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  1. #1
    Join Date
    Aug 2006
    Location
    Utah
    Posts
    23

    A/C sizing - HVAC contractor (1 ton/500 ft2) vs. third party calcs (1 ton/1175 ft2)

    I had a third party (Energy Wise) that specializes in heat load calcs for spray foam insulated homes do the calcs for my home. The calcs focused on A/C since the heating will be done with a radiant system. The calcs showed that I needed 1 ton/1175 ft2. The third party spec'd return ducts in each room and one or at most two registers per room (most were one register) with flow rates of between 60-200 cfm per register. This system is designed to run 35-45 minutes an hour. The calcs can be found here.

    The HVAC contractor has reviewed the calcs, but is insisting on sizing it based on his experience (he didn't do calcs), which ends up being twice the size as the calcs. He would design it to have about 1 ton/500 ft2 of house. He would not put return ducts in each room, but would instead put one or two centrally located returns. He would design it so that the space under the doors of each room is big enough to allow the air to travel under the doors to the returns. He didn't say how many registers he would put in each room, but he did say that he likes to run 800 cfm from the registers (or per room? I'm not exactly sure) so that the homeowner can really feel the cold air coming out. His system is designed to run 15-25 minutes an hour.

    The HVAC contractor's rationale for his system is as follows: (1) he has had 15 years of experience doing it this way and has never had an unhappy customer, (2) oversizing is not a problem in our area because we have low humidity (northern Utah), (3) oversizing is not any more expensive because the cost of add-ons to the smaller system (returns to each room, caulking ductwork, extra labor, etc.) is the same as larger equipment, (4) risk of undersizing is much greater than oversizing because if the system is undersized there is nothing you can do later short of tearing the house all apart to fix it, (5) both systems are equally efficient because even though his draws more energy when it is on, it is not on as long as the smaller sized system, (6) the only unhappy customer he has ever had is one that went to a third party (some local mechanical engineer) to have Manual J calcs done and then the system wouldn't cool the place on the hottest days in the summer. The HVAC contractor is convinced that if I build the home per the manual J specs, I will not have sufficient cooling capacity.

    I'm an engineer myself (chemical) so I am inclined to trust the calcs done by the experts (the third party). However, I would be interested in any of your thoughts on this.

  2. #2
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,287

    Thumbs up

    Quote Originally Posted by dblagent007 View Post
    I had a third party (Energy Wise) that specializes in heat load calcs for spray foam insulated homes do the calcs for my home. The calcs focused on A/C since the heating will be done with a radiant system. The calcs showed that I needed 1 ton/1175 ft2.

    The third party spec'd return ducts in each room and one or at most two registers per room (most were one register) with flow rates of between 60-200 cfm per register. This system is designed to run 35-45 minutes an hour. The calcs can be found here

    The HVAC contractor has reviewed the calcs, but is insisting on sizing it based on his experience (he didn't do calcs), which ends up being twice the size as the calcs. He would design it to have about 1 ton/ 500 ft2 of house. He would not put return ducts in each room, but would instead put one or two centrally located returns. He would design it so that the space under the doors of each room is big enough to allow the air to travel under the doors to the returns. He didn't say how many registers he would put in each room, but he did say that he likes to run 800 cfm from the registers (or per room? I'm not exactly sure) so that the homeowner can really feel the cold air coming out. His system is designed to run 15-25 minutes an hour.

    The HVAC contractor's rationale for his system is as follows: (1) he has had 15 years of experience doing it this way and has never had an unhappy customer, (2) oversizing is not a problem in our area because we have low humidity (northern Utah), (3) oversizing is not any more expensive because the cost of add-ons to smaller system (returns to each room, caulking ductwork, extra labor, etc.) are the same as larger equipment, ...

    The HVAC contractor is convinced that if I build the home per the manual J specs, I will not have sufficient cooling capacity.

    I'm an engineer myself (chemical) so I am inclined to trust the calcs done by the experts (the third party). However, I would be interested in any of your thoughts on this.
    One may generally trust the calculation Method, however, the duct losses in the calc are stated as zero
    so I guess 100% of the duct work is in conditioned space which reduces your system size by ~20%
    in comparison to a residence with ductwork in a ventilated attic.

    Do you trust the 9,382 Sq Ft or the 3,852 Sq Feet which are both listed in the calc as Total Conditioned Floor Area?

    The equipment selection does not seem appropriate for 31,200 BTUh Sensible Cooling as shown in the calcs. I would select a 4-ton 16 SEER unit & at least 2-stage. INSIST on the returns in each bedroom & study.
    Last edited by dan sw fl; 06-13-2007 at 04:09 AM.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  3. #3
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Any contractor that wants to put in 1 ton of AC per 500 square feet in a tight foam insulated house needs to be kicked to the curb.
    If more government is the answer, then it's a really stupid question.

  4. #4
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    are you serious about considering someone guessing [at 1t/500sf]?

    experience hell, he is lazy or dumb or both & does not care how much ongoing energy costs the HO
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,158
    My friend that had a foamed house had 1500 sq ft not counting basement. Under this clowns thinking, she would have needed 3 tons. Probably more as we get pretty humid. But instead she went with 2 ton and it didn't struggle even in the hottest weather with company over!

    You may be dry and oversizing won't hurt humidity control BUT it will kill SEER. More cycles = less efficient so you may think you are getting a 13 SEER or 15 SEER or whatever but in effect you will be much less than that if constantly cycling.

  6. #6
    Join Date
    Aug 2006
    Location
    Utah
    Posts
    23
    Quote Originally Posted by dan sw fl View Post
    One may generally trust the calculation Method, however, the duct losses in the calc are stated as zero
    so I guess 100% of the duct work is in conditioned space which reduces your system size by ~20%
    in comparison to a residence with ductwork in a ventilated attic.

    Do you trust the 9,382 Sq Ft or the 3,852 Sq Feet which are both listed in the calc as Total Conditioned Floor Area?

    The equipment selection does not seem appropriate for 31,200 BTUh Sensible Cooling as shown in the calcs. I would select a 4-ton 16 SEER unit & at least 2-stage. INSIST on the returns in each bedroom & study.
    You are correct that we are putting all of the duct work in conditioned space. We are spraying the foam insulation directly on the underside of the roof decking so the attic will not be ventilated.

    Also, the calcs are divided by floors, with sizing recommendations for each floor. The basement is 3851 ft2 (sized at 3 tons), the main floor is 3861 ft2 (sized at 4 tons), and the second floor is 1670 ft2 (sized at 1 ton). The total square footage is about 9382 as stated on front page of calcs. In the PDF file of the calcs, the basement calcs are pages 1-6, the main floor calcs are pages 7-11, and the second floor calcs are pages 12-16.

  7. #7
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,837
    Can't say enough times, trust the Manual 'J' calculations. ACCA spent years and many dollars so no one would have to 'guess' or use their 'experience'. If three idential homes are built on a given street, do they all need the exact same systems? Probably not if they're oriented differently. Just turning one large window from West to North could change the system size. Square foot calcs only work by accident. Obviously your 3rd floor is considerably smaller than the rest of the home but a true Manual 'J' calc takes it all into consideration. Good for you AND the insulation company that did the calcs. You've got good, solid info. Find a contrcator who agrees or at least runs his own calcs. We do Manual 'J' for every single job be it new install or replacement and we pull out an awful lot of improperly sized equipment. It's simply amazing that in 2007 there can still be so much energy wasted by hacks guessing at the size the equipment should be.

  8. #8
    Join Date
    Sep 2006
    Location
    Angier, NC
    Posts
    102
    That "1 ton per 500 sq ft" is a rule of thumb that alot of people use and have been getting away with for years. The part where he says that there is enough space under the doors that you dont need returns in each room is the really disturbing part. Go with the one who did the load calc or get a third opinion from someone who will do a load calc. Dont go with anybody who uses a "rule of thumb" to determine tonnage.

  9. #9
    Join Date
    Jan 2004
    Posts
    108
    What everybody else said.

    The calculations for my 1320 square foot house predicted 8500 BTUH (0.7 ton) heat gain at 95 deg F outside, so I had a one ton AC installed. The AC only ran 58% of the time when it hit 101 deg F last summer, so my calculations were too conservative. It was my fault. I had neglected the heat loss down through the crawlspace floor. The HVAC guy was reluctant to install such a small air conditioner, so I had a meeting with him and the builder where I said "I will take responsibility for sizing the system".

    We usually cannot hear the AC running, nor do we feel cool air blowing. It's the most comfortable house I have ever lived in.

    Trust the Energy Wise calculations. I suggest that you double check the internal heat gain numbers, especially if you will have a home theater, a row of Sub Zero refrigerators, or other large heat sources.

  10. #10
    Join Date
    Mar 2007
    Location
    Phoenix, AZ
    Posts
    574
    LOL - your posts illustrates the two extremes between the good and the bad.

    If you go with the bad - the "rule of thumb" guy - you'll have a system so grossly oversized it's unimaginable with runtimes of a few minutes, short-cycling protection engaging all the time, pressurized rooms when you close the doors (forcing cool air out every crack and crevice), causing de-pressurization in the rest of the house (sucking hot air in every crack and crevice) and a very uncomfortable home with huge energy bills.

    If you go with the good - Manual J calcs - you'll end up with an energy efficient system with ideal runtimes and neutral pressurization in your rooms and house and low energy bills.


    At least that's how I see it. I could be missing some things, since I'm just a homeowner and not a pro.

  11. #11
    Join Date
    Apr 2005
    Location
    Tampa, Florida
    Posts
    1,634
    I have a rule of thumb as well:

    NEVER use a contractor who sizes using a rule of thumb.

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