Inefficient: A/C set @ <78 °F, heater set @ >68 °F

Geremia

"Setting Your Thermostat for Maximum Energy Savings" by Jeffrey Orloff claims that during warm months, setting an air conditioner less than 78 °F, or greater than 68 °F in cold months, is most efficient.

Another page claimed that going down to 72 °F in the summer months can increase energy usage by 50%. Why are A/C more efficient the lower the temperature they must cool to? Thanks

chrisbaird

You are misreading these articles. The A/C takes more energy to get your house to reach lower temperatures. The article says you can save energy by having the A/C not be "set any lower than 78 degrees Fahrenheit". Average comfortable room temperature is 68° F. The author is advocating that you live in a warmer-than-comfortable room in the summer, so the A/C does not have to work so hard, so you save energy.

Geremia

Sorry, I meant to write:My question is: Why 78 °F and 68 °F? Is he basing those numbers on physics or just pure convenience?

Geremia

If that were the case, why doesn't he advocate, say, 60 °F in the winter so the heater doesn't have to work as hard? Thanks

russ_watters

There is always a tradeoff between energy and comfort. Ultimately, the best temperature for you is a compromise based on personal preference.

I like to flip the issue over and ask people this: are you willing to pay 20% more to be comfortable in your own home?

chrisbaird

It's ultimately a personal preference. My wife is from Idaho and her family cranks the heat up in the winter and sits around in shorts and T-shirts. I am from Massachusetts, and my family sits around in sweaters in the winter. Perhaps its because the drafty old New England houses can't reach T-shirt temperatures in the winter without ridiculous wastes of energy. Personally, I think it's a waste of time to take off lots of layers when coming inside in the winter because the house is so hot, than put all those layers on again if I want to step outside for a minute. It's better to just leave the sweater on, and turn down the heat. My wife disagrees.

D H

From the article,

In the summer months, the air conditioner's thermostat should not be set any lower than 78 degrees Fahrenheit. Likewise, the settings should not be any higher than 68 degrees Fahrenheit in the winter. Keeping to these guidelines will help maximize energy savings.

Wrong! The only way to maximize energy savings is to zero out the expenditures. This can be accomplished simply by switching the thermostat to the "Off" setting. One could go one step further and completely gut the heating and cooling systems, selling the copper and other metals for scrap to realize an additional one-time savings.

Anything other than turning the whole thing off represents a compromise between money spent and comfort. I know some people who value their physical comfort to such an extent that they keep their house at 72 degrees, day and night, all year round. Others are so miserly they keep their house at 65 or lower in the winter, 80 or higher in the summer.

Geremia

Okay, thank you

So, the efficiency of a heater or A/C is independent of the temperature it heats or cools to? I thought the efficiency of an engine depended on difference between the hot and cold temperatures. Is this what you're saying?

Or, another question: Is the amount of energy needed to cool a house from, say, 80 °F to 78 °F is the same as to cool it from, say, 70 °F to 68 °F? Are the A/C's efficiencies necessarily the same in both cases? Why or why not?

Thanks

russ_watters

It does. But the main benefit of running at a higher summer setpoint isn't efficiency, it is reduced load.

D H

The load increases proportionally to the temperature difference thanks to Newton's law of cooling. The energy required to transfer a unit of heat to the outdoors is also proportional to the temperature difference if you had a perfect Carnot cycle A/C. Thanks to the second law of thermo, it's something like ΔT^1+κ, κ>0. Put these together and you get the energy required to run your air conditioner is proportional to ΔT^2+κ.

This means that decreasing the setting by 2°F costs a whole lot more when you already have the house at a chilly 70°F compared to the cost of decreasing the setting by the same 2°F when the house is at a barely comfortable 80°F.


The article never mentioned efficiency. It only talked about "maximizing energy savings", which is a silly notion.

Khashishi

An A/C is a heat pump. The A/C needs to do more work to pump the temperature much lower than the outside temperature because it needs to pump against a larger temperature difference.

A heater can be either a heat pump or a furnace. A heat pump is going to be less efficient if the outside temperature is much lower than inside, and you want to pump heat in. On the other hand, a furnace doesn't really care what the temperature outside is, as long as the house is thermally insulated very well. But, given that all houses leak heat, the rate of heat leakage increases when the temperature difference is higher.

Geremia

Yes, so, for a given outdoor temperature, what temperature should the A/C should be set to that would maximize the efficiency/load ratio?

russ_watters

As DH said, turning the air conditioner off will maximize that ratio (that ratio just yields input power). There is no inflection point in the curve. And even if there was, I see no practical value in the idea.

QuantumPion

The efficiency depends on the outdoor temperature, not your thermostat setting. If you run your A/C when it is 110 outside it will be very inefficient due to trying to reject heat with a small delta-T. If you run your A/C when it is 50 degrees outside for whatever reason it will actually be very efficient because there is a large delta-T between the refridgerant and the environment.

The temperature you set your thermostat to affects how long the A/C has to operate to achieve the desired temperature. Setting your thermostat higher means the A/C will run less and use less energy, but the efficiency is close to the same (ignoring efficiency loss of starting/stopping vs. constantly running, but that is a 2nd order effect more having to do with the size of the A/C system compared to the size and insulation of the house).