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  1. #14
    Quote Originally Posted by beenthere View Post
    I've been basing the FR on metal pipe. If he has flex its even higher yet.
    No flex, all metal! Pretty straightforward, one long main trunk with the furnace in the center, 4 runs on one side, 5 on the other, all running off the trunk sides straight out and up to the floor vents.

    Quote Originally Posted by beenthere View Post
    175 CFM in 6" metal, would be .22" FR. way too high, and too loud for resi.
    What is FR? (Just curious.) As for how loud it is, well, there are two vents that are almost immediately above the furnace -- < 10 feet. They do make some noise. The remainder of them are basically silent -- so I naturally assumed the two loud ones are just a result of being so close to the furnace.

    As far as good air mix. They didn't mislead you, its probably blowing out fast at most if not all registers. Which does give a good air mix.
    Just it can be loud, and use more electric then need be.
    Well, sort of. The 6" runs lead to floor vents that are very large -- smaller ones are 8x10, larger are 12x14. Immediately after the install, I had terrible stratification, somewhere around 5 - 8 degree difference between floor level and the 6 foot mark. (All ceilings are 9 feet.) Feet were freezing, head was warm lol. This is a 100 year old house, and the floor vents are basically built in and "framed" by the hardwood floor, so reducing the size is not an option. We blocked off roughly half the surface area of them from the underside, and it was a huge improvement, there is now about a 2 degree difference. (Is that reasonable?) They are the decorative type old Victorian grilles, so they really do nothing for the throw either. Oh -- and all returns are also at floor level, in the baseboards.

    If they were to increase the size of some of the 6" runs, thereby reducing velocity, would I likely be getting back into stratification problems? Or should they be able to overcome that? I can live with the sound, if I'm going to be more comfortable. It's still WAY quieter than my old unit!!

    Thanks again for all the help!!!

  2. #15
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    Although larger supply runs will lower velocity. It will allow more volume(CFM) to the registers. And with more volume t the registers, the throw will increase.
    You should also have balancing dampers in the supplies

    6" metal pipe is good for around 100CFM. (varies alittle by length, ells, etc)

    Good chance that many of those supplies should have been 7", and possibly 8" on some of them.
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  3. #16
    Quote Originally Posted by beenthere View Post
    A
    You should also have balancing dampers in the supplies

    6" metal pipe is good for around 100CFM. (varies alittle by length, ells, etc)

    Good chance that many of those supplies should have been 7", and possibly 8" on some of them.
    They did install dampers on every run as it leaves the trunk, so there's at least one point. :-)

    The registers all serve almost the exact same amount of space. There is a central wall that runs down the length of the house and divides each floor (~1800sf) in half. Each side is divided into four 12x15' "areas," with a register in each of those areas. (The living room comprises 3 of these "areas," etc.) The ninth run is the front entry hall. Every non-bathroom/kitchen has a return in it as well.

    In my initial skepticism of using the 6"ers, I did purchase HVAC-Calc and painstakingly enter every room, window, and wall in my house. When I used the "Size Ducts" option, and left speed of air at "900 Recommended" it gave a mix of 5" and 6" sizes. Does that make sense? Do you consider 900 FPM too fast/noisy?

    Most of my pre-install conversations with my installer revolved around equipment -- they were the only ones in the area who didn't seem intent of stopping me from getting heat pumps. They flat out told me when they didn't know the answer to some of my questions, but said they'd talk to their Trane distributor and get back to me -- and always did. They successfully hooked up the XLi's and IAQ's correctly on their first attempt, which was one of my worries as they had never installed those combos before. And the whole thing is very neat, ducts are meticulously sealed, etc, overall a great job (in my opinion.) It seems as though perhaps planning the duct work was not their strongest suit, although they did specifically mention "balancing airflow and ensuring proper static pressure," even if they didn't do it!

    Too bad you weren't designing my ducts. If you lived just a bit further North...and West...

  4. #17
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    It the time hasn't run out on it.
    Use 600FPM, and see what size it says.

    Even though those rooms are teh same size. The calc should have had different CFMs for most of the rooms due to orientation, number of outside walls, etc.
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  5. #18
    Yep, changing to 600 fpm gives me a mix of 8", 7" and 6" actually. Is 600 fpm what you would normally design to?

    The room do have varying heat loads showing, but they're not too far off from one another. All have similar exterior wall space, and all four sides of the house on the 1st floor are completely shaded (porches & other houses.)

    I'm home now, and of course now that you've brought it up, the noise is bothering me lol.

  6. #19
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    I use 500 and 600 mostly. Depending on the type of install. What I need the air to do after it leaves the register/grille.

    I very seldom use 8". Once in awhile though.

    2-6", or a 6", and 7" in most cases, instead of an 8". Again, varies with the room.
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  7. #20
    Beenthere -- your advice is much appreciated. I'll update this thread next week when I've had the installer back to take a look!

  8. #21

    Um... i may be crazy but....

    I think we are making this much more complicated than it should be. Load calc in an old house can be somewhat tricky, but pretty much straight foreward. The contractor in questions seems to be trying to make up for stratification of air by adding velocity at the outlets to get a mixing effect. Where there must be a floor return which are very inefficient.

    Also you can throw the fan curve in the garbage. Fan curves are developed in a controlled laboratory environment. They do not take into account what we call system effect, which is the effect the duct run and restrictions in that duct run have on that system. You will get more static from more airflow not less, so if the return is choked down, the result will be less amp draw on the motor. It will not turn harder to pull air through the return, it will pull harder when unlimited return is supplied to it. Cover the filter and amps will go down g'head try it.

    Also forget power factor, that has to do with the horsepower.... on a direct drive motor it is going to do what it is going to do. If it were belt drive i would then worry more about the amp draw, service factor, power factor and horsepower. But this is direct drive so dont worry about that either.

    sounds to me contractor was trying to overcome expensive problems in an old house. furnaces you put in most likely have higher blower speeds which are pushing more air, resulting in more amp draw, But at least an attempt at comfort.

  9. #22
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    Quote Originally Posted by xmatthewx View Post

    Also you can throw the fan curve in the garbage. Fan curves are developed in a controlled laboratory environment. They do not take into account what we call system effect, which is the effect the duct run and restrictions in that duct run have on that system.

    The fan curve shows you teh air flow at the ESP of the duct system. It does take into account what you call system effect.

    You will get more static from more airflow not less, so if the return is choked down, the result will be less amp draw on the motor. It will not turn harder to pull air through the return, it will pull harder when unlimited return is supplied to it. Cover the filter and amps will go down g'head try it.

    Your talking about a PSC motor. A VS motor works opposite. A VS will increase its RPM until its working against the torque for the CFM its set to. Or it reaches max RPM, which ever comes first.
    Don't confuse PSC motor operation with that of a VS.
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