For starters, I live in Atlanta, Ga and I am not an HVAC pro so this is a serious question that I would like answered for one of my clients.  I understand the premise of a zoned system and on the surface, it sounds like a good idea. Then I considered the fact that I have always told my customers that it is a bad idea to close off vents in rooms they are not in because of duct leakage.  Also, the second law of thermodynamics simply states the hot goes to cold, so now the unheated areas are doing their best to rob warm air from the heated areas.  Finally, if your zoning a single system, aren't you creating on oversizing issue because the unit (that was probably oversized in the first place) is now servicing a smaller area than what it was designed for?
I look forward to your input.

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Here is my 2014 experience with a zoned system, a prominent national builder and a client/buyer.

This builder put in a dual zoned (single unit) AC system on a home. 4 ton AC system to support a 3100 sf house and the system is not cooling the house more than 1-2 degrees per hour when it is empty, 89 degrees outside (which is not hot for Texas, guys). We watched and monitored the AC every 15 mins for 1 hour to try to get the air in the house from 77 degrees to 73 degrees. The HVAC system ran for 1 hour and only dropped the temp 2 degree downstairs and 1 degrees upstairs.

Yes, I know it is a somewhat complicated issue and many factors go into the energy efficiency of a home, such as the insulation in the home, the R factor of the walls and ceiling, radiant barriers, doors and window use, number of people in the home, etc. That is all fine and good, but let's talk about the real question: "Can a single 4 ton system push enough cold air far enough, fast enough and hard enough to cool a 3100 sf home in a hot climate?"

In this situation, Just before closing, we discovered that this home did not feel like it was cool properly, especially in the back of house--so, we started investigating the situation. The builder also started an investigation that consisted of two parts: (1) They bought out an independent inspector to look at air flow and leakage (sadly, they did not talk to their inspector who did the study to find out what the information meant and they did not use the data to determine if there was a flaw in the design of the AC system of the house. They did not use the report at all) (2) The asked the sub-contractor who installed the system to come and look at the system. The installer corrected the thermostats (that were flipped), but did not address the inadequate air flow that was discovered by the independent inspector and was clearly noticeable in the back of the house and other areas. The ducts running to the back of the house ran from the garage to the peak of the roof and back down. 12" flex most of the way, an estimated distance of 50+ feet to the junction plenum.

We ran our own set of tests. We set the AC to 77 degrees and left the house for a couple of days to be sure the air was really 77 degrees. Then we tuned the AC unit to 73 degrees on both floors and recorded the temperature every 15 minutes for 1 full hour. After approximately 30 minutes, both floors had dropped by 1 degree in temperature. After another 30 minutes only one floor had dropped an additional 1 degree and the other floor was still sitting at 76 degrees. Outside temperature was showing between 88 - 91 degrees. No a hot day by Texas standards.

I can tell you that if my AC would not cool my house by 2 degrees in 10 minutes on a real hot Texas day, my wife and kids would be telling me that I needed to get our licensed AC tech in to see what was wrong. And while we are on the subject of AC Technicians, I called my guys that I have been using personally for years and have been sending to clients for jobs from basis service to all new installations. For 20 minutes he railed on how these zoned systems are being sized as if the system is only cooling one floor at a time. So, if the house is 2800 sf, split between first and second floor, the system may be rated for just half of the size of the house. The assumptions here are (1) Only half of the house needs to be cooled at any one point in time (2) the duct work that goes to the second zone is short enough or designed so that air flow will carry at greater distances than normal to get to the secondary zone. Big assumptions. In our case, we took note of duct work in the same home floor plan and it appeared as though the builder's AC company was putting in way too much and too large of duct runs for the air to get to the back of the house--which is where the greatest problem was easily noticed.

So, the next question I asked was to one of my trusted inspectors that I have worked with for years. He came from more than 20 years of construction experience before he started inspecting houses and he understands construction. I asked him what he thought of zoned AC systems and he said that he had no issue with them if they were zoned properly to the size of the home (not just sized to one floor). He said what my AC guy said and what seems to be a generally accepted engineering spec for sizing AC systems from any manufacturer or SEER rating, that 1 ton of AC per 500-600 sf of house is the rule of thumb that I have never seen disputed. So, if your house is 2500 sf, you would generally need about 5 tons of AC capacity to heat and cool the house. Literally, to blow enough air in the house. Of course, that has to be distributed correctly to the floors according to sf of the floor and size of the system.

Now, let's talk about builder diversions to the engineering questions:

(1) "Over-sizing is a big problem!" Yes. Over-sizing of a system can be a problem. I have never seen a system that is over-sized, but I am told that it can create some of the same problems of humidity control and electrical use that an under-sized unit can create. So, again, we fall back on the questions of "What is the fundamental engineering spec for an AC system as expressed in AC size to SF of the house and does this house meet that basic specification?" and "Can your builder document and prove that the system is sized properly to the size of the house

(2) "Our insulation is better than most homes!" I have heard this from builders for the past 7 years from every builder salesperson in Austin. They all claim that their methods are unique and better, when in fact the details of what they are doing are almost all identical. There are some exceptions, but buyers need to keep in mind the example of the refrigerator. If you can not move enough air across the house to begin with because the system is not strong enough or the duct work is too long and poorly designed, you can have a 5 start green/no air leak/100% sealed home, and it still would not cool the home. Insulation is not enough to cool a house. (And the truth is, most builders are not doing anything different today with their windows, doors, radiant barrier and attic insulation than they were 3-4 years ago when they were not using zoned AC systems. Check it out. It is the same.

(3) The computer said it was okay. This one is the real leap of faith. This is probably the first thing that you will hear, too. "Oh we don't use engineering spec's anymore. We use a really smart computer program that tells us how to rate the system." Well, if this is true, then the program should be smart enough to tell us how much air flow is needs in each room according to the cubic square feet of the room, how many windows are in the house, which way the sun is coming in from the windows, how much air flow is needed in each room and now long each run of duct can extend without degrading the air flow in home or individual rooms. And then the air flow measurements that are conducted on the house (by people, not a computer program) should verify that in fact that much air is going into and cooling the room. Secondly, the systems should heat and cool the room at a similar rate as it does in any typical home (not 1-2 degrees an hour). Lastly, the house should feel like the temperature that it is set to, or there should be a valid engineering reason why it is not performing as designed.

(4) "Don't worry about it. If it does not work, it is covered by the warranty and we will make adjustments." This is the most dangerous thing you can possibly do because once you sign off on the house, getting something changed after the fact is next to impossible. Yes, maybe they will send a AC tech out to your house from the sub-contractor who will tell you that it is operating fine and maybe you should keep your doors shut, shades pulled down and spend more time in the part of the house that is coolest until summer is over. "After all, this is Texas! What do you expect?!" I hope you were not expecting that the AC company was going to redesign or change your system out for a larger one or two systems--that is not going to happen.

The concern and potential for short term problems is that if your system is under-rated, it will not cool your home and it will run for long periods of time to support minimal or moderate cooling of your home. And when the real heat of July and August hits Texas--oh my! Expect much higher energy bills and turn on those fans. You will need them.

The long term risk should be an even bigger concern. If your system is running at 2-5 times its normal duty cycle, you should expect to have to replace (and upgrade) your system in a much shorter period of time. Motored are designed to last X number of years based on engineering specifications of duty cycles. If you are running your AC system at 2x, 3x or 5x the normal duty cycle, you should expect higher maintenance bills, higher electrical bills and early replacement of the issue. This is not hard to figure out. Example: If your normal $25 hair dryer is designed to be used for 10 minutes a day and you used commercially in a hair salon for 10 mins x 10-20 times a day -- it is probably going to die an early death, as compared to the hair dryer that you use once a day.

So, if you are buying a big new home and when you go outside you only see one AC system outside (compressor unit), maybe you should start asking some questions.

Mr. Builder:

    How was this system designed? What is the engineering spec on the system?

    How many systems are installed on the house and how is the air-flow distributed?

    If using a single zoned system, how has it been sized and designed? What is the engineering spec?

    If you have a computer program telling you it is okay, show us the data that was entered into the program for this house, all the assumptions of data entered into the program, the engineering spec of the calculations used and the air flow data as compared to how it is performing in this specific house.

    While every home may have a design spec, not every installation is the same, so you should learn everything that you can while your house is being built and don't assume anything. Ask more questions.

So, what happened to my buyer and builder situation? We asked too many questions that the builder and HVAC sub-contractor could not or would not answer. Rather than spending the money to fix or change the system, they released my client from their contract. That's the good news. The bad news is that this builder is still building homes all over with the misguided belief that if the AC is working and some technician says that "the computer program says it is okay", that is "good enough" for them to keep selling homes as fast as they possibly can.

The problem isn't just the builder.  Everyone here has some flat earth "conventional wisdom" assumptions that are pure superstitious beliefs.    

A big problem is your perception of how AC should perform, and complete and utter ignorance of Building Science and Home Performance.  You think of a house as if it's a water ski boat instead of a ship.  You want to hit the throttle and have arbitrary level of "acceleration" or the system is bad.  Much of this comes from Thermostat advertisements that promise huge savings if you buy their magic beans.  

Think of your house as a refrigerator.  Would you jerk your refrigerator around?  Do you think doing so would either save meaningful energy OR come without consequences?  Set the thermostat and leave it alone.  Want low bills?  Have a good enclosure (refrigerator).  Want control?  Define it reasonably.  Want a comfortable house?  Don't let it turn into an oven!  

Learn about latent.  Learn about psychrometrics.  Learn about surface temperatures, relative humidity and dew point.  If you don't learn about these things and continue to have absurdly ignorant expectations of performance  you are not only likely to be continuously disappointed, you are risking a moldfest.  

99% of the time zone systems are expensive symptom treatment rather than elegant and efficient cure.  The problem is typically crappy enclosure, oversized equipment, complete system imbalance, and uninformed occupants who mistakenly think they can "game" meaningful energy savings at the thermostat.  You are the 300 lb man who drinks diet soda and telling everyone who will listen that because of diet soda he's losing 5 lbs a week.  But he doesn't own and never steps on a scale.  

How is the absurdity of that not blatantly obvious? 

You can have power or efficiency, choose one.  You want power, put in huge equipment and huge ductwork.  You want efficiency put in small equipment and decent ductwork, and leave refrigerator set so your ice cream is not too hard and not too soft.  

We're finding that after you fix houses, disabling zoning has fair impact on consumption with no comfort penalty.  I suspect much of this savings is due to reduced short cycling.  All that is usually required is a small amount of system rebalancing.  

Instead of rushing to "quick fixes," step back, analyze deficiencies, then recommend comprehensive cure rather than expensive treatment.  

You can't force clients to implement good design, but don't you think it's our duty to always at least present the best path?  If they insist on buy zoning, then the consequences are on THEIR shoulders instead of ours.  

I don't know what a flat earth assumption is, but Realtors have to work with builders and builders are known for their "black box" designs. "Trust me; it works." No proof. Just trust and promises.

As for your side of the equation (homes that are already built)--there are the AC installers at work. Clearly, no one in your world agrees. Some installers say that zoning is great and some of you say it is crap. And then there is the question of dual zoning, one zone up and one zone down--this is what my clients have to figure out, not multi-room zoning that many of you are talking about here. Just two zones.

Unfortunately, the complexity of the questions and the lack of agreement on the technology force me to fall back on common sense, building standards, basic math and physical comfort. If the house is not cooling in a reasonable period of time and the builder will not acknowledge that there is a problem that can be corrected BEFORE buying the house, then I am in the camp that says that dual zoning systems that are splitting the upstairs from the downstairs on a big house is a bad thing. If it runs for an hour and drops the temp 1-2 degrees, it is not reasonable to think that this is going to magically get better when the weather gets hotter and the builder has his money for the house.

Call me a flat world thinker if you will, but I know how an AC system should cool a house in Texas and when that is not happening and the builder is not willing to make a correction, I say it is a bad technology/installation. And it is not worth my client's money. Buy it if you want. Sell it if you must. But it's not for me and I won't recommend it in this application for my clients.

Yep. We have the leak test data and the air flow data and timed temperature data. Sorry. I did not know I had to send you the reports, boss. Clearly, you are the arbitrator of all things HVAC. I guess that is why I am a Realtor and you are someone who spends your time belittling people on this board. Your comments, this one and others, are not helpful and do not promote any sense that professionals in your business like you are really trying to help consumers. For that I am sorry.

But it's so much easier to size by the 500sqft per ton rule that's been used for decades. Seriously, how many houses have you seen that DIDNT use the 500sft per rule for sizing? Most will be 400-600sqft per ton. Newer homes just allow more slack in install quality since they only need 1/2 the cooling of an older house. Sure it's wrong, but good luck getting HVAC contractors to change their ways when they have nothing to loose by keeping the old rules around.

 T. Thornton: Dear T. There are multiple items in your comment that bear some response. First is the zoning issue. I strongly suspect that the reason the house is not getting cooled per your criteria is that the AC is actually delivering way less than its designed capacity to the conditioned space. I strongly suspect it has a bypass that is partially or totally open. Bypasses rob the AC of capacity. 
Here is a report on zoning and bypasses that goes into detail on what happens Perhaps start on page 23. This is an engineering/scientific report. 
One thing that seems to confuse people is the difference between rules of thumb and engineering. Rules of thumb are not engineering. Estimating the probable cooling load on a house based in window directions, solar het gain coefficient, wall insulation, etc. is engineering. 1 ton per 500 or 600 square feet is not an engineering specification, it is a rule of thumb that is obviously wrong since an uninsulated home with bad windows will have a lot larger cooling load than the same size home with good windows and insulation.  
Most air conditioners are oversized. This makes it difficult to design a good duct system that will move the right amount of air and BTUs of cooling to each room. And yes as you suggest when the system is installed it should be checked to see if the design flow is getting to each room.

Thanks, John. My client is a mechanical engineer and he ran the numbers. As it turns out, I suppose this is something that Mechanical Engineers do study in school. He ran the most conservative calculations and compared them to the air flow data that was produced by the builder's third party inspector. The builder did not look at the air flow data, because the word "passed" was on a page related to leakage, so the builder thought the inspector was testing and passing the system use and design. When we spoke to the inspector, we discovered that was not the case. He was producing the numbers so that they could be compared to the spec for the house and the design of the system, duct lengths, etc. The builder did not get past the word "passed".

The system is a 4 ton system that is attempting to cool a 3100+ sf home, most of size which is one the first floor. The temperature outside was only 89-93 degrees during the test days. The air in some of the rooms was running at half of less of the calculated air flow cfm for the sf of the room, before we add in added capacity for very long duct runs, people, additional windows, etc. While there may be more sophisticated formulas available for calculating what the air flow should have been, the system was running non-stop and not cooling the house. The builder waited 3 days after last checking the AC system before letting us come back in to check to see if the house was cool. 3 days to get the house down to 74 degrees. 

On top of that, this particular house was only built one other time in the area and when the builder built it that time it had 5.5 tons of AC (two units; 4 ton + 1.5 ton upstairs). And that house had a smaller upstairs due to options that the buyer did not choose for additional living space. So, more air flow, smaller sf house, dual system coverage. Much higher ratio's of air flow to sf of house. The things that have not changed are building standards. Our energy and insulation standards have remained very close to the same for many, many years. Only 5 star green builders are doing things beyond what is required by building code in insulation, windows, radiant decking, etc. They all claim to be different, but they are all virtually identical.

As for the position of the bypass, I could not tell you. What I can tell you is that we were trying to cool both the upstairs and downstairs at the same time, as a requirement of the buyer and a general requirement of homes in general in Central Texas. When temperatures outside reach 100-110 (yes, we count the number of days we reach and surpass 110 degrees every year--and it can be in excess of 30 days a year here), the AC has to cool the entire house and every room in the house. Most home owners here are not living on one floor and we do not have basements in Central Texas either (never). So, we are talking about above ground floors.

I spoke to my licensed HVAC guy that I have used for years to maintain and install new systems for me and for my clients. He railed on the use of zoned systems in Central Texas, especially in larger homes that need more than 4 tons to get air and cold air across the width of these large homes. This home had 12" duct work that was running from the garage in the front of the house to the peak of the attic and down again to the back of the house. The first time we went into the house you could not feel any air going into the back of the house. The air vents that were closest to the air handler were cooling fine. The air flow was not over whelming, but the rooms felt comfortable when the system was running for more than an hour. Personally, I think that is way too long and compared to my house and my system, I would consider this a failure of design even for those rooms, but it was good for those rooms compared to the rooms where the air was not flowing or moving very slowly.

Honestly, I don't think this house matters since my clients have already canceled their contract to buy the house, but what does matter to buyers and to Realtors are some fundamental questions:

-Can a 4 ton system with long duct runs cool 2 floors at the same time?

-Are large tract home builders and their contractors who are getting far less money to install and implement systems in large homes doing a good job in areas where temperatures reach 100+ degrees for months at a time and have high humidity?

-Should buyers blindly trust that the system is ok when in fact the system is running non-stop and not shifting the air temperature more than 1-2 degrees an hour?

-Should there be verifiable proof for the home owner that an AC system is properly sized and designed for these large homes and systems that seem to be so complicated and so controversial in proper design?

-If these systems are going to remain in use, what can the HVAC industry do to reduce this kind of thing from happening and increase consumer confidence in the short term and long term effectiveness of these systems for the general public who does not want to go through 5 contractors to find someone who can correct the design on the HVAC for their new home?

That is really the crux of the issues and concerns for those of us who are outside of your business, but work with clients everyday who have very simple needs to be comfortable in their homes without having to pay an arm and a leg for energy bills or repair costs.

Thanks for your time. I appreciate it.

Tim, in answer to your questions:

1) Can a 4 ton system with long duct runs cool 2 floors at the same time? 

A 4 ton system that is working properly CAN COOL a well designed and built 2 story house with a floor area of 3100 sq ft. Rick Chitwood is a building performance contractor who makes sure the insulation, air sealing, HVAC, and ventilation is done right. An example of one of his houses is in Redding CA (Record Max Temp 115 degrees F, Average daily high temperature 98 in July). The house is conventional construction and insulation with conventional single speed air conditioner. It is 3500 square feet with a single 2 ton air conditioner. What is not conventional -- he makes sure the work is done properly (insulation, air sealing, AC installation) The duct systems are well designed with MINIMUM LENGTH RUNS that are well insulated. Since he uses a much smaller AC then the ducts can also be smaller and since they are also shorter, the surface area is much smaller. He does use zoned systems -- but not the conventional designs that have a bypass -- rather he shifts part of the capacity from one zone to another (always delivering some capacity to each zone) --NO BAYPASS!!!

2) Large tract builders make good money or they would not stay in the business. They generally hammer the subcontractors for a lowest price, so they get what they paid for -- junk.

3) Buyers usually trust that everything is OK. Without verification however it is at best a crap shoot.

4) Verification should be that the system is commissioned and running properly. The case you put forth is very unlikely to be running properly.

5) The HVAC contractor industry is a "status quo" industry. There are very few contractors that fully understand performance and the rest cover up the lack of knowledge and their standard practices with oversized systems. A few HVAC contractors are stepping up to the issue and learning to be true building performance contractors. That means knowing a lot more about buildings and verifying that things are working right. That costs money and time. Track builders are usually not willing to spend that money, so the building performance industry is mostly in retrofitting existing buildings.  --- then of course what follows are some scam artists who call themselves building performance contractors, but cut corners.


Thanks, John. That is VERY helpful and straight forward. So, net-net, it is possible to do virtually anything with an AC systems that is very carefully designed and implemented, which is not very likely in mass production and clearly did not happen with the house that my client was contracted to purchase. Builders can talk a big game when it comes to quality and energy efficiency, but in the end not all of the product lives up to the hype. Since the AC is such a big part of the over-all quality of the house that consumers monitor on a daily bases, it would be foolish to ignore the warning signs of a poorly implemented AC in a new home. 

Thanks, John. I would definitely hire a professional of your caliber to take care of my homes and clients. Thank you, Sir.

Tim, You are welcome. I think what the others' responses were focused on is the concept that the AC should cool down the house in a few minutes from a hot soak. A well designed and installed system will make a dent, but it can't pull it down that fast. A proper installation is best utilized with a constant thermostat setting. Designed and working well it is efficient enough that a constant setting is the best solution.


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