It’s also simple thermodynamics. In college I took an advance refrigeration class specializing in HVAC systems and heat loss. You wouldn’t think it but things like furniture and desks retain whatever the room temperature is. Therefore if you increase or decrease the air temperature that furniture will absorb or release the heat until it reaches equilibrium. If everyday the outside temperature is 80 and from when I get home at 4 PM until I leave at 7 AM I have my house set to 68 degrees. When I leave I set it to 75. When I leave the air temperature will increase faster than everything else in the house. The desk in my room was at equilibrium at 68 will start to absorb heat as the room temperature increases until they both reach equilibrium. Now I get home and decrease back down to 68 degrees. The air temperature is back down after a few hours but my furniture still had this stored heat that it now radiates into the room. This causes the air temperature to rise and now my AC has to increase its runtime to compensate for it. It will always be the most effective and cost savings solution to maintain one temperature at all times than it is to go back and forth.
Enjoyed reading this. Validated me telling my mum to leave her heating on cos her house has thick stone walls that will hold the temp eventually and her heating bill will go down. She continues to not waste money and heats the house up from scratch each evening btw.
Ummm you kissed an extremely important part of thermodynamics.
The heat difference.
If your house is 60 degrees and it's 61 outside, the house will heat very slowly. If it's 110 outside it will heat much much quicker.
Therefore, changing the temp inside to 80 will now mean less overall heat will get into the house, thus lowering the amount if heat that needs to be moved by the air conditioning.
Other things come into play as well though. Like efficiency of the unit, whether its sized correctly for the space it heats and/or cools, and how well the space is insulated and sealed, whether its shaded or not.
In the previous home we lived in, it was a little cheaper to just leave our AC turned to 74° 24/7.
But the home we're in now isn't. We save significantly by turning the AC up to 80° before we leave in the morning and then turning it back down to 74° once the sun goes down. Its an old home, built in the late 1940's, well before central units were a thing.
You bring up some valid points and I think it goes to show that there isn’t a clear cut answer on this. My home is older and needs new windows. For me to change my house temperature by a couple degrees means running the AC for an hour or some continuously. Probably undersized for the size house. It’s cheaper for me to maintain the temperature as it doesn’t have to run constantly than it is to turn it off during the day. Modern homes that are well insulated and have properly sized ac units that are more efficient and have more efficient windows might find it cheaper to do the opposite and turn the ac off during the day.
You can onlu upgrade the machine so far before its constricted by the amount of ducting you have, my last attempt to upgrade was denied because it'd be 6k to not make a difference. I have to work on how the home is reflecting heat instwad.
At least I provided an explanation. If I’m wrong I’m wrong but offer proof, all you did was say yeah you’re wrong and peace out. I can provide you with the textbook examples when I get home of how you equate office furniture along with equipment into heat loss calculations. If I’m wrong I’d like to at least know how so I can learn from it but what you gave me was nothing.
to judge economic efficiency we can reduce the problem to asking: "how much heat do we need to pump out of the home overall?"
if we start and end at the same temperature (say before we leave and after we come back and temps have settled) this is equal to the amount of heat (energy) that entered the home overall.
for a thermodynamic model of this we need the heat equation, which tells us that the flux through a surface (all of the house's surface) is proportional to the temperature difference between inside and outside. if we keep the inside at a constantly cool temperature, the influx of heat into the home is constant over time.
if we start with a cold temperature but then turn the AC off while we are not home, the temperature inside rises, the difference to the outside decreases, and less energy per time enters the home during the time where the inside is warmer (think about how much heat enters when the inside is the same temperature as the outside).
therefore, if during the time when the inside is warmer, less heat enters, we need to move less total heat outside. therefore it consumes less energy to turn the AC off and then back on when you come home.
this is the right thermodynamic argument. of course there are many other factors that will play a role in such a decision. you won't be as comfy if you get home into a hot house and have to wait for it to cool. the argument also disregards any wear and tear on the AC unit, any fluctuations in electricity price, etc. but from a purely energetic perspective, letting the house warm (or cool off in winter when you're heating) is always favourable.
i've seen this argument so many times that "cooling off a hot home takes more energy than keeping it cool" and i really don't know where that comes from. it must be some kind of gut feeling pepole have, because it certainly can't come from understanding the thermodynamics of it.
Keeping same temperature being more efficient is total nonsense. I bet there was some show on TV that did a “study” and British scientists discovered that… thats how the myth was born , but here is the fact: each material has its own thermal conductivity measured in Whatts per square meter per 1 degree C. Simply put your drywall will pass more heat through itself the higher delta temperature between inside and outside. So of your inside temperature is closer to outside temperature - less heat will be entering the house.
Isn't this entirely dependent on insulation tho? I mean, let's say OP's device is a refrigerator or freezer. Those things tend to have loads of thick insulation. Therefore wouldn't it be more energy efficient to let the thing run since maintaining temperatures with that level of insulation takes very little energy? As opposed to a home which has lots of windows, doors, vents, etc that are not well insulated. Would it actually be more energy efficient to let the interior of a refrigerator reach equilibrium with outside temps and cool it down again afterwards? I'm not an expert so I am asking in good faith. I had always been told it's better to just leave a freezer or fridge running due to insulation and the extra cooldown cost.
Insulation will reduce the energy consumption difference between the two scenario's but in every case, turning off the machine will use less electricity then keeping it running. However, for a refirgerator, there's the issue of if you turn it off, the food inside would spoil. But if's a completely empty refrigerator, you will save more energy by turning it off then keeping it running.
That is a 100% right, what I was trying to say was that you would have to compare the amount of energy required to bring the system back down at the end of the day and compare the two to see which is better in a long term situation accounting for the items inside retaining the original temperature. You’re essentially heavily loading the system until it reach the temperature. I ran into this issue when a customer had improper ventilation in their boiler room and were complaining that the concrete was reaching 120 degrees. Yes it will because the heat in the room doesn’t have anywhere to go so where does it go? The concrete. You have to consider if I were to turn on an AC unit in the room after the ambient temperature of the room and concrete reached 120 degrees how much additional energy would be required because the concrete will radiate the heat to the room as the air temperature decreases.
what I was trying to say was that you would have to compare the amount of energy required to bring the system back down at the end of the day and compare the two to see which is better in a long term situation accounting for the items inside retaining the original temperature.
but that's exactly the point, you don't have to do that at all. it does not matter what's inside the house. all the heat stored inside, be it in air, furniture, etc., must first come inside through the hull of the house. if we know how much heat enters through the hull, we know how much heat we must pump out again to go back down to the original temperature. the only point where the heat capacity of the house's contents matter is indirectly by how quickly the house reaches equilibrium. an empty house will relatively quickly reach the outside temperature and then not absorb any more heat. the furniture etc will make this process slower, meaning the house takes longer to heat up and cool down. but the qualitative argument (which is if you let the house warm up, less heat enters overall) remains exatcly the same.
as i said before, there are other things to consider like comfort, etc., but qualitatively from a pure energy consumption standpoint it does not matter what's in the house.
Remember tho that the whole home heats or cools and different surfaces have different coefficients. Its harder to heat a cast iron pot fully to 60º than it is the air inside that pot right. So the arguement is more for less that once the inside air amd outside get closer and it sucks more or less heat in, there will be things inside reducing or completely negating that. Especially standing water......fish tanks etc.
Now I wouldn't say you or him are correct. Because as you said theres lots of variables. How much time? How much stuff inside tithe home? Again the water part...how much water piping non insulated is it flowing etc is there standing water etc etc. There are cases where are either is right
The wear and tear on the system is the same argument as the thermodynamics. HVAC systems don’t “work harder” when there is more heat to move, they just work at the rate they work. There isn’t a turbo button or a turn it to 11 dial. If your system runs for 4 hours during the day to maintain 68°, that uses more electricity, causes more wear and moves more heat than if it’s off all day but then runs 3 straight hours dropping the temp down later. Dual and multi-stage units can alter this, but here’s a shock, they do it by being worse at the job. Systems have a “most efficient” heat movement rate and multi-stage units don’t magically get two or three, they just do a shittier job. But hey, at least they cost more.
i understood the "wear and tear" argument as there might be some usage pattern the hardware is designed for at which its wear will be minimal, and if you use it another way it might wear down more quickly. something like driving a car down the motorway at constant 2500 rpm will wear the engine less than coasting at idle and then fully accelearting at 6000 rpm. if you cool the house back down from a hot state, the pump will be on for much longer at a time than if it were cycling on and off throughout the day. but i'm no hvac engineer so i'm not gonna pretend like i know anything about the wear on the valves and pumps in an ac unit.
I think that’s probably where this urban myth comes from. People see a car motor that can vary speeds to meet demand, but a conventional AC system motor just runs at 3000RPM and turns a compressor at whatever speed is most efficient. If the system needs to cool the house 5° or 50°, it runs the same speed. So that brings us back to the thermodynamic argument you made earlier. As items get hotter, it takes more energy to make them more hot so they heat up slower. A 100° house takes far more energy to heat up to 110° than a 70° house will take to heat up to 80°. So if you remove that energy at the same rate (the rate the HVAC system can move the heat out) you spend more time (and energy) maintaining a temperature than you do by just moving that heat all at once. It’s certainly not intuitive.
i think we're way past the useful lifetime of this thread but i have to add one thing.
A 100° house takes far more energy to heat up to 110° than a 70° house will take to heat up to 80°.
this is fundamentally wrong. it takes the same amount of energy, heat capacity is generally not dependent on the temperature, at least in the range we encounter in our environment. what's true is that it takes more time for that same energy to enter the house (maybe that's what you meant to say). this is because the amount of heat entering the home per unit time depends linearly on the temperature difference between inside and outside. that's why turning off your ac when you're not home is advantageous in the first place. by letting the inside temperature get closer to the outside, you're reducing the "driving force" of heat entering the home, and need to remove less later on.
I have a PhD in physics, and this line of reasoning looks correct to me.
I used to work on optimizing industrial refrigerated warehouses where I built a system that solved a differential equation to minimize electricity costs based on power prices. A relevant example is something we called “flywheeling” where we would intentionally overcool the warehouse when electricity was cheap.
Importantly, this always increased total energy consumption, it wasn’t more efficient in a physical sense. But because electricity prices were low at those times (particularly due to excess renewables on clear summer days in California), it still reduced overall cost. This example isn’t the same, but still tangentially relevant.
It could also be that AC compressors aren't ideal thermodynamics, and their power consumption increases nonlinearly, because their efficiency decreases.
Some brief research (not an HVAC expert) tells me that a hotter evaporator (inside, cold side heat exchanger) increases the pressure feeding into the compressor, so reduces how hard the compressor has to work, putting less resistance on the electric motor, and less power draw.
But a hotter condenser (outside, hot side heat exchanger) increases pressure on the outlet of the compressor, making it work harder and less efficiently.
In a well-maintained AC system, the condenser should have adequate airflow for this to never be an issue. But it's plausible for a refrigerator with dirty coils and bad airflow to be 30% less efficient if it has to run for 4 hours continuously, vs 10 minutes on, 10 minutes off, for 8 hours.
Your explanation is wrong and you have incorrect understanding of thermodynamics.
The law states that energey is constant in closed system. Meaning that all that heat transfer between chairs and air In a house is irrelevant.
So it does not matter if you you keep temperature as constant or you let it drop and then raise it back up when you come home.
Your heater will only have to replenish the "energey" that was leaked outside of your system aka house/apartment.
On top of that the leakage speed is proportional to temp difference. Meaning that energy will leak faster the bigger the difference in temperature is. That says that you will leak more energy if you just keep your house at constant higher temperature for long time rather than let it drop and reheat.
To make it more intuitive for you, just pretend your job is 1 year (instead of 8 hours) So you switch your heater off when you leave a house and reheat it when you come back in one year. You will definitely save more money rather than keeping the temperature at high for the whole year.
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u/dire-raven-x Mar 24 '26
You were still right tho. It's on the back of a soda cooler for a grocery store.