Results 1 to 13 of 13
  1. #1

    Confused Manual J says 3.34 - Need Advice Please

    Long winded question I need advice on.

    I just purchased a 3/2 home in miami, fl with a 1 car garage. The house is currently under 1324 SF of air conditioned space. The house is a cookie cutter model built in 1988 with the original 2.5 ton split system a/c. The house needs a new a/c before I move in and I will be going with a 14 SEER model.

    I will be adding an extra bedroom inside the garage which will increase the area under a/c to 1468 square feet.

    I performed the calculations in Manual J to figure out what size a/c I will need for the house with the additional room. As of right now, my figures tell me that I need a 3.34 ton system.

    The advice that I need is this. If I were to install an a/c based on 3.34 tons, would I go with a 3.5 ton system? I also plan on building a covered patio on the back of the house where most of the large sliding glass doors are. Once these sliding glass doors get shaded from the patio roof structure, Manual J reduces my a/c requirement to 2.93 tons. Should I get the 3 ton in anticipation of the patio or the 3.5 ton?

    What would happen if I went with the 3.5 ton and a year from now, the patio gets built shading the windows? What negative impacts should I anticipate?

    Thanks in advance for any advice.

  2. #2
    Join Date
    Jun 2008
    Posts
    80
    Are you thinking dedicated A/C or HP? I believe I've seen posts suggesting you can oversize the HP by 25% (please check archives to verify).

    Also, what about a two-speed compressor? Not sure if you can find one that's rated at 3 and 3.5, but instead a wider performance range (e.g., 2.5 - 3.5).

    Maybe others on this forum can give you some idea what type of error tolerances are built into the Manual J calcs. Just in case I would recheck you assumptions such as insulation values, window and door types, kitchen, etc.

  3. #3
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,032
    most important factor is that the air handler & the ductwork is probably sized for 2.5-ton.

    Check the blower figures & duct system if they only support 2.5-ton, then you have to make some more choices. I personally, if need be, would borrow the money and, if possible, would optimize the home & the equipment for 2.5-ton.

    It may or may not be practical, depending on where you live & other load factors, however, an optimized 2.5-ton system should normally handle 1468 sf.

    Here in SW WI with occasional Heat Index of 104-F, had a guy here locally tell me a 2-ton system has been cooling his 2400 sf home without a problem.

    Were it me, I would evaluate all the options toward legitimate payback, before I invested any of my money. - Darrell

  4. #4
    Join Date
    Aug 2003
    Posts
    466
    I am not familiar with what 3/2 home and cookie cutter house refers to, but unless you are referring to a mobile home, 3.5 ton of air sounds a bit much for 1468 square foot of conditioning even for Miami.

    If the existing system is a 2.5 ton system, then the ducting is at best designed for approximatly 1,000 cfm of air. The nominal amount of air required for a 3.5 ton system is 1,400 cfm, or 40% more then your ducting may be able to handle without causing problems.

    Why not go with a direct 2.5 ton replacement for the existing sytem and add a small ductless system to condition the new bedroom? This way you have more control of temperature, less air problems and the savings of only using the cooling in areas where you need them while they are occupied.

  5. #5
    I want to go with a dedicated A/C.

    As far as the Manual J Values, I checked and rechecked all of the values from the house. I can achieve a lower tonnage requirement by shading some of my windows or adding more attic insulation. Since funds are tight, this won't happen until later.

    I just want a basic A/C. I assume the two-speed compressor costs more. Like I said, funds are tight right now.

  6. #6
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,777
    I think your load calc is wrong.

    If a 2.5 ton is currently cooling your 1324 sq ft. The addition of 144 sq ft, shouldn't increase the capacity required to almost 3.5 ton.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  7. #7
    3/2 means is for 3 bedroom 2 bath and cookie cutter means the house is in a planned community where there are hundreds of houses but only a few type of models.

    Maybe the calcs are wrong but what if the builders calcs were wrong when the house was built.

    I'm not sure about the how the duct was designed for the house. I can see by looking at the existing air handler that the plenum upsizes from 11 X 16 at the top of the air handler to where it goes through the ceiling to the attic at 16X16. If the duct is this size in the attic, shouldn't it handle approximately 1500 cfm of air thus a 3 or 3.5 ton a/c shouldn't be a problem?

  8. #8
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,777
    If the builders calc were wrong. Then either your current system isn't keeping the house cool.
    Or its over sized. Does it keep the house cool.

    Your only adding 2 walls, and a ceiling to the load, and your decreasing teh load of teh room that the addition is attached to.

    A simple static pressure test will tell if your current duct system can handle the additional air for a larger system.

    Cookie cutter homes are not known for putting in bigger duct then needed for teh A/C size being installed.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  9. #9
    Quote Originally Posted by beenthere View Post
    If the builders calc were wrong. Then either your current system isn't keeping the house cool.
    Or its over sized. Does it keep the house cool.

    Your only adding 2 walls, and a ceiling to the load, and your decreasing teh load of teh room that the addition is attached to.

    A simple static pressure test will tell if your current duct system can handle the additional air for a larger system.

    Cookie cutter homes are not known for putting in bigger duct then needed for teh A/C size being installed.
    I just purchased the house and don't know how well the existing a/c performs. It does blow cool air. The windows have no window coverings so heat from the sun was obvious.

    You indicate that I am only adding 2 walls and a ceiling to the load and am also decreasing the load of the room that the addition is attached to. I don't quite understand what you mean when you say "by decreasing the load of the room that the addition is attached to."

    If in fact the 2.5 ton system was sufficient enough for the house at the time it was built, do you think the 3 ton system will be okay to install for the additional room I am building? The 3.5 ton does seem quite large. Maybe one of my Manual J calcs is high.

  10. #10
    Join Date
    Jul 2008
    Location
    Kansas
    Posts
    63
    Quote Originally Posted by MiamiRockHead View Post
    I don't quite understand what you mean when you say "by decreasing the load of the room that the addition is attached to."
    You're taking what was an unconditioned wall, and conditioning the other side of it. In turn, the original room need less heating/cooling than it did before.

  11. #11
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,777
    As above. The room that the addition is beside, no longer has any heat gain. so you lost load for that room.

    Many people for get to subtract the windows and doors sq ft from the wall sq ft. So they get a higher load then what is needed.

    You could have 200 plus sq ft of wall lisred, that doesn't exist.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  12. #12
    Join Date
    Aug 2003
    Posts
    466
    Quote Originally Posted by MiamiRockHead View Post
    I want to go with a dedicated A/C.

    As far as the Manual J Values, I checked and rechecked all of the values from the house. I can achieve a lower tonnage requirement by shading some of my windows or adding more attic insulation. Since funds are tight, this won't happen until later.

    I just want a basic A/C. I assume the two-speed compressor costs more. Like I said, funds are tight right now.
    If funds are tight, then you seem to working bassackwards with where you are putting your initial investments. If the current 2.5 ton system is operational, then put your money into insulation, window dressing and shading first and deal with the few extreme temperature days that the system may not keep up as well as you would like it to.

    There are two ways to approach heating and/or cooling a dwelling; decrease the load/loss of heat or increase the conditioning capacity. Which one is going to be the most practical in the long run?

    You are also making an awful lot of assumptions about the existing distribution systems ability to handle more capacity and more air volume. I must still advise focusing on prevention rather then cure for your anticipated cooling problems.

    Just for kicks, let's assume that with 3 bedrooms you will also have a kitchen, dining room and living room. Assuming one vent per room except maybe two vents for the living room, that would be 7 vents at approximately 125 cfm per vent and two bathroom vents equaling a possible 140 cfm total. That would add up to a standard air distribution system for a 2.5 ton system and is about what you are describing. The point is that it is very doubtful that your air distribution system is sized for more then the 2.5 ton system you currently have. Oversizing is not only a waste of initial investment, it is a guarantee of continued higher energy costs for ever.

    Another thing to keep in mind is that if your system was originally designed to maintain, say, 75 degrees when the outside temps are 100 degrees, then the worst that may occur with the existing system is that it may max out on a 95 degree day. Even if this were the case, every day under 95 degree's the system is still essentually oversized.

  13. #13
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by MiamiRockHead View Post
    3/2 means is for 3 bedroom 2 bath and cookie cutter means the house is in a planned community where there are hundreds of houses but only a few type of models.

    Maybe the calcs are wrong but what if the builders calcs were wrong when the house was built.

    I'm not sure about the how the duct was designed for the house. I can see by looking at the existing air handler that the plenum upsizes from 11 X 16 at the top of the air handler to where it goes through the ceiling to the attic at 16X16. If the duct is this size in the attic, shouldn't it handle approximately 1500 cfm of air thus a 3 or 3.5 ton a/c shouldn't be a problem?
    Getting the correctsize system and ducts is critical to effeciency,and longevity of the system.

    You need to slow down,and be sure the calc is correct or find someone that can do it.

    What indoor and outdoor design temperatures did you use in your calc.?

    Are sure of the window type and insulation R-value of the walls.

    It's warm enough now to have an idea how well the 2.5 ton is doing.You have windows without shades,that another factor in Man. J.

    If you shade the sliding glass doors,and build the addition with great insulation,plus maybe add some to the existing attic,chances are the 2.5 ton system may do the job.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event