You must not have heard how, apparently, most Americans do not turn off (or down) the heat / A/C when they're not home. It's pretty crazy yet is a real thing!
If you haven't run the numbers you could in fact be wasting more electricity by fully turning off your system when you aren't home. Depending on your home, system (central air versus room unit), how long you are away, climate, and temperature preferences it can often make sense to leave your AC or heating on at a different temperature versus fully shutting it off.
Having your home heat up to 90 degrees while you're doing some weekend errands only to crank the AC to cool it back down to 70 to 75 will often times use more electricity than if you just set the AC to 80 while you were gone.
> If you haven't run the numbers you could in fact be wasting more electricity by fully turning off your system when you aren't home
I've seen similar statements a few times, even in this HN thread. Most articles just repeat the statement without showing any numbers. This article does show numbers and the graphs. From the article:
> What we found was that even when the A/C temporarily spikes to recover from the higher indoor temperatures, the overall energy consumption in the setback cases is still less than when maintaining a constant temperature throughout the day.
They are multiple reason why it is better to leave AC on. But it really depends on the house.
One main reason is that when AC is off is that humidity increases which causes cooling harder. This is because the cooling process of an AC unit involves removing humidity from the air as well as heat. When the air is humid, the AC unit has to work harder to remove the excess moisture from the air, which can decrease its overall cooling efficiency.
The other reason is that house and furniture will store heat and then cooling will get harder.
Digging into the article, it's a hypothetical home and they are only testing fully off versus ideal temperature blasting.
I don't think it's always true, which is why I listed the factors that can influence it.
But I am also getting into home automation and just watched a video where some one ran through the numbers on their specific house after getting electricity monitoring connected to their heating and Home Assistant. It's not that clear cut and really depends on your specific home.
I don't see how this could be possible, because it should always cost more energy to maintain a higher temperature gradient, assuming a non-zero R-value. If there are other factors you're optimizing for, such as energy cost rather than overall usage, then I could see how avoiding peak pricing could net savings in cost, but the laws of thermodynamics are pretty absolute if you're optimizing for energy consumption.
You're assuming that the system is the same in both cases. It's not.
1. You run the A/C all day: the gas stays both cold, and dry. Cold, dry air is, it turns out, ok as an insulator. Not very good thermal conductivity at all.
2. You turn the A/C on at the end of the day in a warm, humid environment. The air is moist, which causes two compounding problems: first, the volume of air you need to cool just holds more energy than it would if it was dry. But second, because moist air is more thermally conductive, it's better at coupling heat in from the outside, so more energy is coming into the system as you're trying to cool it.
At least, that's the argument. I haven't run the numbers to check orders of magnitude or anything. I wasn't expecting the difference in thermal properties of dry and moist air to be that significant, but there are some very interesting numbers at https://www.engineeringtoolbox.com/moist-air-properties-d_12....
Relaxing HVAC usage such that the temperature increases from 75F to 78F, and then is lowered back to 75F saves energy compared with holding it at 75F. The max temperature can't be too high because the system might need hours to lower the temperature by 3 degrees. e.g. A family leaves at 7 a.m. AC turns off. The high temperature is reached around 1 p.m. Cooling resumes at 2:30 p.m. so it can hit the target temperature by 5 p.m.
I only know of two mechanisms that could make holding a temperature more efficient. Heat pumps in winter time should be left to maintain a temperature. If it is well below freezing outside the heat pump may need to fallback to a secondary mechanism of resistive heating. Therefore it's better to not setback the temperature immediately when the home is unoccupied. Variable speed fans on higher end furnaces use less energy at lower RPM, so running a system at full effect requires more power.
So the label on the package that says "must be stored between 20-25C at all times" is bullshit? Because some random internet people said it was false? Why would they put the label on the package then?
I actually got an e-mail from my apartment manager telling us not to do that because supposedly the A/C uses more power to cool down the room by a few degrees every evening than it would've spent to keep it at that temperature all day.
although personally i disregarded that because eight years of living in overpriced CA apartments that were built during the kennedy administration has conditioned (no pun intended) me to feel more comfortable with the AC off.
Here is an article[1] which cites Logan Kureczka, lead communications consultant at Duke Energy in North Carolina: "If you're just leaving home for the day, it's more energy efficient to keep your air conditioning turned on – but turn it a few degrees higher than you might set it for comfort if you were at home."
This seems pretty legit. I also remember getting similar tips from PG&E here in California. Do you have any citations as to why turning off AC/heater is better than leaving them on when we leave home?
There doesn't appear to be any actually sensible evidence in the article?
> I also remember getting similar tips from PG&E here in California.
Company that sells you energy telling you to not stop consuming energy...
Aside from that, I'd imagine they also want to limit power usage spikes. Everyone coming back from the weekend then running AC on the full blast just consumes a lot of power.
Our office was actually given same advice (just leave AC running over weekend) but it was explicitly said it was to avoid power usage spikes, as cooling back down space that big just takes a lot of power for a long time.
There might be some truth about saving money, if say the rate for power when you'd be turning AC back on is high, vs having that spread over lower rates, but certainly not kWh, at least not if you go out for a whole day or two.
> Do you have any citations as to why turning off AC/heater is better than leaving them on when we leave home?
Basics of thermodynamics ? I think easiest comparison would be boiling water. What makes more sense, to turn off a pot of boiling water after making tea, or to keep it running till you want next cup of tea? You can keep it running to get the next tea instantly ready, or turn it off to not.
It's the same for AC. Turning it off saves power, but it will take longer to get back to the cold state)
You need to keep moving energy (whether heating or cooling) to/from the system to keep it at temperature different than ambient.
The bigger the difference, the more energy needs to be moved.
Keeping AC on constantly is like keeping a pot boiling, you need to pump that energy constantly.
Turning it off means that you stop, and while yes, you have to dump more power to get back to the desired temperature, you save all that power that you'd need to use to keep it at previous temp.
> There might be some truth about saving money, if say the rate for power when you'd be turning AC back on is high, vs having that spread over lower rates, but certainly not kWh, at least not if you go out for a whole day or two.
That might be the main reason behind PGE's suggestion - they charge more for 4-9pm usage and it is much cheaper to pre-cool the house between 12-2pm when the demand is lower.
I want to come home in a comfortable house. My parents used to do that. Turn off all heating. Come home, it's cold outside, cold inside and you have to wait until the radiators start to heat up. That takes a while.
But it worked, radiators can heat up a room quite fast.
On the other hand, if we consider heating with a heat-pump, or cooling with a heat-pump (i.e. just a normal AC) they rely on continuously heating up (or cooling down) air (or water in the case of an air-to-water or water-to-water heat-pump).
So you just can NOT quickly change the temperature in a room. So I leave the AC on 78 all day. If I go away for a couple of days I'll set it to 85, but anything higher than that is not recommended.
This is a non-issue with Nest thermostats (and maybe other brands as well). They have a pre-heating/cooling feature which figures out how long it takes for your heating system to get up to temp, and starts heating/cooling early so it can reach the target temp by the scheduled time (eg. when you get home).
It's not crazy at all. For example my house has floor heating. It takes a looong time for that slab of concrete to cool down. There's no point at all to mess with the temperature for a short time.
I don't understand. What stops you from changing it yourself and swapping it back when you move out? And you're ready to propose more legislation.
Most programmable thermostats need batteries, which now have to be changed on a maintenance schedule. One more task for the superintendent.
These batteries often fail without warning. Which means pipes can freeze, leading to property damage. Most homeowners don't even know they have batteries till the heat stops working and they call the HVAC guy.
Shady property managers consider security deposits an additional revenue stream and will cite any minuscule damage as reason for forfeiting it. For example they'll cite a need for "professional carpet cleaning" that coincidentally costs as much as the deposit, and then just pocket the money without delivering clean carpets for the next resident.
When property management does not pay the utility bill they will furnish apartments with the cheapest energy hog appliances. When they do pay the utility bill they will lock the thermostat.
If you select an premium apartment and the market is not tilted in favor of landlords as it is in most cities then they tend to care about the customer experience. Since they're making their margin on the higher rent already and they know too much BS will lead to people switching away.
I've always been told it was cheaper to maintain than to cycle on/off within a day as long as the insulation is decent and you aren't trying to get to 60F during 100F weather. My guess is that comes from a belief that the units work inefficiently to get the temp down as fast as possible when turned on, but work more efficiently to maintain a temp, but I have no idea if that's true.
considering the mass of a home, what’s the efficiency difference between letting the temperature stay in uncontrolled for 8+ hours and then trying to rein it in quickly vs just keeping it in place the whole time? remember to account for wear and tear on the relevant HVAC equipment.
>considering the mass of a home, what’s the efficiency difference between letting the temperature stay in uncontrolled for 8+ hours and then trying to rein it in quickly vs just keeping it in place the whole time?
The energy lost from your home to the environment is dependent on the temperature difference. A warmer house (during the summer) loses less energy than a house that's cooled the entire time. Therefore at least from a pure energy use perspective, it's always cheaper to only turn on the system when you actually need it (ie. when you get home) than trying to maintain the same temperature.
>remember to account for wear and tear on the relevant HVAC equipment.
Without an accurate wear/tear model of HVAC equipment this can easily go either way. You could argue that having the equipment run for longer cycles shortens life because it has to work "harder", but you could also argue the opposite because most of the wear originates from the system starting/stopping and/or thermal stress from it being on/off.
