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Thread: duct sizing

  1. #1
    Join Date
    Jul 2006
    Location
    Columbia, SC
    Posts
    96
    I've been reading alot here and have quite a few questions. My current system is a 10 year old Carrier 12 seer 2.5 ton package unit. It is undersized on the a/c and oversized on the heat. I have ran the hvac calculator and that gave me a lot to think about. I want to upgrade sometime in the future, just not now. Will I be able to keep the existing duct work if I go to a 3 ton 13 or 14 seer dual fuel heat pump? Current sizing is 16in. return, 20x25 filter grille, and a 16in. supply trunk that branches off what looks like 10 or 12in. lines that feeds 2 rooms. All end up at 6in. lines to each room. I did the cfm and line size tool on the calculator just to check but that just added to the questions. One other thing, my t-stat is in the hallway near the return. There is no register there, does it need one? I was thinking of having one installed if it will be helpful. Just trying to be prepared and informed when the time comes. For now I'm going to see about more insulation and adding window film to the south and west windows. One other thing I noticed my unit doesn't have a trap on the condensate drain. I guess they didn't put one on cause it's at ground level on the slab. How important is it to have one put on?

    [Edited by jl1 on 07-30-2006 at 08:53 PM]

  2. #2
    Join Date
    May 2006
    Location
    Philadelphia, PA
    Posts
    1,398
    You are in a really good situation. Your system is operating so you are not in a rush to change it.

    Use the information in HVAC-Calc to make improvements to your house that will make it more energy efficient (like the insulation and window tint you mentioned). This way you can see how changes to the bldg envelope will affect your system sizing and make a more informed choice on what improvements make enough difference in heating and cooling loads to pay back the investment.

    HVAC-Calc also shows duct sizing. Use this information along with your contractor's advice to determine if changes are needed.

    T-stat sounds like it is in the correct place. it should be in a centrally located area away from direct sources of heat and cooled air.

    I install traps on all condensate lines. There is a potential for the condensate pipe to become air locked and not drain during operation and you are also blowing conditioned air through the pipe. Some manufacturers require it and some only suggest it. Some contractors install them and some don't.

  3. #3
    Join Date
    Jul 2006
    Location
    conroe tx
    Posts
    19
    How is the air flow in the house now? How many square feet in the house? Changing to the 3 ton will add about 200 cfm to the duct system.If there are not any air flow issues now the 3 ton should go in without any problems.You don't need a register in the hall, cool air is moving down the hall to the return. Is the unit draining now? If it is don't worry about to trap.

  4. #4
    Join Date
    Jul 2006
    Location
    Columbia, SC
    Posts
    96
    House is 1500 sq. ft after an addition which made the unit too small. Typical run times are 8- 12 hours a day nonstop when it's hot. Condensate is pouring so I won't worry about that. I keep the t-stat set at 77 and the system will maintain 78-79 through the day just keeps running. Temp drop is about 20 degrees from filter to registers. The airflow is pretty good in most registers. I have 2 that have issues due to the way they were installed through the floor(who knows when). They are partially collapsed due to being so close to the floor joist. Can't really do much about it. One other thing, if I had the fan speed increased(assuming it can be) would this help the cooling?

  5. #5
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,793
    Just improve your insulation, and or infiltration and the 2.5 ton will be fine.

    Why pay for a bigger unit and pay higher electric bills when you can decrease your heat gain and loss, and save money year round.
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    How many times must one fix something before it is fixed?

  6. #6
    Join Date
    Jul 2006
    Location
    conroe tx
    Posts
    19
    How is the insulation in the ceiling & walls? 20 degrees is a good split assuming it's correct.Could the return possibly be open to unconditioned air? What tempatures were you recording?

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Size using the improvements you are making,very important or you could oversize the system.

    Airflow from the floor supply grilles should be "felt" at least six feet high off the floor.If not the temperature near the floor can be 72 when it's 78 at the stat,causing long cycles and increased energy costs.

    How far up can you feel the supply air??

  8. #8
    Join Date
    Jul 2006
    Location
    Columbia, SC
    Posts
    96
    Wall insulation is R11 as best I can tell. I added to the R19 in the attic. Used R19 batts over the top so I figure R30. House was built in 1969 so energy efficient it ain't. Nothing in the crawl space so I have to add R19 one day. Very important for winter I know. The ducts are actually in the wall about 6 in. from the floor. But they come up from the crawl space. Can't figure that one out. Some registers I can feel the air about six feet away but the registers I have direct it 3 ways so it's hard to tell. Is there anything I can measure the airflow with? The room I added, enclosed carport is always the hottest, it's also the fartherest away from the unit. One duct going to it is partially collapsed. If they would have only put it in 6in. to the left it wouldn't be that way. The other duct I added a booster too cause the airflow was marginal. The room is 12x20 so 2 6in. ducts are probably not enough but really have no way to add any. Oh today it's 99 outside and 80 inside. After being in a hot shop all day I guess I can't complain.

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