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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, syst

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.

andruby 3y ago
> 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. Full data on https://theconversation.com/does-turning-the-air-conditioning-off-when-youre-not-home-actually-save-energy-three-engineers-run-the-numbers-188694 https://theconversation.com/does-turning-the-air-conditionin...
- Larrikin 3y ago
  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. https://youtube.com/watch?v=Dn8NRC1XrrA https://youtube.com/watch?v=Dn8NRC1XrrA
  - tadfisher 3y ago
    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.
    - regularfry 3y ago
      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_1256.html https://www.engineeringtoolbox.com/moist-air-properties-d_12....
- tlogan 3y ago
  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. Also AC runs more efficiently if its constant.
mynameishere 3y ago
Odd how common this notion is. It would be like leaving your oven running all day because you make food in it occasionally.
- ilyt 3y ago
  Or water boiling all day. I think that's just power companies not wanting everyone to turn on AC at roughly same time is the reason for those power "saving" tips.
housemusicfan 3y ago
Not to mention that most prescription medication specifies that it must be held at room temperature (72-ish degrees) or risk spoilage. Your refrigerator will also work harder as the ambient temperature climbs.
- olddustytrail 3y ago
  That's not a thing. Any prescription medication can be stored in a refrigerator if it degrades with heat.
  - housemusicfan 3y ago
    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?
    - olddustytrail 3y ago
      I've literally never seen that on any medication ever, and it's impossible for pretty much any household to meet such stringent measures.
supertrope 3y ago
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.

andruby 3y ago

> 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. Full data on https://theconversation.com/does-turning-the-air-conditioning-off-when-youre-not-home-actually-save-energy-three-engineers-run-the-numbers-188694 https://theconversation.com/does-turning-the-air-conditionin...

Larrikin 3y ago
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. https://youtube.com/watch?v=Dn8NRC1XrrA https://youtube.com/watch?v=Dn8NRC1XrrA
- tadfisher 3y ago
  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.
  - regularfry 3y ago
    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_1256.html https://www.engineeringtoolbox.com/moist-air-properties-d_12....
tlogan 3y ago
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. Also AC runs more efficiently if its constant.

mynameishere 3y ago

Odd how common this notion is. It would be like leaving your oven running all day because you make food in it occasionally.

ilyt 3y ago
Or water boiling all day. I think that's just power companies not wanting everyone to turn on AC at roughly same time is the reason for those power "saving" tips.

housemusicfan 3y ago

Not to mention that most prescription medication specifies that it must be held at room temperature (72-ish degrees) or risk spoilage. Your refrigerator will also work harder as the ambient temperature climbs.

olddustytrail 3y ago
That's not a thing. Any prescription medication can be stored in a refrigerator if it degrades with heat.
- housemusicfan 3y ago
  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?
  - olddustytrail 3y ago
    I've literally never seen that on any medication ever, and it's impossible for pretty much any household to meet such stringent measures.

supertrope 3y ago

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.