not exactly - your car uses a lot less gas if you park it then start it up again an hour later…
Thermodynamics is a zero-sum game. what heat your system loses goes out to the wide world.
Especially an electric furnace, with no chimney or effluent, all the power consumed creates heat which then remains in the system (house) until it leaks out into the wild blue heatsink yonder. Except for state changes, when ice melts or air liquifies, heat is linear. 1 calorie raises 1 g (ml) of water 1°C. For other substances, it depends on the specific heat how much it raises the temperature. But the calories are the same calories, whether it raises the air from 60F to 61F or 70F to 71F. So the only math is - how much heat did you put into the house using electricity, and how much leaked out to the wild through conduction of the walls and ceiling?
For automobiles, the math is more complex. There’s rolling resistance, air resistance, the design of the engine, etc. But essentially, speed-up-slow-down doesn’t work sometimes because when you slow down, your gasoline engine does not recapture that energy to re-apply to the vehicle when it accelerates. In fact, it is more efficient in most scenarios. Slow from 70mph to 55 without using the brakes because of air and rolling resistance and bearings friction, etc, and then cruise a while at 55, long enough so you make up for the energy lost during deceleration and then return to 70 - you’ll use less fuel because air resistance is less at 55.
Possibly, but this was a wall-mounted unit designed to be fitted in living spaces. I suppose there are also design constraints at play such as a)containing enough bricks to store the heat and b)not taking up too much space with insulation.
The ones in my house comprised an inner box made from some sort of mineral board (probably not asbestos), containing the bricks and heating element, and the outer metal and plastic cosmetic shell, insulated in between these two shells with about 5cm of glass wool. (I had to take them apart to uninstall them as they are too heavy to carry in one piece)
Except for modern high-end units, residential furnaces have two settings: on or off. When they’re on, they operate at a particular efficiency, regardless of any other factors. So total operating cost depends only on what % of the time it’s running, which in turn depends on the time-averaged temperature difference between indoors and outdoors. Keep the latter low, and you keep the former low, nothing more to it. One way to do that is to let the indoor temp drop as low as you can tolerate it, whenever you can tolerate it, before heating the house back up.
Cars are more complicated. Engine efficiency varies with RPM and accelerator pedal position (engine load). Engines tend to be most efficient at a high percentage of full load, and this is especially true for conventional gasoline engines. So if you achieve a particular fuel economy while cruising at a steady speed on a flat road, you can actually get better fuel economy while cruising at that same steady speed in hilly terrain (provided the downhills aren’t so steep as to require the use of your friction brakes). On flat terrain, your fuel economy could be improved by a similar strategy, albeit one that requires allowing your speed to vary: accelerate hard to your max acceptable speed, then coast to your lowest acceptable speed before repeating the cycle.
In formal fuel economy engineering competitions, the typical winning strategy is to use a very small engine operated at high load to accelerate the vehicle up to a target speed, and then shut the engine off to coast; at the minimum acceptable speed, the engine is turned on again.
Highly specialized vehicles operated with the best strategy can get thousands of miles per gallon, but as that link shows, even vintage carbureted production passenger cars are capable of hundreds of miles per gallon under optimal circumstances.
Assuming there are only two rates for day and night, the algorithm is fairly simple:
First, define a max_tolerable_temp, a min_tolerable temp and a preferred_temp.
If the house is well insulated enough to not lose more than (max - min) through the day period, then heat the house at dawn so that it spends the most time closest to preferred_temp and never turn on the heat during the day, start reheating the house as soon as the cheaper rate kicks in.
If the house is not that well insulated, heat the house to max_temp at dawn, let it coast down to preferred_temp, keep it at preferred_temp during the day until the end of the day and let it coast down to min_temp right before the night time rate starts.
If there are more gradations of rates but the house is well insulated, then just heat the house to max_temp at the cheapest rate time. If it’s less well insulated, then it’s a bit more complicated but you can just keep on adding more heating periods from the cheapest rate first until the house never drops below min_temp.
Something I’ve wondered about is the use of more effective/efficient control algorithms for heater/AC control. For example, instead of a bang-bang control system like most home thermostats, would a PID or something similar be more effective? That could smooth out those peaks- a PID is going to take into account how fast the temperature rises and falls in your home, and run the heater accordingly to keep the temperature where you want it.
I think that greatly depends on how you used your dumb thermostat. If you just left it on the same temperature year round (or at least throughout the season) 24 hours a day, it’ll save money by adjusting the temp when you’re gone. However, if you’re like me and were already doing that, it saved some, but not as much. The reason being that when I left for work in the morning and turned (if it’s winter) my heat down, I might only turn it down to, say, 68 so I wouldn’t have to sit in a freezing cold house for two hours while it warmed up. With the nest, or any programmable t-stat, I can let it get considerably colder and the Nest knows when to kick the furnace back on so it’s at my preferred temp right as I’m getting home.
The Nest, however, isn’t without it’s quirks. I absolutely loved it when I got it. It worked perfectly. But for whatever reason they’re consistently updating the software and changing their algorithms. One by one I had to disable certain smart features to the point that I essentially just have a wifi enabled t-stat. And to top it off, Nest would pretend like they had no idea what I was talking about. I could go to their facebook page, ask them why this or that odd thing happened and they’d tell me they’d never heard of that before and C&P some troubleshooting steps. Seems fair enough, but then you’ll see a dozen other people complaining about the same thing, on the same facebook page. Clearly they’re aware of the problem.
PS. One thing I really do like about having a Nest t-stat and Nest Protects (CO and smoke detector) are that if the smoke detector sense smoke, it’ll shut down the furnace/AC to help from spreading the smoke (or fire) all over the house. Similarly, if it sense CO, it’ll shut down the furnace/AC both to help keep it contained and in case the furnace is the source of the CO. Plus, it’s nice to know if there’s a CO leak in my house, I’ll know about it within minutes, not when I get home and wonder if the alarm’s been going off for 10 minutes or 8 hours.
Side note: Something that would be nice is a gas valve that you could install on your main gas line that connects to the Nest. That way if it detects CO, it can shut off all the gas and stop all production of CO within the house. It wouldn’t help with external sources (car in garage, something outside), but at least when you get the alert, you know the water heater/dryer/stove/furnace etc are all shut off.
I have a moderately smart TM. I can set up to six time zones with different temperatures for each and easily alter the current setting which reverts the next time. It also has a vacation setting that turns it down to the minimum for a set number of days.
We are retired, so at home all day, so I only use night and day, dropping a full 5 degrees at night from the 22 °C we have it set at during the day. On warm days I switch it off altogether as it does sometimes come on when windows are open.
What I would like is a more sophisticated system that sensed outside temps and responded by making predictive changes. Our heating is by wet radiators, so relatively slow to respond and insulation is above average but not great,
My previous programmable thermostat, a Honeywell from 15 years ago or so, did that, and my current one, which is just a newer Honeywell programmable, will also do it. In the default mode, if I say I want it to be 72 at 7pm, it will start running the AC before 7pm, so that it by the time it gets to 7pm the temperature is at 72. Supposedly it learns how long this takes, so it can start running 30, 50, 90 minutes early.
As with all thermostats, when set to 72, it doesn’t turn on the AC at exactly 72.01, but at 74 or whatever. I know the old one was supposed to learn something about how fast it would reach the set point from the trip point, so it would adjust the trip points. The logic was then “the actual temperature is 10 minutes away from the set point” rather than “the actual temperature is 3 degrees away from the set point.”
I have all of that behavior disabled on my current thermostat, because I have time of use electric billing, I view the temperature sets as permissions, rather than desires. I don’t desire 72 at 7pm, rather at 7pm when the low rates kick in, the AC has my permission to cool as low as 72. During the high rate times it only has permission to cool to 80.
If your furnace has continuously variable heat delivery, then yeah, a PID would probably allow for tighter temperature control, but I’m not sure it would improve efficiency.
Modern residential gas furnaces are two-stage, i.e. they have two non-zero levels of heating power available; I’m not sure how much a PID control can do with that. Older gas furnaces are single stage - they’re either on or off - and I don’t think a PID can do anything useful with that.
No idea what the typical capabilities are for a residential electrical furnace.
That sounds very much like a PID controller. The big advantage as I see it would be that it would “know” the rate that the house warms up as well as how fast the AC can cool it off, and pre-emptively run the AC to keep the temp where it’s supposed to be, without overshooting or undershooting.
Probably not a lot of efficiency gain, but still some I imagine.
That’s one of my beefs with my Nest - it theoretically will heat the house preemptively, I.e. so that it’s at the desired temp at the desired time - but does not seem to accomplish that. I don’t think that “overshooting” is a problem with furnaces. Given the size of the house and the amount of heat in the vents, etc - it’s not going to overshoot by more than a degree or so. The bigger problem is thermostat location - adjust the balance in the ducts so that every area gets even heat, so the thermostat perceives the same heat as everywhere else in the house. . Another problem - my Nest is located in a corridor, but sometimes I may not walk down that corridor very often, unless I’m doing laundry. So the nest gets a warped view of “when someone is home” and starts to make decisions based on that.
I think we’re working our way toward central home management systems like Google or Apple, but for now most appliances don’t talk to the central control. Worse than that, most don’t have the connectivity to even be controlled. I think there are shortcuts - for example, combine a smart power outlet with an item like an Airtag that does temperature, and you don’t need a smart refrigerator - but would still be able to vary the power use somewhat to a schedule. (I.e. don’t run the fridge during the daytime provided the temperature stays below a certain threshold.) My current alarm system has motion sensors and door sensors, but there’s no way to tie it to the Nest (or anything) so the Nest knows people are home, nor will it email me - a central monitoring service has to do that… My garage door opener has remote control, but currently is not wifi-controlled. (newer versions are). Like most people, i don’t think the gains from central control are worth the cost of replacing many of my appliances. The next generation of devices will come with such connectivity built in, but the next generation may have to wait for a generation.
That’s actually why I haven’t bought one. Our thermostat is in a hallway, and would do exactly what you describe.
I think they could do a lot of stuff to possibly make things better with technology- multiple temperature sensors and the ability to selectively cool/heat/circulate air for different parts of the house is one that seems kind of obvious. I mean, your kitchen may be hot as balls because your oven has been running, you’ve been cooking, and it’s on the sunlit side of the house. Meanwhile, the bedroom at the other side of the house may be 20 degrees cooler, and 5 below the thermostat’s temperature. Right now, neither would be remedied if the thermostat is in a hallway where the temperature reads what the thermostat is set to. But having temp probes and the ability to circulate the air around the house as needed might actually get the temperature more even without actually needing to run the AC or heat so much. Or you could just spot heat/spot cool as well.
That is the main reason I got a new thermostat. Most new ones will have a circulate feature, where they run the fan at least 15 minutes per hour. I think it helps eliminate hot and cold spots. It is just a dumb timer, though. If the AC ran for 5 minutes this hour, then it runs the fan for an additional 10 minutes, whether the temperature is unbalanced or not.
Some smart thermostats also have remote sensors, so they can know if that one bedroom is really cold, but I’m not sure what they do about it in a typical single zone residential setup.
My Nest has the feature (and these newer natural gas furnaces are intended) to run the fan all the time. This keeps the whole house close to the same temperature, even if some rooms have more windows or less vents than others. (it’s a 2-speed fan, low and high; low when not actively heating) In my old house, the living room with the crappy double pane picture window from 1962 and three walls exposed to the outside, facing north, it got cold faster than the rest of the house.
Definitely for AC you want to run the fan for a while after the compressor shuts down, as the coils are still cold. Better to distribute that cool. I assume the same goes for electric furnaces and the heat element. The heat is there - spread it around.
In general, as others have said upthread, it’s better to let the temperature dip. Your house will lose less energy when it’s cooler inside.
But…
One thing you have to be careful of is how your heating system will react to that tip. Many heat pumps have a much less efficient auxiliary heat that kicks on if the temperature difference between the current temperature inside the house and the desired temperature is too great. This heats the house up much more quickly, but does horrible things to your electric bill.
Newer systems and newer thermostats often have settings to prevent the aux heat from turning on and ruining your savings. You just need to make sure that you set/program the thermostat correctly.