Air Conditioner Performance In Extreme Heat

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Air Conditioner Performance In Extreme Heat

Will our residential cooling units still perform well in a future that includes extended heat waves?

Posted on Jul 14 2017 by Martin Holladay

During the last week of June, many major U.S. news outlets sent reporters to Arizona to issue updates on the area’s extreme heat wave. Outdoor temperatures hit 119°F in Phoenix. Some airplanes were grounded because the hot air was too thin for small jets to take off. Car steering wheels were so hot that some drivers wore oven mitts. VinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). records delivered by mail arrived warped. Emergency room physicians reported an increase in burn cases: hands were burned when people touched their cars, and children’s feet were burned when they went barefoot outdoors.

The reports from Arizona conjured apocalyptic images of a science-fiction future — one in which rising outdoor temperatures make it difficult for humans to leave the safety of an air-conditioned building.

The question arises: Can typical split-system air conditioners handle these conditions? Or will extended heat waves require air conditioner manufacturers to design new types of equipment?

The compressors will still work

To get answers to my questions, I turned to John Proctor, the president of in San Rafael, California. Proctor is a professional engineer and a nationally recognized expert in residential air conditioning systems.

The short answer to my question is that the outdoor units (the condensers) of split-system air conditioners should have no problems handling a heat wave in Arizona. If homeowners feel hot during a heat wave, the problem is rarely due to a condenser or compressor that can’t handle high outdoor temperatures. Instead, problems typically arise from installation problems (for example, restricted air flow to the outdoor unit or poorly designed duct systems).

“If you have an old compressor that is unhappy — one that is getting ready to go out — what happens is that at high temperatures, the head pressure goes up, the unit doesn’t cool as well, and the compressor goes out,” Proctor told me. “It can happen. But in an Arizona heat wave, you aren’t really up against an upper limit for performance.”

Proctor isn’t sure whether an annual checkup is a good idea. “A checkup is a mixed blessing,” Proctor told me. “Everyone says to get your air conditioner checked out at the beginning of the summer. But the chance is pretty high that they will tell you that you need something you don’t really need.”

Desert experience

Proctor has worked on air conditioning projects in Saudi Arabia, so he’s familiar with high outdoor temperatures. He told me, “In Saudi Arabia, I noticed that people sometimes put aluminum foil on the outside of their windows.”

I asked, “Ordinary aluminum foil from a supermarket?”

“Yes,” said Proctor. “Ordinary aluminum foil. They tape it up.”

Needless to say, aluminum foil isn’t transparent or translucent. But it certainly cuts down on solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss.. If you are having an air-conditioner emergency, and your grandma is having a health crisis, then by all means, go ahead and tape foil, cardboard, or almost any opaque material on the exterior side of your window glass.

Testing by manufacturers

“Manufacturers are required to test their equipment at high temperatures,” Proctor told me. In a follow-up email, Proctor explained that he was talking about the ANSIAmerican National Standards Institute. National nonprofit membership organization that coordinates development of national consensus standards. Accreditation by ANSI signifies that the procedures used meet the Institute’s essential requirements for openness, balance, consensus, and due process. /AHRI Standard 210/240 test.

“The test requires equipment to operate continuously for one hour at a temperature of 115°F,” Proctor said. “For equipment to be certified, they have to do the test. The manufacturers aren’t required to record or report anything about the cooling performance of the machine at this temperature — all they have to record is the answer to the question, ‘Did it die?’ ”

While it’s possible to criticize the ANSI test, the bottom line is that standard split-system air conditioners should work OK during a heat wave.

No problems up to 125°F or 130°F

The next expert I interviewed was Van Baxter, a senior research and development engineer at Oak Ridge National Laboratory.

“Most manufacturers can provide operation or performance data for outdoor temperatures up to 125°F or even 130°F,” Baxter told me. “The design point in the U.S. is mostly at 95°F, but manufacturers recognize there are a number of hours that exceed that. So yes, they are designed to operate at those temperatures. But are they designed to operate for 165 hours a week at those temperatures? That may be an issue.”

Baxter described some recent testing performed by his colleagues; he was a technical reviewer of the paper. “They tested a lot of systems — minisplit air conditioners and rooftop air conditioners. Some used R410A refrigerant and some used alternative refrigerants. The systems were tested up to 125°F or 130°F ambient conditions, and we didn’t run into any issues with the R410A and most of the alternatives. The systems with R32 refrigerant only worked up to 125°F. The alternative refrigerants in some cases gave 2% better performance.

“For all systems that I am aware of, the compressors are rated for a maximum discharge temperature at different ambient temperatures, and they have safety mechanisms that shut the system down if those temperatures are exceeded. The equipment includes a safety device that just won’t let the compressor go there. We didn’t run into those issues with any refrigerants but R32.”

What happens if you spray your condenser coils with a lawn sprinkler?

Roof-mounted commercial air conditioners often circulate water over the outdoor coils to improve cooling performance. If a homeowner set up a sprinkler to spray the outdoor unit, wouldn’t that help cool the outdoor coils?

I posed the question to John Proctor, who responded, “Running a sprinkler works for a while, until you either corrode or scale the outside coil. If you try that trick on a regular basis, your machine won’t last very long.”

Proctor’s advice

Proctor noted that problems with condenser performance are often due to obstructed air flow to the outdoor unit.

“Make sure there is free air flow,” said Proctor. “No fences. No weeds. No bushes. Clear that all away. I’d love to see the area wide open like Kansas. If you have to install a fence, make it 3 feet away from the unit, and make sure it has slats that allow airflow.”

Proctor continued, “There’s something else that can be done to improve the airflow: you can add a diffuserIn a forced-air heating/cooling system, the diffuser is a register or grille attached to ducting through which heated or air conditioned air is delivered to the living space. In a tubular skylight or an electric light fixture, the diffuser is a cover plate through which scattered light is delivered. on top of the fan outlet. It’s pretty simple. We’ve got .”

Proctor’s research team has shown that the installation of a simple sheet-metal diffuser — basically, something that looks like a funnel — on top of the outdoor unit increases the rate of air flow through the outdoor unit. That's good.

It seemed surprising to me that a jury-rigged addition to a new air conditioner could significantly improve air conditioner performance. I asked Proctor, “If a diffuser improves performance, why aren’t air conditioners designed and manufactured that way?”

He answered, “Manufacturers never want to do more than they have to. If there is a diffuser above the fan, the diffuser makes the unit more difficult to ship. I’ve talked to them about it. There are ways they could include a diffuser. But they don’t want to do it.”

How does the diffuser work? “As the air exits the fan, the air flow is very messed up,” Proctor answered. “Adding a diffuser smooths out airflow. It directs the energy upward. There are a lot of other benefits: the diffuser reduces the power usage, and it’s quieter.”

For instructions on making a diffuser for your outdoor unit, see .

Some experts are worried

After I spoke with Proctor and Baxter, I interviewed a third air conditioning expert, Craig Messmer. Messmer is vice president of engineering at Unico, a cooling equipment manufacturer with headquarters in Arnold, Missouri.

I asked Messmer whether residential air conditioners face an upper limit for outdoor temperatures. He answered, "There is a limit. I don’t think anyone has a hard and fast limit. It happens at around 120 degrees in our experience."

What happens? "The capacity stops dropping off at 105 degrees," said Messmer. "At 120 degrees, the unit can shut down. But it depends on what type of equipment it is."

Messmer continued, "The refrigerant has to be hotter than the air to give up the heat. At high ambient temperatures, you don’t get much refrigerant flowing through the system. The flow rate slows, and that reduces your capacity.

"Equipment is usually rated at 95 degrees. We also do a report to the state of California, reporting on the performance of our units at 105 degrees and and 115 degrees. We report that performance data to AHRI, and they report it to the state of California. So there is a database out there that lists performance for units at high temperatures."

I asked Messmer whether there are any engineering solutions to these high-temperature limits. He answered, "The short answer, is of course. We can come up with new designs. But if your question is, ‘Is there anything in the back room waiting to come out?’ the answer is, I don’t know.

"There are strategies you can try if you have a hot environment. In extreme situations, try a water mist. You wouldn’t want to do that on a normal basis, though. That’s a strategy — not necessarily what you might want to encourage on a regular basis.

"Really, 115 to 120 degrees is the practical limit for today’s equipment. I think that 125 degrees would be really pushing things. When outdoor temperatures get really hot, you actually raise the refrigerant temperature. Of course, the condensing temperature has to be hotter than the air. If you want the equipment to perform at 125 degrees, maybe you need a condensing temperature of 135.

"The superheated gas — what comes out of the compressor — might be 40 degrees hotter than the condensing temperature. If your condensing temperature is 125 degrees, the superheated gas might be at 165 degrees. So if you raise the condensing temperature to 135 degrees, then the temperature of superheated gas is up to 180 or 190 degrees. That is hot. That has negative consequences for the oil, because at those temperatures, the oil can start to break down. So you can really limit the life of the product. Maybe the unit won’t quit working, but if you break down the oils internally, you will get less cooling and you start to get long-term problems. It would shorten the life of the equipment, for sure.

"From a manufacturer’s perspective, the designs will change, because we want to provide products that work in these desert areas. Maybe the heat exchangers will get bigger. We can slow speeds down at hot temperatures. That means that you get a reduction in capacity, but it would work. It would become a matter of putting in a larger unit to anticipate the reduction in capacity."

Back to basics

What's the bottom line? If you live in a hot climate, and if your air conditioning system isn’t providing enough comfort during heat waves, you probably don’t need a new air conditioner.

Instead, focus on the basics:

  • Make sure that your air conditioner has been properly commissioned, to verify that the system has the correct rate of airflow over the indoor coil and the correct refrigerant charge.
  • Make sure that your distribution ducts are inside your home’s thermal envelope.
  • Seal duct seams with mastic to prevent duct leakage.
  • Verify that your forced-air system includes adequate return-air pathways.
  • Plug air leaks in your home’s thermal envelope.
  • Install exterior shading devices to keep the sun off of your east-facing windows and west-facing windows.

And as long as you are preparing for the next heat wave, you might as well buy a few extra pairs of oven mitts.

Martin Holladay’s previous blog: “PV Systems That Divert Surplus Power to a Water Heater.”

Tags: , , , , ,

Image Credits:

  1. Proctor Engineering Group

Jul 14, 2017 6:53 PM ET

Edited Jul 14, 2017 6:57 PM ET.

SW and Extreme Temperatures
by Armando Cobo

Would it help in the SW to shield the condensing unit from the sun with a "shed" roof, with enough clearance for good airflow? (Maybe a flat roof to follow architecture)
I know that condensers installed on the West and North side of the houses run better, or it appears to!
I can just see those diffusers full of leaves and junk in no time! ;-))

Jul 14, 2017 9:43 PM ET

Multiple sources I see
by Jon R

Multiple sources I see indicate that compressor oil temperature isn't an issue until > 300F.

Jul 15, 2017 1:09 AM ET

AC Diffuser
by Lucy Foxworth

I like that idea. Now I need to find a vane anemometer to measure the increase in flow. I think I can do this.


Jul 15, 2017 1:12 AM ET

Vane Anemometer - not cheap
by Lucy Foxworth

Oh well. Not sure about the vane anemometer because they are not cheap. I may have to go with using my hand to estimate air flow.

Jul 15, 2017 2:04 AM ET

There's a bit of scattershot info herein
by Curt Kinder

Info in this article is kinda all over the place in terms of actual operating conditions and reality. A few examples:

1) Annual checkups - While John Proctor is right that some companies troll for unneeded service work under the guise of periodic preventive maintenance, it must be acknowledged that complex systems operating in severe environments are vulnerable to deterioration. When do you want to find out that the capacitor that has drifted out of spec will no longer start your outdoor fan or compressor...during spring maintenance or during an outdoor heat wave?

Disclaimer - I run an HVAC company. We check our clients' systems semi-annually using an 80 point checklist designed to stave off unplanned failures and expensive repair costs. One improperly secured wire can cause the system to blow its refrigerant charge or burn out the compressor. Pay us now or pay us later...your call.

2) "No problems up to 125°F or 130°F" Maybe or maybe not. At that outdoor air temperature AC compressors are at the ragged edge of design gas discharge pressures and temperatures as well as compressor motor current. If outdoor unit air flow is maintained as to volume and the installation is such that the unit won't "rebreathe" its own discharge air, it should keep running, albeit at a significant reduction in capacity and efficiency...both will drop by 1/3 or more compared with ratings at industry-standard 95* ambient.

Then again, competent system specifiers in the desert southwest should already have applied correction factors for operation at temps well above 95*F. It's called Manual S, and this isn't the first year Phoenix has been well above 95*F outside.

3) The Funnel. I'd sure like to know more about that. I highly doubt it increases air flow through the outdoor unit since it must by definition introduce a bit of restriction to air flow. What it likely does do is channel widely swirling discharge air further away from the system so that it is less likely to rebreathe its own discharge air when it has been installed in a location confined by vegetation, walls, fences and / or overhangs.

4) Capacity dropoff - there is no magic temperature such as 105*F; it's actually a continuum. Systems are rated by national standard at 95*F. All of them have progressively more capacity at outdoor air temperatures below 95*F, and progressively less at temperatures above 95*F. Efficiency follows the same track. Again, I refer readers to Manual S, the long recognized procedure for sizing HVAC equipment that takes into account actual outdoor air temperatures.

Jul 15, 2017 6:20 AM ET

Edited Jul 16, 2017 12:00 PM ET.

Response to Armando Cobo (Comment #1)
by Martin Holladay

Q. "Would it help to shield the condensing unit from the sun with a shed roof?"

A. Basically, no. We've addressed this question many times on GBA. I addressed it, for instance, in a 2014 response to a Q&A post titled Does an AC compressor located in full sun work harder than one located in the shade?

The following year, in 2013, GBA blogger Allison Bailes wrote an article on the topic called How Much Will Shading Your Air Conditioner Improve Its Efficiency?

Q. "I can just see those diffusers full of leaves and junk in no time!"

A. Good point. I imagine that anyone who installs this type of diffuser will need to perform, at a minimum, an annual cleaning at the beginning of the air conditioning season (February 1st in Florida, and July 10th in Vermont). In most cases, the condenser fan will probably blow out the occasional leaf that falls into the funnel during the summer.

Jul 15, 2017 6:30 AM ET

Edited Jul 15, 2017 6:44 AM ET.

Response to Curt Kinder (Comment #5)
by Martin Holladay

Thanks for you detailed and helpful comments. The fact that the three experts I interviewed are "kind of all over the place" and that their advice amounts to "scattershot info" is an accurate representation of a few differences of opinion from engineers who are experts in the field. You have added a fourth voice, at times congruent and at times divergent. It isn't unusual to find that engineers don't always agree.

Jul 15, 2017 11:40 AM ET

cheap anemometer
by Charlie Sullivan

Expensive anemometers are not cheap, but cheap ones are cheap. Here's one for $14.

I am not particularly recommending that one, but am just noting that there are cheaper options.

I also want to note that just having that instrument doesn't make measuring airflow easy, as the flow varies across the surface, and the reading of the meter depends on things like turbulence in the air as well as the actual flow. But you'll probably get a more accurate assessment than with your hand.

Jul 15, 2017 9:58 PM ET

Anemometer and outdoor unit air flow
by Curt Kinder

Most competent HVAC companies take a stab at measuring system performance in the course of PM procedures. It is common to measure return and supply air temps and also report the delta. A properly running system in cooling mode will typically exhibit an indoor Delta-T in the range of 18 - 22 *F

Our techs go a step further and calculate airside temp split across outdoor compressor section. Typical values are 8 - 12*F, with higher SEER units coming in on the lower end of that range. If the entering air temp is above ambient outdoors, then the system is rebreathing outdoor air, likely owing to adjacent walls, fences, overhangs, vegetation or fratricide (another unit operating nearby)

In round numbers, the outdoor fan moves approximately twice as much air as the indoor air handler blower. We constantly stress the necessity of attaining proper air flow on both sides, indoor and outdoor, of a split heat pump system.

Assessing the efficacy of a retrofitted funnel (picture looks quite like a galvanized sheet metal agricultural tub with bottom cut out) should be fairly easy:

Operate the system continually for 15 minutes to achieve steady state. Attach good quality AC current meter and high side refrigerant pressure gauge, preferably a modern digital manifold.

Note readings, attach funnel, monitor readings. If compressor amps fall along with high side pressure, winner winner chicken dinner. If OTOH, current and pressure rise, remove funnel.

Jul 16, 2017 11:08 AM ET

Edited Jul 16, 2017 11:11 AM ET.

Response to Kurt and Lucy
by John Proctor

The diffuser absolutely works. We have tested it many times. We incorporated it into our two high temperature prototypes. it does not increase restriction, it does increase airflow. The science of diffusers is well known and used for years and years. You do not need to measure the increase in airflow unless you are a energy nerd like me. My recollection without going back to the textbooks is that the diffuser helps change the pressure increase across the fan into velocity rather than providing a "backpressure" on the fan. What is more the watt draw per 1000 CFM drops. If you want to see some of the tests go to Table 21. Also the performance of an air conditioner degrades as the temperature increases nothing magic about 105.

Jul 16, 2017 4:09 PM ET

I find figure 47 interesting
by Jon R

I find figure 47 interesting - small differences in charge caused large differences in efficiency.

Also figure 28 - thermostat hysteresis has a significant effect on efficiency?

Jul 17, 2017 6:21 AM ET

Response to Jon R
by Martin Holladay

I have reproduced Figure 47 from Proctor Engineering's paper () below.

Your reference to Figure 28, however, is a little confusing, since Figure 28 depicts a tube axial fan. The fan illustration doesn't appear to have anything to do with thermostat hysteresis.


Proctor paper - Figure 47.jpg

Jul 17, 2017 7:59 AM ET

More on diffusers
by Martin Holladay

I just received an email from Van Baxter (the researcher from ORNL).

In his email, Baxter noted, "Nice article and interesting commentary. Just goes to prove that engineers are a lot like economists: if you ask three of three of us for an opinion on a given issue you may likely get >3 opinions. I do agree with Dr. Proctor about the diffuser. Such a device will absolutely improve the performance of the outdoor unit fan."

Jul 17, 2017 8:47 AM ET

A challenge to GBA readers
by Martin Holladay

Who wants to be the first reader to send us a photo of a newly installed home-made diffuser? I hope that someone takes on the challenge -- perhaps next weekend.

Jul 17, 2017 8:55 AM ET

DIY diffuser
by Curt Kinder

Diffusers piqued my just now occurs to me that many, if not most or even all new commercial VRF system outdoor sections feature discharge air diffusers. Those appear to be hyperbolic, rather than straight sided funnels.

There's probably something to that, given that cooling towers sometimes incorporate a similar feature, and cooling tower performance very much depends on maximizing air flow for a given fan power input.

Jul 17, 2017 9:02 AM ET

Engineering opinions are converging!
by Martin Holladay

It's nice to see that the herd of cats can agree on a few important points.

Jul 17, 2017 11:32 AM ET

Whoops, figure 48 (and table
by Jon R

Whoops, figure 48 (and table 23), not 28. Which I interpret to show that if you don't need the latent capacity, it's efficient to turn it into sensible capacity by running the fan after the compressor stops. And run short cycles so you do this often. Maybe it's better to just run a humidistat controlled humidifier in the house?

Not living in a desert, I typically need all the latent capacity I can get. So my question is "what effect does a thermostats "cycles per hour/swing/differential" setting have on AC efficiency?"

Jul 17, 2017 12:46 PM ET

Here's Figure 48
by Martin Holladay

Figure 48 from the Proctor Engineering paper.


Proctor paper - Figure 48.jpg

Jul 17, 2017 5:18 PM ET

Short cycling = bad.
by Curt Kinder

..."And run short cycles so you do this often"

That would be bad - it takes 10+ minutes (some say 15-20) for a split heat pump to get to steady state conditions, before which efficiency is poor. It's also very hard on the compressor to short cycle - windings overheat, insulation degrades. Home HVAC systems all use hermetic compressors - unrepairable, and very expensive to replace.

Be kind to your system compressor, unless you like giving your boat payment to your friendly neighborhood HVAC contractor...who probably likes boats too!

Jul 22, 2017 2:06 PM ET

Comment 14
by Calum Wilde


I took a look at my RSL3H series Fijitsu mini splits to see if I could take advantage of this. To my surprise they already have a short integrated diffuser. It's only a about 2 inches deep as an estimate, but it's good to see manufactures aren't leaving something so simple and easy on the table.

Jul 22, 2017 3:05 PM ET

Mitsubishi City series
by Dana Dorsett

Mitsubishi City series compressors have substantial built-in diffusers.

The lowest possible impedance profile for easing the pressure difference is a logarithmic arc (the same shape as with ends of brass musical instruments, police megaphones, and antique Victrola record players) but any reasonable approximation helps, even a cone section (such as the cheap un-powered fan megaphones at football games or a crew team coxswain's megaphone.) The DIY version is a cone section, easy to build, but clearly not the highest possible performance.


Mitsubishi diffusers.jpg

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