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  1. #1
    Sorry this is so long. I wanted to give all the info I thought might be useful, and also outline the various options I've thought of, although I'm inexperienced at HVAC. I'll probably call in a pro, but I want to have an idea of what to expect....

    I own a 3 year old, one and a half story house that has had problems heating and cooling the second story since I bought the house a year ago. Since it's currently summer here in Kansas City, MO (current temps in low 90s), I'm currently focusing on improving the cooling aspect. The second floor stays about 10 degrees warmer than the first floor, which I keep about 78 degrees.

    Since the house is one and a half stories, the second floor basically sits in the attic. The house is a basic rectangle. The stairs to the second floor are in the front middle of the house. At the top of the stairs is a bathroom at the back middle of the house. There is a bedroom to the left (east) and another bedroom to the right. Each bedroom is approx 15'x17'. The door to each bedroom is near the back of the room. Each bedroom has a single supply register at the front of the room. The bathroom also has a single supply register at the back. There are no return registers.

    The gas furnace/blower is in a utility closet in the middle of the first floor, under the second floor bathroom. It has a common return on the first floor. It is a downflow furnace to a trunk in the basement that supplies the first floor. There is an 8"x16" vertical trunk that runs up next to the furnace to supply the second floor. This vertical trunk supplies what I think is about an 8"x16" trunk under the second story. This trunk runs about 12' both directions from the intersection. At each end is 14' of 6" flex to the supply register in each bedroom. The bathroom register is also supplied with about 7' of 6" flex from this trunk. All 3 registers are 4"x10".

    I'm experimenting with one of the bedrooms before working on the other, since they're basically the same. As a first step, I tried a register booster fan from Home Depot. It sits on top of the supply register and turns on when it detects cold air flow. It hasn't helped at all.

    The next step I'm thinking of trying gets a little unusual: Behind the bedroom kneewall, I found an unused 8" flex duct that runs from the back of each bedroom (opposite of the supply register) to a vent in the ceiling of the utility closet, where the air can enter the common return. They never cut a hole in the bedroom floor to put in the register for the duct, though. I'm wondering if completing this return register in each bedroom could improve airflow on the second floor, even though each bedroom would still have only one supply register. Currently, the return air has to go through the bedroom to the door, down the stairs (3 ft wide and not open on the sides), do a 180 degree turn at the bottom, go 13' through the living room to a large open door to the dining room, and then about 5' to the common return. I'm not sure if this is too far for a good return. On the other hand, I could also turn the unused flex into another supply register for each bedroom. I could run a duct off the 8"x16" vertical duct in the utility room that already supplies the second floor. This new duct would go about 8' across the utility room ceiling to where the unused 8" flex is that goes to the back of each bedroom. In addition, I could put an 8" duct booster fan in each of the currently unused flex ducts to help supply or return air. Would any of these options be very helpful? Is one better than the other?

    My prefered solution would be to zone my current system. The second story would be a zone, another zone would be the first floor west duct from the furnace (master bedroom, master bath, guest bath), and the third zone would be the east duct (dining, kitchen, living rooms). However, I'm afraid the current duct sizing won't handle only a single zone being active. I could upsize the first floor ducts since they're in the basement. The second floor ducts are mostly inaccessible though. Even if I add two more supply registers, making a total of five second floor supply registers, would the 8"x16" supply duct be too small to handle the full airflow from the blower? I could probably increase the vertical duct size to 8"x22" and then go back to 8x16 after branching off to the duct to the two new supply registers...would that be enough? If it's close, could put bypass dampers in the basement to compensate?

    The furnace is a Lennox Merrit Series, model G40DF-36B-090. This is a 3 ton unit with 88000btu heating. Per the engineering specs from Lennox: at high speed, the blower does 1295cfm at 0.50 in.w.g. With a zoned system, I could reduce the blower speed to compensate for the smaller duct capacity of a single zone. This is done on the circuit board and can reduce the blower to 1200cfm, 1055cfm, or 900cfm (all at 0.50 in.w.g). I suspect this would really hurt efficiency, though, and could be a problem if 2 or 3 of the zones are open. Thoughts?

    Thanks,

    Josh

  2. #2
    Join Date
    Mar 2005
    Location
    burlington county n.j.
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    9,763
    if the 8" flex ducts are already drawing return air i would cut return boxes into knee wall as high as you can.
    have you tried closing downstairs registers down some to push more air to 2nd floor?

  3. #3
    Join Date
    Jul 2005
    Location
    Orange County, NY
    Posts
    936
    Josh

    The first and most important step is to perform a LOAD calculation. A complete study of the building envelope and construction is what you first need to correct any possible problems. To get this it is recommended that you call a qualified HVAC company in your area. Sure there are some basic things that can possible help like adding returns in each room, zoning, and additional supply ducts. But until you know what the Load calculations are in each room anyone is just guessing. Keep in mind that the rated CFM you see is at a predetermined ESP. Your duct system has its own operating curve, which has to be calculated, and rated to the system fan curve. Also keep in mind you have a furnace, any alterations must be inline with the units "Temperature Rise" across the exchanger.

    Your best bet is to call a contractor.

  4. #4
    Join Date
    Jul 2002
    Location
    Zelienople, Pa
    Posts
    2,965
    3 words...

    zoning, zoning, zoning...
    How tall are you Private???!!!!

  5. #5
    core - I figured I would have to call a pro eventually, especially if I want to consider retrofitting with a zone system and need all the calculations ran. At this point, I'm looking for some educated guesses for my various options. I'll probably try roughing in the unused flex as a return duct to see if it helps before even considering cutting holes for registers. If the stairway path should be providing adequate return air flow (?), then I might even try roughing in the other supply register.

    Yellow Dot - I like those 3 words as well, but it's not an option if the concealed second story ducts can't handle it.

    Thanks,

    Josh

  6. #6
    Join Date
    Jul 2002
    Location
    Zelienople, Pa
    Posts
    2,965
    Unconceal them or problem will remain with alot of spent energy trying to fix...
    How tall are you Private???!!!!

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Simple test,uf the upstairs can be cooled satisfactorily with the doors left open,It's a return duct/Path problem.

    If leaving the doors open doesn't solve the problem,adding a return won't either.

    Man J, By ACCA,to determine the supply air needed,and a way to return the air.No return,little or insuffient supply air to a given room.

  8. #8
    Join Date
    Jul 2005
    Location
    Orange County, NY
    Posts
    936
    Josh

    If installing the return duct is an easy job then yes do it. Return air, to me, is more important then supply air clearly a fan can only put out what it gets in. Also you may want to check your supply diffusers on the second floor. Cheap diffusers create excessive drops in pressure. Look for diffusers that have the greatest FREE AREA.

  9. #9
    Join Date
    Jul 2005
    Location
    Orange County, NY
    Posts
    936
    Originally posted by dash
    Simple test,uf the upstairs can be cooled satisfactorily with the doors left open,It's a return duct/Path problem.

    If leaving the doors open doesn't solve the problem,adding a return won't either.
    Dash

    I have to disagree with you here. If your Return air ducts are restricted/ or pressure drop is to high opening the doors is not the same as adding a return in each room nor will it yield the same results.

    with all respect;

    Core

  10. #10
    I've been leaving the doors open since that's the only return path.

    I think I'll try roughly hooking up the return vent first. I don't want to cut holes in the carpet and floor until I know what works. I can get to the return vent in utility room ceiling from behind the knee wall on the second floor. The new return vent would be very near the access hatch to the knee wall, so I can hook up the flex and pull it through the access door temporarily. I'll put plastic over the access door to seal it up with the flex running through it. It will probably only take about 6' of flex so I won't be out a bunch of money if it doesn't work.

    I honestly don't expect the upstairs to ever be cooled/heated very well unless I zone it, though. Per core's advice above, I'll call a pro and ask for their evaluation.

    Can anyone venture a guess if the currect ducting could handle the full blower if it's the only zone open? There's duct sizes and lengths in my original post.

  11. #11
    Join Date
    Feb 2005
    Posts
    655

    My Opinion

    Heat rises,return air is critical for second floor.

  12. #12
    Join Date
    Dec 2003
    Location
    Location!, Location!
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    929
    RETURN- You need it upstairs to ventilate properly.

  13. #13
    Join Date
    Mar 2005
    Location
    burlington county n.j.
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    9,763
    do NOT put returns in floor. you want to pull your return as high as you can on the knee wall. if you do not the cold air from floor supplies will just get sucked back thru returns

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