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  1. #14
    Join Date
    Nov 2004
    SW FL

    Thumbs up Windows Location Location Location Orientation

    Originally posted by chris7
    But then to replace it with a 2 ton which would put total tonnage on our 2200 sf house at 5.

    [Edited by chris7 on 04-13-2006 at 06:12 PM]
    And _______ ?

    That's good

    for some parts of the U.S.
    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

  2. #15
    Join Date
    Apr 2006
    Austin, TX
    Thats what i'm confused about. How would I know it's good if they have know interest in doing to load calc.

  3. #16
    Join Date
    Jun 2004
    4H: Hot, Humid Houston H.O.

    The thinking barbarian argument

    Chris, I am a homeowner in S.Texas, not so very far from your climate. Let me play devils advocate for a minute. What would be the tangible benefit of doing a Manual J? What would be the downside of using same-size equipment for your replacement?

    My thinking is with an existing house, there are things that you know now, which give you an advantage over someone equipped with just a model. You know how it feels to you now, the same-size equipment would give you similar or slightly better cooling (and presumably comfort). You know whether humidity is a concern to you, and it is not -- the biggest argument in favor of Manual J sizing is better humidity control because of long runtimes. You already plan to seal leaks in your existing ducts, which could only help efficiency and humidity control.

    I would be extremely wary of upsizing airflow without properly designing the whole system, largely because of my experience with duct issues. I suspect it is foolery to put a 2-stage bigger system on an inferior duct system "because it will mostly run on low airflow anyway". I don't think that is something you will plan to do.

    As an alternative litmus for some level of skill, ask your tech if he would perform a test for ESP (External Static Pressure). Ask him if he can measure airflow at each duct and balance the system. A too-high ESP reading would suggest an inferior duct system for your level of airflow. A lot of homes have return duct size that is less than ideal, so adding a new return perhaps in the master bedroom, might be recommended. A good tech should be able to advise.

    You could probably look up companies trained by the National Comfort Institute, surely there are a couple in your area:
    To read the website sounds like heaven, in practice some NCI trained companies are still unwilling to do the sophisticated work. Keep your fingers crossed!

    Myself, I have done HVAC-CALC Manual J calculations twice, once whole-house a couple of years ago and more recently room-by-room. Believe me, there is nothing about having your two unusual AC systems, that complicates doing a heat loss calculation on your house. Heat loss is a function of several things but has nothing to do with existing AC systems.

    Like you I find it extremely difficult to find AC techs who are able and willing to do the advanced tests we hear recommended. I have located a few who would be willing to do Manual J, for up to ten times the cost of HVAC-CALC. None are eager for the business, they don't expect you to want to pay the price. FWIW my own house is 3300 sqft, lots of single pane windows and I calculate 4.5-5.5 tons cooling with various leakage assumptions and even assuming all my trees are destroyed by the next hurricane. That is well below what is considered "normal" in my neighborhood.

    However that is before selecting proper equipment using Manual S techniques, with proper attention to high temperature derating, and to humidity removal. You cannot read too much into a tons/sqft number. The actual equipment recommendation might nudge upward some, but I already have 6.5 tons and am honestly afraid to downsize more than a little.

    FWIW my duct system was "designed" by someone who did not think things through. While both halves of the house have nearly identical heat loss per Manual J model, one half has 290 sqin of supply duct area and the other half is a separate system with 400 sqin. The result is the smaller duct system is stressed and makes excessive noise due to air speeds. Manual D recommends 600 ft/min max for flex duct, and with the full 400 cfm/ton airflow some ducts are pushing 860 ft/min. The lesson is to watch your air speed, to be extremely wary of upsizing.

    Hope this helps -- Pstu

    [Edited by Pstu on 04-14-2006 at 10:31 AM]

  4. #17
    Join Date
    Apr 2006
    Austin, TX
    Thanks for your insight Pstu. So far I like #3 best because he did address the ducts. On his quote it has -

    Replace duct system: Increase register size on 2 ducts and upsize ductwork feeding the supply registers. Add return air in upstairs room. Install new plenums with manual dampers for air balancing. All new ductswork will be R-6 insulation and sealed with inductrial grade mastic.

    He also pointed out the places where the ductwork makes turns and said every turn really restricts airflow. He said he would straighten out the ductwork since he would have the room in the attic. He said that as long as they can reach the two farthest ducts (they should be able to) we'll have all new ductwork. This guy (compared to the first two) seemed the most thorough and conscientious. I have at least one more to go and I have high hopes just because of what the companies web site says they do in evaluating a home. In the end if no one else does manual J I'll probably do it before making a final decision just so I feel confident that we are getting the right size system. Thanks again.

    (he meant that sizing of the unit, versus sizing of the ducts, was not as critical since the unit would run at 1.5 ton most of the time and then kick up to 3 when required)

    [Edited by chris7 on 04-14-2006 at 02:17 PM]

  5. #18

    Load calcs rare

    None of the 4 HVAC companies I spoke with mentioned anything about load calcs. They were content with installing something similar to prior equip.

    Supposedly, duct sizing is MOST critical for heat pumps. Ask your contractor to show you the CFM specifications for the proposed air handler(s). Then tell him your duct sizes/number of registers and at least ask him to "rough ESTIMATE" the airflow of your duct system. This comparison (along with HVAC calc & excellent advice from the pro's on HVAC-Talk) prevented me from getting 1/2 ton oversized. Oversized heat pumps/ac do NOT acheive their rated efficiencies.

    My Trane dealer says I can get a 300 dollar tax credit for 15 seer. I have an order in for XL14i (acheives 8hspf matched to my furnace), but I'm sort of wondering about XL15i 8.3 hspf. Search for "HVAC Investor" under and you can estimate payback for higher seer/hspf for your quoted systems.

    My variable speed blower is quieter and helped me get higher efficiency ratings for the heat pump.

    Based on all your posts and especially the following quote, having a new 2 ton system and then years down the road a replacement for the other system might be your most economical.

    "Well, #3 has come and gone. I will say he was much more thorough than the first two. He was the only one who really looked at the duct work and told me that it might be sufficient for the 2 ton we have but would have to be replaced for anything else b/c it's just to small".

  6. #19
    Join Date
    Apr 2006
    Austin, TX
    I agree that it would definitely be the most economical approach and if it turns out that a two ton is the right size to heat/cool the bulk of our house than we'll go for it. If the guy who's coming out Monday does not do a load calc for the house than I'll go ahead and do it myself. I just need to know what the right size is since the system with the bad compressor is our primary system. I'm looking for a whole house solution even if it costs more up front. Right now we have a two ton doing most the work with a 2.5 ton that has only two bedrooms completely dependent on it. Would rather spend the money (if necessary) and get the right size system to keep 80% of our house comfortable and then just let the 2.5 ton keep going as long as it can on the other 20%.

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