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  1. #1

    Attic furnace install: Floor mount or suspend from rafters?

    I am considering bids from three contractors regarding complete replacement of my A/C and heating in Houston, TX townhouse. The inside equipment will be in the attic, which is typical for this area. The original equipment installed when the home was built was supported on the attic floor.

    Two of the contractors propose installing the new equipment on the floor like the original system was installed.

    One proposes suspending the new furnace from the rafters.

    Both camps claim that their installation method will reduce noise into the house. All three offer a Lennox G60V (this is Houston, remember--not so cold in winter, so the G61V is not really worth it), which is supposed to be about as quiet a blower as you can get. Since even my current 25-yr-old Magic Chef isn't audible in the living space of my house, I'm really not too concerned about noise coming through the floor.

    Unless I've missed a thread, I've gathered that as long as the install is done competently, both methods can be acceptable. Do I have that right?

    I'm a little skeptical of the roof mount. I don't know how to judge the integrity of the truss system, and don't know if it could be compromised by hanging a few hundred pounds off it. Since I haven't had a rafter-mounted system before, I don't know if that might introduce sound to the house where there wasn't any before.

    The floor-mount guys also say that rafter suspensions introduce the risk of metal fatigue at the gas line connection, if the furnace vibrates or sways. Is that a real concern, or just scare tactics?

    Thanks,
    Jim

  2. #2
    Join Date
    Nov 2007
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    N.C.
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    170
    I'm in N.C. and most of the furnaces that I install, they mount on the floor on blocks vibration pads b/w the blocks and the furnace. Also they could brace it to the rafters so that would keep it from vibrating when it's running.

  3. #3
    Join Date
    Mar 2005
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    burlington county n.j.
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    we do them on platforms all the time, last one we did the customer wanted it suspended. construction official said if we wanted to hang it we would need an engineers report that the roof rafters would hold it............
    that one went on a platform to..

    g60v furnace is so quiet you will never hear it, especially if you never heard the old one. we use rubber pads between unit and supports and have never had a complaint.

  4. #4
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Funny how things vary around the country.

    Here in Florida ,code requires them to be hung from the top chord/rafter of the truss,to floor mount you need engineered design,as they say the bottom of the truss is only designed to hold the weight of the drywall.

  5. #5
    Join Date
    Mar 2005
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    burlington county n.j.
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    Quote Originally Posted by dash View Post
    Funny how things vary around the country.

    Here in Florida ,code requires them to be hung from the top chord/rafter of the truss,to floor mount you need engineered design,as they say the bottom of the truss is only designed to hold the weight of the drywall.


    most of what we do attic systems in are older stick built houses, last truss house we did was new construction and builder beefed things up for our platform.

  6. #6
    Join Date
    Dec 2007
    Location
    Frognot TX
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    836
    It really doesn't make alot of differance, when they first came out with inshot burners they would get rather noisy if installed on the deck, but they're alot quieter now.

    If there is concern about whether the rafters or deck will hold the weight of the unit,you have alot more problems than deciding on how to mount it.

  7. #7
    Join Date
    Jul 2004
    Location
    Massachusetts
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    We generally install on a platform. We're in snow country so nobody really cares which way it's done as both the ceiling joists and roof rafters are plenty strong to suppor the weight of a furnace and we do find them suspsended. But out issue is mainly the overflow drip pan. If the unit is mounted in the pan and raised up on blocks or other non-conbustible materials, you know the pan is not going to move and be misaligned with the furnace or AC coil. When they're suspended, we usually find the pan installed on the collar ties, under the unit and subject to being knocked out of position. I did see one recently in a commercial applications where the was not attic above and the installers had suspended the furnace and left the threaded rod long enough to include a second layer of Unitstrut beneath the first holdling the overflow pan. That was a good solution but they never put anything under the drip pan to provide support. So if the pan had ever filled with water it would have creased in the unsupported area. We replaced the furnace, used the same arrangement but put a 1/2-inch plywood sheet under the drip pan.

    I'd got with a Goliath drip pan mounted on the floor personally. But honestly, there are cable hangers that will not transmit any noise. Gas piping, well that's another issue entirely. I've never seen gas piping suffer from a unit being suspended but there's always a first time, eh?
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  8. #8
    Join Date
    Feb 2003
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    Huntsville,AL
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    4,125
    MC-- what is the closest city, in which state?

    hard for us to judge your present rafter or truss loading from here!

    but, one can always beef- up either framing member & 'spread' the load.
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  9. #9
    Join Date
    Jun 2005
    Location
    Near Chicago, IL
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    3,317
    Quote Originally Posted by MysticCobra View Post
    One proposes suspending the new furnace from the rafters.
    Roof truss systems are usually speced for the minimum weight and wind load requirements needed. Drywall, OSB sheathing and asphalt shingles weigh a lot.

    That means there isn't much extra capacity for the weight of an HVAC system, especially if there are only 3 pairs of anchor points distributing the weight (two on the air handler, one on the cased coil).

    The weight is distributed better if a length of Unistrut spans four or five trusses, and the equipment is hung off of that... but that costs more.

    If the original location worked, then that's where I would insist on having it installed.

    A plywood/extruded foam/plywood sandwich does wonders for noise isolation.

    Make sure the pan under the equipment is large enough to do the job.
    Proper Prior Planning Prevents Piss Poor Performance

    "There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper, and the people who consider price only are this man's lawful prey. It's unwise to pay too little.
    When you pay too much, you lose a little money -- that is all. When you pay too little, you may lose everything, because the thing you bought was incapable of doing the thing it was bought to do.

    The common law of business balance prohibits paying a little and getting a lot -- it can't be done. If you deal with the lowest bidder, it is well to add something for the risk you run. And if you do that, you will have enough to pay for something better."

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  10. #10
    Quote Originally Posted by skippedover View Post
    We generally install on a platform. ... But our issue is mainly the overflow drip pan. If the unit is mounted in the pan and raised up on blocks or other non-combustible materials, you know the pan is not going to move and be misaligned with the furnace or AC coil. When they're suspended, we usually find the pan installed on the collar ties, under the unit and subject to being knocked out of position.
    That's an interesting consideration. In the installs I've looked at here in Houston, I've never seen a unit "mounted in the pan". Even for floor-mounted furnaces, the evap coil is usually strapped to a roof member for support (so there's no bottom support to interfere with the drip pan), and the drip pan is arranged on the floor underneath with a tilt to encourage flow to the overflow drain.

    I've never given a drip pan a "tug test" to see how much it could move. Now that you mention it, I suspect a good swift kick to the overflow drain would send most of those drip pans flying. I guess that's one reason why I generally watch where I step around an A/C!

  11. #11
    Quote Originally Posted by cem-bsee View Post
    MC-- what is the closest city, in which state?
    From my original post: "...in Houston, TX townhouse. ... (this is Houston, remember--not so cold in winter, so the G61V is not really worth it)".

    Quote Originally Posted by cem-bsee View Post
    hard for us to judge your present rafter or truss loading from here!
    I understand--as I mentioned in my first post, I'm right here and it's hard for *me* to judge my rafter / truss loading. But I didn't really ask for an evaluation of my specific situation; my questions were very general:
    I've gathered that as long as the install is done competently, both methods [floor mount or rafter suspension] can be acceptable. Do I have that right?
    ...
    The floor-mount guys also say that rafter suspensions introduce the risk of metal fatigue at the gas line connection, if the furnace vibrates or sways. Is that a real concern, or just scare tactics?
    I've posted several questions on this forum now, and gotten some fantastic help from many of the professional members. Many thanks to all who have replied--this forum has been a fantastic source of information, and I really appreciate it!


    Jim

  12. #12
    Join Date
    Sep 2003
    Location
    McKinney, TX
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    470
    In dallas they have to be hung, and I would suspect in houston it is the same. Whenever I see one sitting on stands in a attic it is usually a very old installation. Having it hung is usually quieter. I guess if I was a installer I would rather mount it on the floor using stands because it is easier. Have it hung man.

  13. #13
    Join Date
    May 2002
    Location
    Houston Texas
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    6,326
    In Houston they do not need to be hung and either process is acceptable however many Townhomes have 2"X4" rafters ot 2"X6" so flor mount is acceptable.

    I also now know who one of your bids is from and if I am correct they have dropped in quality of work over the last few years.

    My furnace is hung and I occasionally hang furnaces but it genrerally is not the best option or worth the extra expense or effort.

    Most furnaces today make more noise than the old Magic Chef unit you currently have because they have so much more going on. The main reason for hanging the furnace is to eliminate vibration from transfering to the ceiling joist. As long as the blower wheel stays clean and doesn't through a blance weight that should not be a problem.

    As to another statement made in another of your post flex duct in Houston must be supported every four feet to pass inspection.

    As far as a potential problem with the gas line that is just bunk!

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