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Why Your Air Conditioner Is More Efficient in Fall and Spring


It’s cooled off a bit here in Atlanta. A couple of weeks ago we even had temperatures down in the low 50s Fahrenheit. Ah, that felt nice, especially since I’ve been running again this year.1 With the fall equinox two days away, we should start seeing more of those nice cool days. We’re back up close to 90° F now, though, and that means air conditioners are still humming. But the good news is that cooler weather reduces the cost of air conditioning in more ways than one.


Astounding scientific fact #1: Heat gain through the ceilings, walls, windows, and floors is less when it’s cooler outside. Of course! Lower outdoor temperatures means your house doesn’t warm up as much. Whether your house is uninsulated or superinsulated, heat flows from warmer to cooler (second law of thermodynamics). Whenever it’s warmer outside than inside, heat is going to move into your house. The insulation level affects the rate of heat gain (which is what a load calculation tells you), but there’s still heat gain.

And now, here’s astounding scientific fact #2: An air conditioner removes heat faster and more efficiently when it’s cooler outdoors. It’s true. It’s true. Again, it goes back to the second law of thermodynamics. An air conditioner is simply a machine that moves heat from one place (indoors) to another (outdoors). And heat acts according to the second law, so that means it flows from warmer to cooler. Right?

Heat flowing from cooler to warmer?

Wait a minute! On hot days it’s cooler indoors and warmer outdoors. How can this be true? Do air conditioners violate the second law? Is there a loophole?

Nope. You read it right. Heat from the cooler indoors flows to the warmer outdoor air without violating the second law of thermodynamics. That’s because the compressor of the air conditioner pumps up the temperature of the refrigerant, which carries the heat from indoors to outdoors. The refrigerant coming out of the compressor has a condensing temperature of maybe 120° F. Here in Atlanta, our design temperature is 92° F. So the heat flows from the 120° F+ refrigerant into the condensing coil and then into the 92° F outdoor air on a design day.

The diagram above is from my article explaining the refrigeration cycle. Step 2 is the compressor doing its job. Step 3 is the high temperature refrigerant going through the outdoor (condensing) coil. If you want to understand how the refrigeration cycle in more detail, check out the article. Actually, I wrote two articles, one without too many technical terms and one that’s more technical.

Two benefits of cooler weather

When the outdoor temperatures start dropping in autumn, the refrigerant has an easier time of dumping its load of heat into the outdoor air. That does two things: First, it means you actually have a bigger air conditioner. It has a greater cooling capacity. That’s not really of any help, though, because you have less cooling load.

The second thing it does is to make your air conditioner more efficient. Assuming you have a single capacity air conditioner — not a multi-stage or variable capacity system. If we further assume the air conditioner uses the same amount of electricity to run the compressor (the biggest energy user in an AC), that makes the air conditioner more efficient. If you get more capacity (bang) for the same amount of energy (buck), your efficiency is higher. But actually, the compressor uses less energy, which makes the efficiency gain that much more. Woohoo!

This same scenario plays out over the daily cycle of changing temperature as well. Your AC has more capacity and is more efficient at night when it’s cooler than it is during a hot afternoon. That’s why HVAC designers are always aware of the three types of heating and cooling loads.

As your electricity bills begin to drop with the onset of cooler weather, now you know that it’s not just because less heat is getting into your house. It’s also because your air conditioner becomes more efficient. Air conditioning is an amazing thing! Having been born in Houston and spent most of my life in Texas, Louisiana, Florida, and Georgia, I have a great appreciation for it.


Related Articles

The Magic of Cold, Part 2 – Intermediate Air Conditioning Principles

We Are the 99% — Design Temperatures & Oversized HVAC Systems

3 Reasons Your 3 Ton Air Conditioner Isn’t Really 3 Tons

The 3 Types of Heating and Cooling Loads




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This Post Has 25 Comments

  1. Does it also follow that if
    Does it also follow that if your outside condenser units are always in a shady location – but of course having free airflow around the sides – they will generally always run more efficient? I always cringe when I see outside compressor units on the southside of a home because I think it would handicap their operation.

    1. Charles, it seems that

      Charles, it seems that putting your air conditioner’s outdoor unit in a shady spot or building a shade structure for it should save energy. The Florida Solar Energy Center has tested it, though, and found the savings are very small, typically less than 3%.

      One way to understand that is to think about the volume of air being pulled through the outdoor unit. If you just have a small shady area, the condensing unit will quickly eat through all the cooler air from the shady area. You’d need a whole lot of shade to effect the temperature of the air being used.

      Here’s the article I wrote on this topic:

      Will Shading Your Air Conditioner Save You Money?

    2. Charles — in theory (and in
      Charles — in theory (and in practice) the outdoor heat exchange improves some with shade, but tests have shown the effect of the sunlight is extremely small. However the air temperature and airflow (as you mention) are extremely critical. Free airflow includes a lack of restriction above the unit as well. Nearly all the air conditioners we have tested have some degree of recirculation of the condenser exhaust back into the condenser intake. The diffuser mentioned in Allison’s previous article really helps that.

  2. Actually the temperature of
    Actually the temperature of the refrigerant coming out of the compressor is higher than 120 degrees. You should state that it is coming out of the compressor at a condensing temperature of 120 degrees. You should correct this error.

  3. Now that football season is
    Now that football season is in full blast, I’ve continued with my annual experiment… Beers stay cooler in the shade and with cooler outdoor temperatures, and there is less condensation! I’ll follow up at the end of the season with some data. ;-))

  4. Summer’s end brings not only
    Summer’s end brings not only lower “thermometer” (dry bulb) temperatures, but, with cooler nights, lowered dewpoints, reducing latent load on HVAC systems.

    In addition, drier outdoor air increases the temperature at which people feel comfortable, allowing higher thermostat setpoints, and, eventually, heaven forbid, turning the HVAC system off entirely and opening a few windows.

    I’d really like to see a poll measuring percentage of residences in which occupants actually embrace pleasant outdoor weather by opening windows…betcha these days it’s under 20%, maybe even 10%

    Is there any data out there on that?

    1. There was a survey done in
      There was a survey done in California a few years ago about the frequency of people opening windows in residences, but I don’t have the reference available. I can’t remember specifics, but the use of operable windows is declining. The main reasons seem to be security and noise. I have notice a trend in newer construction to go to more fixed glass and fewer operable windows which is probably a net gain in terms of annual energy consumption. I haven’t opened my windows for “fresh air” for at least the last 20 years. One reason is that in humid climates, you are often raising the indoor humidity level during mild outdoor conditions. Another reason is as discussed in this blog, it is cheap to mechanically cool your house during these conditions. A third reason is that my house doesn’t smell like “stale” air, so I see no need for “fresh” air. I do like “fresh” air, but I go outside to enjoy it.

    2. A subject near and dear… I
      A subject near and dear… I live in the high desert in southeast AZ, which of course has a dry climate most of the year. I do enjoy opening my windows — the bedroom window in particular. In the winter, the sun tends to overheat the bedroom so I can cool it quickly by cracking the window before bedtime.

      I rely heavily on the bedroom window in fall and spring for evening and overnight cooling. I have a nice box fan with an delay-off timer, and a filter that mounts between the fan and the window screen. In this climate, I can typically go at least three months (annually) without A/C or heat.

      As a mechanical engineer/designer, I’m considering how I can automate an economizer mode for my new house, soon to be built. A fully integrated built-up economizer would likely cost more than I would ever save. My baseline cooling costs are less than $250/yr for my not-so-efficient existing home. Without windows, I’d spend between $300 and $350/yr. I’m considering a simple in-duct fan (by fantech) with a filter box and simple mechanical stat with a wide hysteresis (would not be integrated with central HVAC). OTOH, the box fan works quite nicely.

      The only downside of opening the bedroom window overnight is that I sometimes end up over-cooling the room in winter, and on more than one occasion I woke up to a rain storm :^)

      1. David, I am just curious, do
        David, I am just curious, do you live in a high population density neighborhood? I live on a 1 acre lot in a nice rural subdivision with no history of crime, but I would still be nervous about leaving the windows open at night for security reasons. I think that I have gotten to the point where the noise (wind, wildlife, traffic, etc.) would bother me too. Finally, we do have a lot of dust and pollen in the air and I am pretty sure that my screens are MERV 1 or less. I am not trying to argue against you or anyone else opening windows, I am just trying to figure out if you have more amenable surroundings, or if I am just getting too soft. By the way, I grew up with hot summers and no air conditioning and had to “survive” with operable windows and fans. It didn’t seem that bad at the time, but I sure wouldn’t want to do it again.

      2. Roy, I live on 8 acres in a
        Roy, I live on 8 acres in a very rural area. But I routinely opened the bedroom window at night when I lived in town on a 1/4 acre lot. Never gave it a 2nd thought, in terms of security. I always make sure the windows are locked when we travel, but that’s it. When I’m home, I have other means of security 🙂

        Where I live, dust is a fact of life. Even when windows are closed up tight for weeks at a time (mid-summer), we still get a lot of dust inside the house. As I mentioned, I use a filter with the window fan, which pretty much eliminates any *extra* dust load from the fan, relatively speaking. My next home will be super tight, so hopefully that will cut down on dust! And my ‘economizer’ setup will have a high MERV filter. Our pollen season (mostly from grasses) typically occurs after it’s too hot to cool with outside air.

        As for noise, where we live now, we occasionally awake to coyotes howling, sometimes within yards of our bedroom window, so we hear them even when the windows are closed. In town, our neighborhood was quiet. I didn’t hear any sounds over the white noise created by the fan. Oh, I forgot. About six years ago when we lived in town, a local defense contractor was testing a Predator (UAV) with a new diesel engine developed to simplify supply chain logistics. The open ended test stand was close enough (about a mile) that I had to keep the windows shut for about 4 weeks during nice October weather.

        I spent most of my adult life in North Carolina, where humidity limits the ability to substitute windows for A/C during the shoulder seasons. But I monitored the dew point at the nearby airport. I would always open the windows when the outside dew point was lower than about 55F.

  5. Someone needs to invent a way
    Someone needs to invent a way to run your AC late at night and store it up to be released throughout the day time.

    1. Such systems already exist.
      Such systems already exist. Off-peak ice making equipment and chilled water storage are used in some commercial applications. However, the associated costs are not nearly justified for residential. Even if energy were priced at zero overnight (one utility actually does this!), the additional equipment would likely cost more than the lifetime energy savings for homes built to today’s codes.

      Last year I developed a cooling system with a chilled-water “thermal battery” (6,000 gallons below-ground tank) for an off-grid home near Phoenix. With off-grid, the levelized cost of power is typically north of 60 cents/kwh, but even at that, the cost was only able to be justified because my engineering team ate most of the R&D costs.

      In large buildings, peak demand charges can provide a huge incentive to load shift. If mandatory peak demand charges ever become commonplace in the residential market, we may begin to see some R&D in this area and through economies-of-scale, perhaps the cost of thermal storage and phase change systems will come down. But for now, it’s far more productive to simply reduce your cooling load. My new construction clients typically consume less than 0.75 kWh per ft2 per year for cooling.

    2. Those inventions already
      Those inventions already exist. Thermal storage has been developed and widely used for large commercial buildings for quite a while now. Many researchers, including me, have looked at it for residences. The main problem is that the net energy savings from running equipment during the night when it is cooler is not necessarily positive. Ice is a great storage medium, but it means that you have to run your evaporator at least 20 degrees cooler at night to freeze the water, and that is comparable to the energy savings from a 20 degree cooler nighttime temperature. There are phase-change materials available with higher melting temperatures than ice, but you still need a lower temperature to solidify that material than you need to cool the house directly. Night-time energy storage starts to look economically feasible when night-time electrical rates are significantly lower (or daytime rates are significantly higher), but it takes a large difference to justify the cost of the equipment. I currently pay $0.25/kWh during the day and $0.12/kWh at night, but that is only costing me an additional $60/month for about 3 months per year. I can’t afford to buy much thermal storage at that price, or battery storage either. A Tesla PowerWall costs thousands of dollars.

  6. In areas with large diurnal
    In areas with large diurnal swings, WHOLE HOUSE FANS ARE BIG WINNERS. The CVRH project showed very large savings from whole house fans. Sometime I need to write that up. The ones used in that project were too elaborate and also require massive increases in attic venting — a good thing in summer BUT NOT IN WINTER.
    Yes you need to open your windows (we sleep with them open every night summer and winter in northern California). If you don’t want to open the windows — get a dampered intake hooked to the WHF.

    1. I agree with John and David
      I agree with John and David that opening windows makes sense under some conditions, especially in drier climates. I guess that I am just too lazy;-)

      However, I am interested in looking into residential economizers. AC and furnace manufacturers will eventually be using variable-speed motors on all of their indoor units, so adding a cost-effective and well-controlled economizer should not be hard to do, assuming that there is a market for it. Hopefully, this market will grow everywhere for new houses since higher-performance envelopes result in lower outdoor balance points, thus increasing the opportunities for economizers. These economizers could also be designed to provide the year-round mechanical ventilation requirement in tighter homes.

      I wonder if we can come up with an economizer that is cheaper to build and install than operable windows and whole-house fans? If not, there might still be a market for lazy bums like me.

    2. It takes more than a variable
      It takes more than a variable speed blower. In order to integrate 100% outside air cooling with main HVAC system (as opposed to ventilation mixing with compressor-cooled air), we need a rather large** 3-way damper assembly at the return plenum, a differential enthalpy sensor (compare indoor and outdoor dew point) and a thermostat designed specifically for this application. In particular, the t’stat would ideally support a low-limit set-point to allow some over-cooling to further delay the need for compressor-based cooling. Utilities like that.

      ** sized to accommodate full system flow, ideally without adding any static drop

      Such a system was developed more than 10 years ago ( The reason it didn’t take off was the high cost. Even in a dry climate with large diurnal swings, payback is elusive for a system like this, especially for homes built to today’s codes and beyond with already low cooling costs. Forfeiting integration and automatic control greatly simplifies things.

      I agree with John’s point re: whole house fans. In my case, I’m only interested in cooling the master bedroom. We like the bedroom “cold” when sleeping (73F in summer). Otherwise, we keep the stat on 77F or 78F, aided by ceiling fans. As it turns out, any time the house gets warmer than that, it’s usually too hot to use outside air for cooling.

      1. David, I have a little
        David, I have a little experience in this area too. I have probably been dabbling in this field almost as long as you 😉 My point about the variable-speed blower is that it will soon be effectively free on new systems since it will be standard. I am not interested in looking at economizer applications that require high air flow rates when there is a small temperature difference between the indoor and outdoor air because I still think that operating the air conditioner is a better alternative for us lazy people (you and John can open your windows and run your fans). I am more interested in high-performance homes that may still need some cooling to cover internal loads when it gets down to 50 F or lower outdoors. Of course this still requires dampers, but the airflow rates would be much lower, and outdoor humidity levels would not be an issue. So we are now talking about kind of a hybrid economizer/ventilation system since the economizer airflow rates would be somewhere between normal heating/cooling airflow rates and ventilation rates. I have a sneaking suspicion that a large residential AC manufacturer could do this much cheaper than a 3rd-party add-on component supplier. They just need a market. As for controls, that is practically free now since the hardware is already there and we just need the additional software logic.

      2. Sorry, I put this aside and
        Sorry, I put this aside and it fell through the cracks. I just wanted to clarify that the reason the NightBreeze 3-way damper assembly is ‘rather large’ is because it has to handle the full system airflow when the outside air path is closed. I wasn’t talking about bringing in anywhere close that that volume of fresh air (attic fans are another matter). Of course, there are other ways to design an integrated economizer.

        What you’re proposing is more akin to CFIS as proposed by Armin Rudd at BSC (see and check out Armin’s comment and mine), except yours would be amped up beyond the prescribed ventilation rate to take advantage of favorable conditions. But why do you say outside RH wouldn’t be an issue?

        I remain skeptical that any domestic unitary equipment manufacturer would go after this market. But you lived in that world for 25 years so you would know that better than I.

        1. I am familiar with CFIS and
          I am familiar with CFIS and you are right, this would be an “amped-up” version of it.

          If you do not switch to economizer mode until it is around 55 F or cooler outside, then the outdoor air is plenty dry (dewpoint at or below 50 F) even if the if the outdoor air RH is 100%. Thus, there is no reason to know outdoor air humidity.

          I understand your skepticism about manufacturers selling an “economizer”. It could happen for one of two reasons: 1) a significant market develops for it based on its own merits (doubtful), or 2) it is required by code or law in a significant market (e.g., California). My main point is that HVAC manufacturers can do “accessories” better and cheaper than 3rd parties if there is sufficient market to justify the development, especially in terms of the required controls.

        2. Re: humidity not an issue…
          Re: humidity not an issue…
          Ok, so the variable blower & damper would react in concert based on ODT? I can see how that would work: max ventilation flow if ODT < threshold, otherwise min ventilation. Threshold could default to [setpoint - 20] or thereabouts to ensure boost mode doesn't introduce excess moisture. Boost mode would be disabled if t'stat not in cooling mode AND indoor temp < [setpoint - 5] or thereabouts. The challenge of course would be maintaining the min ventilation rate when ODT > threshold. Setup and field commissioning has been the Achilles heel for CFIS. Moreover, if source equipment is multi-stage or variable capacity and/or duct system is zoned, the ECM and vent damper controls would have to be smart enough to maintain correct flow. One way to accomplish that would be to maintain field-selectable static pressure drop across the vent damper.

          I would buy that.

          1. Yea, I think that using the
            Yea, I think that using the economizer for temperature control would be relatively easy, but also controlling it to get the right ventilation airflow rate is another story for the reasons you identified. I think that commissioning ventilation flow rates on the order of 100 cfm is always going to be problematic, especially when the ventilation is tied into the HVAC system.

  7. How much does a heat pump
    How much does a heat pump system efficiency drop in sub-freezing temperatures? How does HSPF relate to SEER ratings of heat pumps? I replaced my 14 year old system last year with a new 16 SEER 9.5 HSPF system, but the cold weather performance doesn’t appear to have improved. While in heat pump only mode, register temperatures are lower than my old system.

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