Merry Christmas . . . uh . . . Happy New Year! . . . ahem . . . Ok, so it’s been a while since my last blog . . . Sorry for the hiatus. I was put on a support contract to help California Energy Commission staff write the updated (2013 code) version of the Residential Compliance Manual and that took me off line for a few months. I have a newfound respect for the hard working folks at the Energy Commission. They truly want the code to be fair, practical and enforceable. That’s a much harder task than any of us on the outside realize. All in all, there are many improvements to the code this time around. The compliance software is still a big mystery. The new dynamic forms will be very cool once the bugs get worked out.
The topic of today’s blog post is summer outside design temperatures and why you shouldn’t stress over them. More precisely, there are other things far more important to stress over.
A few years back, I literally had a contractor refuse to install my HVAC design solely because I chose a summer outdoor design temperature that was two degrees lower than what he thought it should be. I was using the temperature required by the energy codes and recommended by ACCA and ASHRAE. Granted, the project was a few miles away from the city that the temperature was measured for, and granted, it was on the other side of a small ridge. Even if the “true” summer outside design dry bulb for the precise location of the project was 2 degrees higher than what I used, how big of a difference does that really make? According to him, all the difference in the world.
His argument was that it would cause the A/C to be undersized and result in homeowner complaints that HE would have to deal with, not me. First of all, as a licensed mechanical engineer, I am totally responsible for the performance of any mechanical plan that I stamp and sign. Yes, he would be the first one they called, but if it turns out that my design was the cause of the problems, I would be responsible for fixing it and for paying for his time to respond to it.
The first thing to realize is that typical residential A/C systems only come in a few sizes. 1.5, 2, 2.5, 3, 3.5, 4 and 5 ton sizes. Roughly speaking these represent sensible cooling capacities of numbers something like 12,000, 16,000, 20,000, 24,000, 28,000, 32,000, and 40,000 btuh. Basically, what happens is that you do your load calcs and then you pick the next size up. So, if your sensible load is 13,000 Btuh, you would have to pick a two-ton because a 1.5 ton system would not be enough. (Note: these sensible capacity values are very crude. They are probably a little low. I’m making them up because it’s easier to work with round numbers and I am too lazy to get some real numbers; however, real numbers would illustrate the exact same point. Actual sensible capacities come from detailed performance tables published by the manufacturer and depend on indoor temperature and humidity, outdoor temperature, airflow across the coil and, of course, make and model. Please don’t use these as rules of thumb. I disavow any responsibility for them. If you want to learn how to determine these for real equipment, read ACCA Manual S.)
For a typical 3 to 4 ton load in a fairly new home, changing the summer outside design temperature from 98 to 100 adds about 1,000 to 2,000 btu to the sensible cooling load. The reason that it is a fairly small number is because the indoor and outdoor temperatures establish the temperature difference (delta-T) between the inside and outside of the house. This delta-T only affects loads caused by conduction through the building shell (heat transfer through solid walls, ceiling, etc.) and convection into the conditioned space (outside air leaking into the house). This delta-T has no impact on the largest single source of heat entering the house in the summer – solar gains. Solar gains can be 30-40 percent of the sensible load and do not change due to outside temperature. Neither do internal loads.
Did you know that a house with an indoor summer design temp of 75 and an outdoor summer design temp of 100 will have about the same cooling load as a house with an indoor summer design temp of 65 and an outdoor summer design temp of 90. (65 is not a reasonable indoor summer design temp. Don’t use that. Use 75. This was just another dumb example to make a point.)
So, let’s say your sensible cooling load calc at 98 deg is 29,000. That would suggest a 4 ton system. If you re-ran it at 100 deg and it went up to 31,000, a four-ton system would still work. Your load calc at 98 deg would have to be over 30,000 before changing the temperature to 100 deg would even hint that you needed to go up to the next size equipment. In this case, that would be a five-ton system. So, lets just say for laughs that my load calc at 98 deg came out at 31,000. If I caved to the contractor and reran the calcs at 100 deg, they would come out at around 33,000. Too big for a 4 ton, so we would have to go to a 5-ton that delivers 40,000 btuh, sensible.
But, are we really doing the homeowner a service by putting in a system that is oversized by 7,000 btuh (21% excess capacity). Wouldn’t it make more sense to really look at what this means and maybe try to find a way to make the 4-ton system work by dropping the load of the house back to 32,000? (The answer is YES. It would make tons more sense to do that. Pun intended.)
Also, what exactly does the summer outdoor design temperature number represent? Currently, we use a value called the “1% Summer Design Dry Bulb”. It can be found for pretty much any city in California (there are about 750 listed) in the 2008 Joint Appendices, Appendix JA2.2. What does the 1% mean? Well, you can scan through Reference Appendix JA2 and by looking at cities you are familiar with, you will notice right away that it certainly doesn’t represent the hottest day of the year. It’s usually well below that. What it means is that the outdoor temperature is higher than that number only 1% of the time over however many years the data was collected for. (Also notice that they list a value for 0.1%, 0.5% and 2%. The code requires that you use the 1.0% value.)
Another way of looking at this number is that 99% of the time, the actual cooling load is less than the load calculated using that temperature. The system is perfectly sized for the few hours where the temperature is exactly the design temperature. Let’s be generous and say that’s about 1% of the time. This means that 98% of the time the system is oversized and will cycle on and off (or not run at all).
So, if our cooling load and cooling capacity were exactly the same, let’s say 32,000, then 1% of the time the load is greater than the capacity of the equipment and it cannot remove Btus as quickly as they are coming in. When this happens, the temperature in the house will creep up.
If you didn’t know this already, a perfectly sized air conditioner will run continuously when the outdoor temperature is at or above the design temperature. This is a good thing and the reason why is a discussion for a later blog, perhaps. Just suffice it to say that cycling on and off is about as good for an air conditioner’s efficiency as stopping and starting is for your car’s MPG.
Let me also say this: There is no such thing as a perfect load calculation. They are a SWAG, which is only little better than at WAG (A WAG is a wild-ass guess. A SWAG is a scientific wild-ass guess.) Trying to calculate an exact sensible cooling load is like trying to measure the average diameter of a cotton ball with a micrometer. Where do you draw the line? The best you can do is document your assumptions and hope that you are right most of the time (99% is pretty good, by the way).
So, what ultimately happens during that 1% of the time when the A/C cannot keep up? The indoor temperature shoots up like your car parked in the sun, your favorite leather chair burns skin off of your back, all the plants wilt, the goldfish are parboiled, and the kid’s crayons melt into pretty little puddles of color. No. None of these things happen. What actually happens is the indoor temperature will creep up a few degrees. If the set point is 75 degrees, it will rise up to 76, 77, maybe 78 degrees. (Seventy-eight degrees was the indoor design condition for many years by the way). How fast it takes to do that depends on the house. One of the biggest factors is how much insulation and thermal mass the house has. Thermal mass stores Btu’s in the winter and Bcu’s in the summer. A “Bcu” is a Bubba’s Cooling Unit and it is equal to -1 Btu, see an earlier blog on that topic.
A fairly new, reasonably well-built house will rise about 1 degree per hour. Whether or not that becomes a big problem depends on how hot it gets outside and how long it stays above the design temperature.
The above graph shows a hypothetical house that has the equipment sized exactly to the load. Remember that this usually doesn’t happen when you pick the “next larger piece of equipment”. Normally, there is some excess capacity in a properly sized system.
The red line is a typical pattern for outside temperature in the summer for a fairly hot city like Fresno or Sacramento. This graph shows two consecutive “hot” days where the outdoor temperature exceeds the design temperature by several degrees for a few hours. Remember, this only happens 1% of the time.
When it does happen, what happens to the indoor temperature?
The indoor temperature is represented by the blue/green/pink line. When the line is blue, the indoor temperature is below the thermostat set point, of 75 degrees, for example. This happens when it is cooler outside than inside. When the line is green, the indoor temperature is right at 75 degrees. This happens when the outdoor temperature is above 75 degrees but below the outdoor design temperature. When the line is pink, the indoor temperature is above 75 degrees. This happens when the outdoor temperature is above the outdoor design temperature. Notice that if these were weekdays, the pink bump is happening mostly when no one is home.
Something else to realize is that when the line is blue, the A/C is not running at all. When the line is green, the A/C is cycling on and off. When the line is pink, the A/C is running continuously. Interesting? I think so.
So, does that graph represent something that the typical homeowner would complain about? Possibly. Homeowners have a right to be picky. They are spending a lot of money on their home. Are there things a homeowner can do to make sure this doesn’t happen (without changing the size of the A/C)? Absolutely. Remember, this only happens on the few hottest days of the summer and in a system with no excess capacity. Most homeowners know when hot days are going to happen and can take reasonable precautions.
Most cooling loads are calculated with the assumption that some or all of the interior shades (drapes, etc.) open. Keeping all of the drapes closed during hot weather makes a huge difference. Planting shade trees around a house makes a big difference. Even neighboring buildings provide shade not accounted for in the load calcs. Pre-cooling or overcooling the house can help too. This is when you set the A/C down a couple extra degrees, cooling the house down a little extra at night, and letting the thermal mass of the home help keep it cool during the hottest time of the day.
Did you know that a house with less thermal mass will have a taller hump in the pink part of the line. A house with more thermal mass will have a flatter hump.
The vast majority of homeowner complaints about cooling that I have dealt with did not stem from undersized equipment and certainly would not have been solved by using a higher outside design temperature. They stemmed from poorly built homes (leakier than expected, poorly installed insulation, etc.), underperforming cooling systems (poor refrigerant charge, low airflow due to undersized ducts, leaky ducts, etc.) and poor thermostat operation (turning system on and off and not letting it reach equilibrium).
This graph represents a more normal hot summer day. Where it does not quite reach the design temperature outside. These are far more common than the previous example. Notice that there is no pink bump where the indoor temperature drifts up. Realize though, that the lower the outdoor temperature is, the more often the system will cycle on and off. This reduces efficiency. The ultimate question then becomes, is it worth having a less efficient system the vast majority of the time just to prevent the indoor temperature from creeping up a few degrees on very hot days (which can be prevented with simple precautions). I vote no, but that’s just me.
By the way, the same contractor that I mentioned at the beginning of this blog also told me that a 16” return duct is fine for a 4-ton system (hint: that’s not even close). So, the moral of that story is: stop quibbling over things that we cannot control, like the weather, and start quibbling over things we can control, like quality construction, quality system design, good air flow, and proper thermostat operation.
Duc-Bloc register blocks for duct testing
HVAC 1.0 Book – Introduction to Residential HVAC Systems
Kwik Model 3D HVAC Design Software
Russell King, M.E. 
Feb 28, 2013 @ 05:30:05
Thanks Russ for the great blog as usual!
I was just having this same conversation 2 days ago at a conference in Connecticut (with HVAC contractors).
To embellish a bit on your article: when the building is “peaking” it is due to solar gain. Unless you have excessive glass exposures both on East and West ordinals, the solar gain loads goes away in a short period of time and the system should have plenty of capacity to recover from the sensible “delta” induced load.
A “technical” point I like to illustrate to contractors – especially in the context of Manual S, is that compressors gain capacity and efficiency when the suction pressure goes up; this of course is caused when the evaporator [can] absorb more load. Therefore if the indoor temperatures go up, the load goes up, and the compressor actually moves more refrigerant (higher gas density = more mass flow). I try to stress the point of making sure any load increases are coming from the living space (not elsewhere).
Another point I like to make is related to something I “re-learned” on my many trips to your state, in particular the Palm Desert area: when outdoor temperatures get high, you do not want to maintain a “colder” indoor temperature. Anyone who travels or lives in a desert area knows you keep the thermostats at 80 degrees (or higher) – the reason: too large a delta causes your body temperature to rapidly drop when you go from outdoors to indoors. This can cause hypothermia (yes in the summer), heart attacks and other blood pressure relates acute emergencies in the elderly, and discomfort with children. It is related to a biological relationship called “Dubois Law” which correlates metabolism to body mass and an organisms surface area relative to their mass. When you ask yourself why the elderly wear sweaters in the summertime, remember Dubois’ Law, or if your a fan of “A Christmas Story”, remember Ralphies little brother! A larger portion of our society is at risk than those who might be put off by the thought of a few degree rise.
The point is when someone complains about “outdoor” design temperature I usually ask them if they’ve ever been to a desert community, and if they have actually addressed the things they can control (as you point out in your blog) – which usually leads to conversations about how they design and test ducts, airflow, etcetera’s.
Feb 28, 2013 @ 06:28:24
Hi Buck!
Thanks very much for the reply. All of your points are great supplements to this article. You are really getting into the nitty gritty technical issues and if anyone wants to dig into this topic deeper, that’s exactly what they would find. Obviously, I tried, unsuccessfully, to keep my article short by only covering the basic stuff.
The point about senior citizens and colder indoor temps was new to me, though. I’ve done a LOT of design work in Las Vegas. My general opinion of seniors, based on my dealings with them, is that they keep their houses cooler than the average family. Most of them were “snowbirds” from up north. I’m not sure if that matters. I even had one gentleman get my number from a production builder before he bought a house from them and ask me if I could design a system to keep it at 65 degrees when it was 115 outside! I said, “Yes. I could.” but told him that he had to take it up with the builder (knowing that the builder would refuse). I never heard back from him.
The suggestion in my blog is to set the thermostat down a couple degrees only on those few days where the temperature may drift excessively, just as a precaution, not as general practice. Hopefully that will not cause any issues. The other thing that I forgot to mention for those days is to not let the thermostat “set-up” during unoccupied periods because it can be harder to recover. Thanks for making me clarify that.
Russ
Feb 28, 2013 @ 07:43:10
I can certainly see the snow birds wanting/stating that initially!
We only tested 2 houses occupied by snow-birds; they told us they and other snow birds typically left to go “home” because it gets too hot after March/April out there. The lowest thermostat setting we came across was 78 degrees. The locals (“year-rounders”) would typically be happy with fans and swamp coolers until it got over 95 degrees – then they’d use their DX units. I have no idea if that is actually “statistically” true, but that was a typical response we got from both residents and HVAC contractors – although we also heard from the HVAC contractors about the extreme cases as well. Food establishments seemed to generally have the lowest set points I experienced. Hotels were typically set above 80 degrees. The 3 contractors we visited told us that was pretty typical out there.
I learned about Dubois’s Law in my undergraduate studies; then saw it applied with customers for whom we built houses where the petite wife stated she was uncomfortably cold in a room of the house, while the “larger” husband said it was too warm. This always results in complaints about thermostat locations or “the windows are cheap”. Its a no win situation for the builder and mechanical contractor!
Apr 03, 2013 @ 09:06:23
how can i found air flow vailocity
Apr 03, 2013 @ 09:18:53
Hi, Thanks for the question. Air velocity is measured using a tool called an “anemometer”. There are several types, depending on where and how you want to measure velocity. Hot wire anemometers can be inserted through a small hole and are great for measuring velocity inside a duct. Vane anemometers are good for measuring velocity of air at the face of a register. Average velocity inside a duct can also be calculated if you know the flow (CFM). Velocity(FPM) = flow(CFM) / area(sf). Where “area” is the cross sectional area of the duct in square feet.
May 24, 2013 @ 14:43:07
Hi Russ… I’m still trying to figure the design rules for cooling in California– so please correct me if I’m wrong– but It seems that there is always too much latent cooling calculated in the design, resulting in understated sensible capacity and over sized equipment. Shouldn’t it be designed to 100% sensible?
1) 2008 CA Energy Code (Appendix RA1.2.13): “The latent factor shall be 1.0. A latent Factor of 1.0 results in a design sensible cooling load calculation.” I interpret this as a sensible heat ratio (SHR) of 1.0, or basically 100% sensible and no latent. This is due to the low humidity levels found throughout all of Calif. I’m in the low desert, which is the extreme. Humidity levels of 10-14% are common and well below the manufacturer’s design of +-50%. Rarely do we have high humidity– August or a July thunderstorm– and even then, it’s only for a day or two. That’s why many people run evaporative coolers, the increased humidity is not a big factor except for those few August days, but that’s another discussion.
Most manufacturers design for approx 70% sensible/30% latent, so this must be adjusted according to the equipment, design conditions, location, etc. Therefore, the drastic reductions in sensible capacity that you mention would be grossly overstated in California, or unwarranted altogether. If your 5-ton example of 40,000 btu sensible were figured at 100% sensible, this 5-ton system would be downsized to 3.5-ton and still deliver the 40,000 btu.
2) (Appendix RA-1.2.3): “The indoor design temperature shall be 75F…An indoor design temperature swing of 3F shall be used.” Buck also made the point that even though design temp is 75F, that most use a higher setting– some for desired comfort, and some to merely save money on cooling costs. I set my T-stat at 82-84F, and sometimes don’t even turn the system on until it hits 88-90F. We are used to warmer weather. Out-of-town guests can’t handle that, so we always drop to lower settings when they’re visiting, but I’ve never set my T-stat to 75F, and never had a complaint. So, the point is that even though you design for 75F, you can also have reserve capacity with higher set points.
But from my experiences, the biggest problem that I run into is lack of airflow. It doesn’t matter what label is on the equipment, if the system can’t deliver, then it is under performing. Manufacturer design is typically 400 cfm/ton, but I rarely see it. In California, if you use the 100% sensible design, then airflow should be even higher. In the very dry desert, 500 cfm/ton is conservative, but I’ve yet to see a system deliver that.
I ran into one recently where the installer had a 16-SEER 5-ton with one18″ and one 6″ return duct. In addition, he had a 90 degree- and then a 180 degree turn in the 18″ duct, a MERV 16 filter, and dampers installed for the supplies. He thought he was upgrading the system by adding the second return, better filter, and dampers. What he got was a system that was delivering barely 250 cfm per ton with a 16-SEER sticker.
Even though the design element is a SWAG, and you do the best you can to fit the square peg into the round hole, you can still count on the installer to muck up the whole process by plugging the hole.
May 25, 2013 @ 10:56:07
Hi Roy,
You’ve obviously been putting a lot of thought into this. Your basic question is how to deal with latent load vs. latent capacity in a very dry climate. The short answer is that the latent load is so minor that there will always (99.99% of the time) be enough latent capacity to deal with it. The next question is how do you determine the “true” sensible capacity of the equipment in these super dry climates. You cannot assume 100% sensible capacity (12,000/ton). I’m not exactly sure why, but it just doesn’t work like that. There will always be “some” humidity. Remember, we are talking about indoor humidity, which can be substantially higher than outdoor humidity. I measured 60% relative humidity in a house in Las Vegas one time. It was 8% RH outside.
ACCA Manual S covers cooling capacity in detail. So, I suggest you take a look at that. Manufacturer’s expanded data tables (aka, detailed cooling capacities) give capacities at different entering wet bulb temps. 72, 67, 62, and 57 are common EWBs. As the air gets dryer, the sensible capacity goes up. Even at 57 EWB (about 30%RH at 75 DB), there will be some latent capacity. There is a rule of thumb (cringe) that says you can take your excess latent capacity and convert half of it into sensible capacity. That’s probably safe.
Your comment about airflow is dead on. All of these calculations are completely worthless if the airflow is not there. It’s inexcusable how some so called “professionals” charge money for systems that have such low airflow. That’s like an auto mechanic putting a brand new engine in your car and then permanently setting the parking brake by putting a bolt through it.
Oct 10, 2013 @ 05:15:57
I came across your post in a random search, so I don’t know if you blog only about Las Vegas or desert air conditioning needs, or more generally about any climate.
I am not an expert in air conditioning, I work in a totally unrelated field. So please forgive my ignorance if my comments seem foolish.
However, a couple things struck me. First, those “snowbirds” you mention are probably not just people from colder climates, unused to the heat. I suspect that many probably suffer from chronic obstructive pulmonary disease, or congestive heart failure. These are diseases of the elderly, which make it difficult to breathe in warm weather. A few degrees too warm means little to a healthy person, but to someone with COPD or CHF, it can make the difference between being comfortable and gasping for air. This is especially important at night, because the condition is worsened on lying down or reclining back, when fluid fills the lungs.
So I would say such people have valid concerns. And there will be a lot more of them as our population continues to age. If A/C manufacturers or landlords or building constructors are not cognizant of their needs now, they will be eventually.
Second, your post seemed to deal mostly with desert weather – hot dry days, cool nights. Certainly the graphs at the bottom seemed to represent such a climate. I don’t know whether this was by intention, but I live in Chicago, and Midwest weather is not like that. We can have days or weeks in the summer when the outside temperature NEVER dips below your thermostat set point. Heat waves are pretty common, every summer there are at least a couple. By definition, a heat wave is a week or more of unusually warm weather, and sometimes they’re 2 weeks or even longer. That’s 2 weeks when the temperature never drops below 80 even at night, and in the day it can hit 95 or 100.
I know a lot of people’s A/Cs can’t keep up with that. I’ve seen several people go out and buy additional window units even though they already have central air, because it can’t keep up with the heat wave. Usually they install the window unit in a bedroom, because warm temperatures at night kill a lot more people than warm temperatures in the day.
I don’t know if this is because someone under calculated their cooling needs, or because the A/Cs are old and need maintenance, or because we have more heat waves than we used to. I know people who’ve thrown a lot of money away at repairmen, thinking there was something wrong with their A/C. But it didn’t solve their problem, so I think it was probably just an underpowered A/C, someone miscalculated the cooling needs.
So I’d like to say that while it’s true that a right-sized A/C is okay for 95% of the year, the consequences of those 5% days when it’s not okay are not the same. Few people would care overmuch if their indoor temperature hits 78 instead of the target 72 a few days of the year – if they live in Las Vegas. But if they’re in hot and humid Chicago, they will care.
And if they have elderly people at home who are unusually susceptible to warm weather, it can be a serious health risk to have an inadequate A/C. Lots of people die every summer in Chicago. Usually it’s not healthy adults who worked in the sun longer than they should have. Usually it’s old people sitting at home with none or inadequate A/C.
I understand the problems of overpowered A/Cs. Constant cycling causes wear and tear and is energy inefficient. It results in poor control over humidity. On the other hand, an underpowered A/C can leave you gasping for air and endanger your health, if you live in parts of the Midwest or gulf coast where it’s hot and humid.
Seems to me that manufacturers need to come up with more elegant solutions to the problem.
Oct 10, 2013 @ 07:53:55
Hi David,
Thank you for your thoughtful and well spoken comments. You have some excellent points. They are a little different than the main point I was making in my blog, but very much worth addressing. My point is to not oversize air conditioners. Your point is to not undersize air conditioners. These are not mutually exclusive.
I and most of my audience work primarily in “hot-dry” climates, which is why I usually do not address humidity much. Humidity requires a whole extra load calculation, called latent load, which will result in a larger air conditioner than a house in a dryer city with the same outdoor summer design (drybulb) temperature. I did not want to get into wet-bulb and dry-bulb temperatures or sensible and latent loads, but that is where humidity gets accounted for.
With regard to folks with health issues, I totally agree. That is a special condition that should be taken into account when doing the design. The problem arises when designing a system but you don’t know who is going to live there, such as in a subdivision. Should you “over design” in case someone with health issues may live there? In my experience, I would say no. Over designing wastes energy. Out here in the west, excessive peak loads causes regional brown outs and black outs resulting in far more health problems than a small percentage of systems undersized for some people.
People with health issues need and deserve special consideration. Obviously, custom homes should be designed specifically for the occupants. Subdivisions intended for seniors should be designed differently than a “regular” subdivisions. If someone with a health issue moves into a house not designed for their special condition, then adding supplementary cooling, such as a window unit, makes sense; but before adding cooling capacity, I would first recommend reducing the load (reduce demand before increasing supply). This can be done by shading windows, increasing insulation, reducing leakage, improving attic ventilation, etc, etc.
Hopefully you still agree with the main point of my blog: it is more important to worry about other design issues, such as proper airflow, than worry about a few degrees on the design temperature. A system with properly sized ducts and designed to 90 degrees outside will cool a house MUCH better than a house with undersized ducts and designed to 100 degrees outside. Over the years, contractors out here have tried to solve cooling problems by increasing equipment size by arbitrarily jacking up the outdoor design temps. The problems were caused all along by poorly sized ducts.
Thanks again for your comment. Don’t hesitate to contact me if you have any questions.
Russ