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  1. #1

    Question Balancing or Hard Pipe Duct?

    Hi,

    I am in the process of getting bids on a new HVAC system in my home. I've gotten bids from four contractors and different answers from all. In addition to getting a newer, more efficient system, I am interested in better cooling my living room which is an addition. The original house is 1600 sq feet and there is 300 sq ft addition that was built on. It has three external walls, two of which are mostly windows each with a glass door. There is no attic on this room. THere is one supply register at the end of the room (the end that opens to the rest of the houe).

    I live in Austin, TX where it is very hot, a lot (especially this summer). I've asked the contractors providing me the new HVAC bids how we can better cool this room (in the afternoon it is about 8 degrees warmer than where the thermostat is). Two of the contractors had interesting ideas that were somewhat in conflict (and the other two said there was no solution).

    One contractor recommended changing the flex duct that goes to the addition to hard pipe duct. He said that this would enable more cool air to pump into the room because of less restriction on the air flow. This made sense to me. He said that we could push about 40% more CFM with the hard pipe. (By the way, expanding the existing duct is not really an option due to there being a very small space in the attic where the addition connects to the original house).

    Another contractor recommends installing balancing where each duct from the plenum would have a damper (that's my term, not his) installed so that the air flow could be controlled, thus pushing more of it to the duct that supplys the hot room. This same contractor said that changing the flex duct to hard pipe would not provide any significant increased cooling for the room (although 40% CFM seemed significant to me).

    One more piece of info, I've tried to push more cool air to the hot room myself by closing or partially closing various supply registers in rooms that are cool enough or not frequently used. I've heard that this could be a bad idea. How is this any different than putting dampers on the ducts and controlling the airflow this way?

    So, in conclusion, my questions are:

    1) Can hard pipe really increase CFM 40% over flex duct? The duct is about 15 feet long. And, would this be significant enough to better cool a room?

    2) Using the theory that it is not good for your system if you try to balance it by closing supply registers, why would it be okay to do effectively the same thing by balancing via dampers installed in the ducts in the attic?

    3) What questions should I be asking my prospecitve contractors?

    THanks!

  2. #2
    Join Date
    Feb 2004
    Location
    New Mexico
    Posts
    5,473
    [QUOTE=sehoop27;4043862]Hi,

    So, in conclusion, my questions are:

    1) Can hard pipe really increase CFM 40% over flex duct? The duct is about 15 feet long. And, would this be significant enough to better cool a room?


    CFM's can be increased with hard duct but how much depends on how the flex was installed. If it looks like a snake it should be stretched out to correct this and stretching reduces the dips so the air path is less like a bumpy road. Generally a space with a lot of glass I install supplies on all walls with glass as glass is probably where most of your load comes from. Window treatment can help.
    As far as the room goes, when a space load is very different than the rest of a home the solution is often adding another unit just for the space. As long as the load is being controlled from another area the equipment probably won't cycle enough to solve your problem.

    2) Using the theory that it is not good for your system if you try to balance it by closing supply registers, why would it be okay to do effectively the same thing by balancing via dampers installed in the ducts in the attic?


    I don't under stand what is meant by good for a system. Balancing at the registers is often avoided because of increased noise. Dampers farther up stream aren't noisy.
    Balancing might provide more air to your problem room but still not solve your problem as the main house is controlling cycling. The room has different load requirements. Sometimes a constant fan operation helps a little. While units called mini splits might look a little like a motel they do solve problems like yours nicely.

    3) What questions should I be asking my prospecitve contractors?

    It sounds like your asking and understanding your questions.
    Just when a statement is made "not good for a system" ask why. See if the answer makes sense.
    The contractor that said changing to hard duct won't help might have been because of the load variations.
    Tracers work both ways.

  3. #3
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
    Posts
    629
    In order to push the extra 40% down the new ducting, you'd have to significantly increase the pressure in the supply ducting. This in turn could cause a chain reaction with your current system.

    As stated by hvacker: the heat load of this room would still not be met. Try a minisplit. They are made for applications like this.

  4. #4
    Thanks for the replies. I think I will look into the minisplit. Any recommendations regarding minisplits? I can always ask the contractors I've gotten quotes from, but they don't really seem to be in the business of installing minisplits since only one guy mentioned it (and he didn't seem real interested in doing it).

    Just clarify (sord of) my previous statement about closing the supply registers not 'being good for the system'....I can't repeat exactly what was explained to me, but I think it had to do with air getting pushed back to the coil and maybe the coil freezing over??

  5. #5
    Join Date
    May 2009
    Posts
    196
    My guess is that you'd need a new vent on the far (outer) wall of the new room to see much of an improvement. Installing sheet metal ducting probably won't help much. Installing dampers to allow for balancing may or may not work depending on room heat gain and available static pressure from your blower.

    Closing off air vents to divert air flow is no more damaging to a blower than closing off dampers. You just have to know what you're doing to prevent starving your coil of air or overloading your blower.


    If there's any reasonable way to run a new duct to supply a new vent in your addition then by all means do it. Otherwise, you're probably best served by a minisplit.

    Daikin's Quaternity is the most sophisticated minisplit on the market at present. Also look at Mitsubishi's minisplits but beware that capacity figures stated in their marketing literature for some units are considerably higher than actual capacity measured by the ARI. Both Daikin and Mitsubishi have dealer finders on their respective websites.

  6. #6
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
    Posts
    629
    I think Cold Feet answered your follow up well...

    One other thing to look at is if there is the possibility of adding an external chase to run an extra duct through. I don't know if it would be an eye sore or not but may worth looking into.

  7. #7
    Thanks again for all of the advice! A lot to ponder and to talk to my prospective contractors about.

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