Best Place For A/C Air Handler?
We’re building a house in northern NJ and we’re faced with loads of confusing options. One is about the best place to put the A/C air handlers (heat will be hydronic baseboard and radiant under tile). There will be two zones, one upstairs and one downstairs. Some contractors bidding the job propose putting everything in the attic. They say cold air falls so it’s better to start in the attic. One proposed putting the upstairs unit in the attic and the downstairs unit in the basement. The claim is that there will be less cutting of framing to run duct work this way. Everyone assumes they will install flexible duct work with R6 insulation.
I think it’s a bad idea to put the unit in the attic since it’s the hottest place in the house during the summer. I also heard that rigid duct work is better.
What’s the real scoop on this? Where is the best place for the air handlers and why is rigid duct work considered better?
Replies
alot like it in the attic to save space but you have to cool 130 degeree air. closet take space and noise. fiberduct are quiet but fibers come loose and hard to clean. I have metal round duct but they are noisey but stay clean eailer
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Haga su trabajo de fricken
So do you recommend basement? What's your opinion on the claim "you'll cut less framing if the unit is in the attic?"
Thanks
I like to keep all duct work and air handlers out of unconditioned spaces like the attic. Crawl space is much cooler and if you use a sealed crawl system it qualifies as conditioned space. On slab houses we do put them in closet-like spaces in the walls with the return straight into the base of the unit. It's noisier, but we use the variable speed units and insulate the heck out of the homes so it's not so bad.
I have no problem with R-6 flex but prefer the foil-flex and be sure to have them use duct mastic (duck-butter) on all joints. We generally run our distribution lines in large diameter metal duct and insulate the exterior.
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"You cannot work hard enough to make up for a sloppy estimate."
Thanks, Shelter. I'll be sure to spec mastic.
both.
one in the attic and one in the basement.
but your attic needs to be conditioned space.
carpenter in transition
Ok, thanks. What's your recommendation for the best way to condition the attic? I've seen some builders use spray foam. By the way, if the attic is conditioned, what do you do about venting?
By the way, if the attic is conditioned, what do you do about venting?
You don't. It won't need it. The same way the spare room (sewing room, guest room, whatever) doesn't need venting to the outdoors, either.
There's some really good physics in not heating up all the roof and upper ceiling framing to summertime temperatures, too. Mostly, it's not trying to push all that expensively 20 deltaT-ed air through R6 insulated ductwork every time the stat cycles. (Here in Texas, it really would be cheaper to run the ducts on the outside of the house instead of the attic, they'd be in 25-35º cooler air.)
For a/c, really, you want the registers high, so the cooler air has to move down to get to the floor (or whatever lowest level it can get to). Realistically, you need return air ducting to bring air back to the handler (even if the hvac dude forgets to think about that). So, you wind up with ducting high and low. (Or you can have doors with really big--1 inch--gaps under them, and a dustfall from under that ceiling mounted return . . . )
So, the scheme would be to take the insulation plane, start it in the basement, carry it right up the walls, then into the roof framing to fully enclose your house. About the only improvement on that is to divorce the insulation from the framing entire--but, that's getting into PERSIST, and that's often just too far "out there" to pass AHJ approval (or contractor "buy in" for that matter).Occupational hazard of my occupation not being around (sorry Bubba)
What's your preferred way to insulate the attic? Is it spray foam or something else?
preferred way to insulate the attic? Is it spray foam or something else?
Depends on the project, and the AHJ.
Spray foam over roof deck and rafters seems to "saam" down to the wall insualtion plane best, as there is less fussiness with differing overhangs--at least in existing construction.
Unless the local AHJ then insists that any foam has to have a non-flammable surface over it. Then, rigid sheets between rafters, foamed to seal the edges, and a layer over the rafters with whatever the minimum thickness gypsum panel the AHJ will green tag you for.
For new construction, you need a "sit down" with the insulators and the framers. Isocyanurate sheets over the roof deck are pretty quick, but the trick of that is managing how the wall insulation plane melds with the roof while also "playing pretty" with overhangs & similar roof framing. Which means, spray foam can just be easier (and maybe an entire page of details easier in the plan set).
The ones I've had a oar in lately have all been retro fit work, which pretty much mandates using spray foam if only to control labor costs (and even a stingy person executing their own sweat equity will start recalculating that labor rate the eleventyth time they have to un-wedge themselves from that darn skinny space betwixt rafter and ceiling joist to get right out to the exterior wall in the very fiddly space behind the roof overhangs--that same 5% space that is never quite insulated right . . . <sigh>)Occupational hazard of my occupation not being around (sorry Bubba)
Any unconditioned space is a very bad location. You'll have higher operating costs forever.
Since you are building the house there should be a place designed into it for the services. Any design that doesn't is poor. The best thing is to keep designing until all your needs are met, that includes not only your living and social needs, but functions of the building such as HVAC, plumbing, wiring, light, etc.
Remember that units come in many configurations and that vertical ones are available that don't take up much more space than a closet.
Rigid ductwork is better because the joints can be better sealed, because there is less flow restriction on the smooth interior, and because the plastics of flex ducts become brittle over time and eventually crumble. I suspect that the contractors are pushing flex duct because they aren't very skilled in rigid duct, or because they think they can get the job by cheapening the estimate, or because the design of the house is lacking and there is no good place to route proper ventilation ductwork. Again, this last item comes down to proper design of the home. The design should include proper location of the building services.
Don't settle on a final design until everything about the house works.
Thanks for the explanation of the advantages of rigid duct. Makes sense. I think the HVAC contractor is just trying to spec whatever gets him done fastest. The builder said rigid duct would add significant cost.
Maybe I jumped the gun assuming that the entire attic has to be conditioned. Do you recommend some sort of insulated closet in the attic to house the unit? Where does that leave the ductwork? R6 isn't all that much...
Well, an insulated space in the attic is better than nothing, but what the other poster already said, your ductwork will still be subject to considerable loss.
The best way really is to have the entire system inside the conditioned space of the building. If it were me, I would not settle on a design that did not allow it. To me building a house without a place for the HVAC would be similar to saying that an indoor bathroom wasn't really necessary because you could use an outhouse and save money.
WayneL, I'm a little confused. I understand you're saying to put the unit and duct work in the conditioned space, but which conditioned space? Insulate the entire attic and put it there, insulate the basement and put it there, or create some other space in the house? In a 2 story house with basement, where specifically do you recommend putting the units? To me, the basment makes sense, but the duct runs to the upstairs will be pretty long...
Generally a basement is in the conditioned space. An attic generally isn't, but with a "hot roof" it can be, or one can enclose the part of the attic surrounding the equipment.
If your view never changes you're following the wrong leader
Thanks, Dan. I'm leaning more and more to insisting the units go in the basement. I'll also speak to the builder about the hot roof. My understanding is that a hot roof is built by spraying insulating foam directly to the underside of the roof sheathing and there is no venting at all. Is this accurate?
That's one way to do it. Another is to apply rigid foam on top of one layer of decking, then another layer of decking on top. There are also modified versions you might call "warm" that involve essentially a double roof.
If your view never changes you're following the wrong leader
Ok, Dan, thanks for the input. If we go the hot roof route, I'll leave the implementation details to the pros, but at least now I can have a more intelligent conversation with him...
Put the first floor unit in the basement and the second floor in the attic, you want to keep the runs as short as possible. Why run all that trunk when you don't need to. This would be for a typical colonial style house, yours might be very different and running the basement trunk might be difficult, it depends on the living space, etc. in the basement.Do a search on this site for unvented attics or roofs and for two spray foam companies, icynene and corbond. Those are the two foams I've used most, icynene is open cell and corbond is closed. You do not need to vent with the foam but make sure your building inspector is ok with this, some are still in the dark ages and don't allow it. Also, you need to talk to your builder about the shingles you'll be using, only certainteed and elk accept an unvented roof with spray foam. The foam companies will have additional information as well as warranty papers for the shingle brands.
You said, "Why run all that trunk when you don't need to." He needs to. The reason to run the duct is that the alternative is to put the unit in the attic, which is the worst place it could be put. There's no problem running duct from the basement. It should be run in the area of the house designed for it.
"The reason to run the duct is that the alternative is to put the unit in the attic, which is the worst place it could be put."spray foam the attic and its a great place for it. Conditioned space and short runs. Also, no need for a chase for the trunk, so you keep the space rather than losing it.
Edited 9/22/2007 10:57 pm ET by DDay
Ductwork run from a basement to the second floor can be worse than putting it in the attic. Lots of extra metal, fittings, extra unsealed joints, additional design compromises...
Things like 6" oval duct only flows the equivalent of 5" round pipe.
The best alternative is to put the actual unit in a utility closet on the second floor with just the duct in the attic.... but no one wants to give up the floor space.
Putting the HVAC in the basement is the standard way. The duct runs, if properly sized, will not be too long.
It doesnt' make sense to insulate the attic just for a place to put the HVAC unit. But basements nowadays are nearly always insulated and conditioned anyway as inexpensive living space, even if the rooms are not finished off.
It's best to have the air handler and all ductwork inside the conditioned space. If it's not in the conditioned space then it's extremely important for all the ductwork that's outside the conditioned space to be very tightly sealed; otherwise you can end up trying to air condition the outdoors.
As other have pointed out the units should be inside the conditioned space. DanH said it best, without going to a "hot roof", the basement is the location of choice.
Rigid metal duct is more expensive in material cost and installation cost than flex. Move up to a heavier gauge metal duct and you reduce the noise factor significantly. Contractors like flex because it is cheap and easy to install, thus reducing thier labor cost. I have never seen a good flex duct distribution system in residential work and very seldom in commercial work. Flex is fine for short runs from a main or branch trunk of metal duct to a supply or return register. Less than six feet of tigthly stretched flex is a goal I shoot for.
If you are in the design stage, take your plans to a good HVAC contractor before you start framing. Pay them, if neccessary, to draw in you your distribution system. Include duct sizes and exact locations of everything they are proposing. Give this addition page or pages to the framer and let him frame around the system design. Beats the hell out of having the HVAC contractor hack up your framers work becuase his work was an after thought in the design.
Dave
One significant advantage of an attic installation is that a main trunk of rigid can be easily run the length of the house and then short flexes run to the individual registers. It's cheap, labor-wise, and produces fairly good distribution without taking up space (especially headroom) in the liveable quarters. Interior distribution requires actual design work, involving the archy.But the attic install is a real energy pig unless done with considerable attention to detail.
If your view never changes you're following the wrong leader
Dave your suggestion to have the HVAC guy draw up plans BEFORE framing is right on target. I'll speak with everyone and get that moving.Thanks, everyone, for all your great advice!
Anyone care to see the bills from an attic installed unit that had a 2nd drain pan under the FCU and installed in the 2nd pan was a float switch that failed.
Don't preach about PM. They are done every year at the start of the season. Looks like the stars lined themselves up just right.
What a freakin mess... As for the house keeper, I swear see can't see above eye level. She had the gall to tell me she just left the room. Clearly didn't see the plaster ceiling hanging 10" lower than the rest of the space.
It hit the floor and went SPLAT!! Just like poop. :-(
Duct work would be cheap on most homes.
Preferred place for equipment and ductwork is within easily accessible conditioned space.
Metal duct is better because it is much less restrictive than flex duct. Downside is the joints need to be sealed and it needs to be insulated.
Sealing can be done with mastic or butyl tape. Butyl tape is ready for use immediately, mastic should be allowed to dry before insulating the duct. Butyl tape will save time compared to mastic. UL listed aluminum foil tape can also be used, but butyl sticks *way* better. Water based mastic isn't as "stiff" as solvent based, and water based stuff takes longer to dry. Fumes from solvent based stuff will knock you out, though....
Use proper FSK (foil, scrim, kraft) tape to attach insulation... do *NOT* use duct tape.
In fact, do *NOT* use duct tape *anywhere* on ductwork. It is worthless.
*ALL* ductwork must be sealed and insulated.
It is best to build an insulated room for the air handling equipment to minimize temperature extremes.
Use a media filter with a gasketed door to minimize the chance of hot attic air being sucked into the system past the filter- that kills efficiency really quick.
Poorly designed fittings at the air handler will also kill efficiency.
Flex runs (the inner liner) must be sealed with mastic to the metal start collar before attaching the nylon strap. That means the mastic is brushed onto the metal collar before the flex liner is installed and fastened.
Put a nice, big *metal* waterproof drip pan under the equipment, being sure the pan is under anything that can generate water. Do not put a hole for a drain in the pan. Put a float safety switch in there, wired to interrupt the 24v from the stat- this will disable the equipment during heating or cooling season if there is water in it- forcing you to take a look when the HVAC equipment isn't working. A 32" x 91" x 2" pan will hold *plenty* of water (about 25 gallons).
Did I mention that the ductwork must be sealed and insulated?
Radiant barriers seem to work well for duct wrap.
Sometimes attic installs can't be avoided. There is nothing wrong with them if there is attention to detail during the installation process.
Sealing your home with foam will create the need for forced mechanical ventilation (ERV).
Edited 9/20/2007 11:36 pm ET by danski0224
Thanks for your comprehensive post. This one goes straight to the HVAC guy...