Suspend AC/Furnace from 2x4?

I have gotten several quotes on a new 3 ton Central AC/Gas Furnace. Different brands. The current unit sits on a platform in the attic that sits on a load bearing wall.
Two different installers want to hang the unit from the roof joists. They are on 24" centers, and are 2x4. One layer of asphalt shingles. The lengths of the joists vary, not knowing exactly where they want to hang it, but I'd guess 30 feet.
I'm worried I'll end up with a sagging roof after a few years. Any thoughts?
http://www.dfwblues.com Soul, R&B & Blues listing for the Metroplex
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They want to hang to sound-isolate it, a major cause of callbacks for attic-mounted systems. You must live down south, well south of snow line. Your rafters (not joists, those are the things in attic floor) would be considered marginal for a shed up north, unless they were part of a many-crossmemebered truss. Are there lots of vertical or angled 2x4s in your attic, attached to the rafters and joists with big silver nailer plates?
Anyway, to the question at hand- even in a truss roof, I would be highly hesitant to hang anything directly from the rafters, at least not without adding long crossties across several rafters to spread the load. (and if it is trusses, don't let them remove the vertical members w/o adding structure back in to make up for it.) I'd be much more inclined to built a vibration-isolating platform to place the HVAC on, over a load-bearing wall, again spreading the load over several joists. Build a cage and hang unit from giant muffler hangers, put it on a thick industrial rubber mat, find a purpose-built platform with springs, whatever they can come up that fits in the hole. Or, just level and align the damn thing properly as it is installed, and live with the minimal noise it produces (sorta like being on a ship) Did the old unit produce an unacceptable amount of noise and vibration?
aem sends....
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A furnace and coil weigh maybe 160 lb Im guessing Plus the weight of ducts so maybe 200. This is not much weight and could be spread to many beams by adding a 2x4 to spread out the load. Roofs are strong and designed for much higher loads.
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On Sun, 08 Aug 2004 22:29:57 GMT,

Yes, in Fort Worth, TX. OK, rafters. No bracing, no silver nailer plates, house was built in the 40's. Roof get's snow maybe every other year, melts in a half day usually.

The ~25 yr old unit in the attic doesn't make enough noise to bother me.
I was assuming it must be easier to hang it, since the 2 lowest bidders want to do that.
http://www.dfwblues.com Soul, R&B & Blues listing for the Metroplex
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Yeah. Have someone put a REAL roof on your house.
2X4s?!! Sheeesh!
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Hi Jack,
Though I do not have an attic mounted furnace. Some of the homes built like mine(and yours) in WI also put them in the attic. I also have 2x4 construction in the attic with 16"O.C. spacing and not enough bracing. The pain of furnaces in the attic, is access. Changing filters, turning on the power and gas for the winter season and etc... I would also guess that the elements within the attic could lead to a shorter life span than inside the home. But that is not your question... I say go for the cheapest route if the sound didn't bother you before, it sure won't bother you with the new setup. Unless you're buying a "pulse" furnace.
Luck, Brian

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Well, code says I gotta install a pulldown stair, and you can walk around in my attic, so access not so bad.

That's the odd thing. 2 lowest bidders want to hang it.
http://www.dfwblues.com Soul, R&B & Blues listing for the Metroplex
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Jack,
As long as it spreads the weight out over several studs and joined close to the peak. Then hanging it, I don't think it would matter to me as long as I can get around it when needed.
Luck, Brian
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This is Turtle.
I can't see it from here but here is what I think.
If you had 2" X 6" or 2" X 8" rafters I would say yea go with it.
Now you say you have 2" X 4" rafters and hanging 200 pounds from it which will weight point at point of attachment. Well I really question the weight being put on these rafters being 2 X 4 size. You will just not have a weight deviding load by hanging it and not just setting flate over a larger area.
I do this kind of work but it just seems to be pushing it to load the 2 X 4 up like that. I would have to say no. It maybe alright but you will still be pushing the limits of the rafters.
TURTLE
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Yea, don't hang from the 2x4s.
~kjpro~
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Jack wrote:

You might check with an engineer or architect (I have friends who are architects which has been handy for "is this a totally stupid idea?" feedback.
2x4s. Huh.
Can you run a couple 2x8's into there easily? Anchor them at the floor and at the peak and afix them to the 2x4s?
You have to not get snow. 2x4's on 24" seems really lightweight.
Low bids aren't always teh best (or the lowest after you fix it). Contractor I know often bids knowing it's high but explains that, say, "I'm going to have to reinforce the rafters so we can hang it." for instance.
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Thanks almost everyone for the responses. The AC will not be hanging from 2x4. It will sit on top of a wall.
http://www.dfwblues.com Soul, R&B & Blues listing for the Metroplex
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wrote:

This is Turtle.
Good thinking.
TURTLE
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I'm guessing when he says he has 2 x 4 rafters, he is refering to engineered roof trusses. Sometimes with shorter spans, trusses are made with 2 x 4 lumber with a "web" of lumber to reinforce the truss. I'm also wondering if there is enough free space to crawl up in there to put the ducts in and access the furnace
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