> 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-conditionin...

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

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.

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.

I'd imagine that has more to do with price of power than absolute amount.

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.

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).

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.

If anything it should be less efficient the colder you go.

Cold house running during middle of hot day should be lowest efficiency during the day

>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.

Do they though? If it’s hot, sure, but they seem able to tolerate temperatures of -50 centigrade, and sleep outside at the South Pole.

So to save environment we should ban pets