Central Air: Attic Ductwork Layout for Single Zone.

[bjferri]

My other thread was: Central Air Questions for Old Home.

I am going Single Zone with unit placed in attic.

New Question:
What is the best way to run the ductwork from within the attic? I’ve heard several ways. One said ductwork around perimeter of attic then tap off that for different rooms. Another said a strait branch with shoots for different rooms off that. Are those the same?

I’d like to keep some storage in the attic if I can.

I asked if they can run from the blower outside, whatever they normally up the side of the house, through the house instead. Is this okay? I’m also asking that they put registers in the walls from closets rather than the ceiling, if possible. This all makes my head dizzy. I don't want to lose the character of the home.

I just don’t want to say to myself…I should have done it this way instead.

BTW – I’m going 16 SEER Rheem

[Dana]

A decent manual-D software package would probably tell you the answer. Doing the perimeter approach is probably less work, but may require the perimeter duct to be larger. The trunk & branch approach is what I see most often. (And mastic-sealed seams & joints make an efficiency difference, in either approach.)

But whatever you do, if the mechanicals & ducts are all in the attic, insulating & sealing the attic at the roof deck turning it into semi-conditioned space makes a large difference in operating efficiency (a bigger difference than 14 vs 16 SEER). In a vented attic, any duct leakage results in air infiltration of the worst-sort: Solar-heated attic air than's several 10s of degrees hotter than outdoor air. A sealed & insulated attic is by far the preferable approach, since all potential leakage then occurs inside the insulation & pressure envelope, and the air handler & ducts are then surrounded by room-temperature air, not solar-heat-attic temperature air, so even the conducted losses to the surrounding air go toward cooling the conditioned space, not the overheated attic. See:

http://www.buildingscience.com/docum...s-vegas-nevada

For other considerations/issues around sealed attics see:

http://www.buildingscience.com/docum...d/attachedFile

If it's just the ducts in the attic (and not the air-handler), sealing them with mastic and burying them in a foot of cellulose works too. (Cheaper than foam-sealing/insulating the roof deck.)

[bjferri]

Thanks for the technical information. I actually read most of it - understood some. I have a vented attic with an attic fan so maybe my system won't be as efficient as I thought. Something else besides the ductwork is going in the attic, the blower or condenser - I forget. I try to learn DIY when I can but HVAC is not my forte...

Nice to know the trunk/branch approach is fine.

[Dana]

Thanks for the technical information. I actually read most of it - understood some. I have a vented attic with an attic fan so maybe my system won't be as efficient as I thought. Something else besides the ductwork is going in the attic, the blower or condenser - I forget. I try to learn DIY when I can but HVAC is not my forte...

Nice to know the trunk/branch approach is fine.

This definitely calls for a sealed-insulated unvented attic then. See what it takes to spray ~R19 worth of half-pound foam (Icynene, Sealection 500, Demilec, etc ) between the rafters to seal it all up. It won't be cheap, but it'll :

A: reduce the cooling load- you may be able to drop a ton (or even two) on the compre$$or sizing and...

B: It'll allow the system to run at it's true optimal efficiency in an ~80F rather than degraded in a 120F+ environment.

There may be tax credits & other local subsidies for adding insulation as well.

The roof probably accounts for ~30%+ of your peak cooling load- cutting that 30% in half reduces the whole-house load by ~15%. Most Manual-J estimates are 25% oversized to the true load anyway, so reducing a 4 ton (derived by Manual J) to a 3 ton is usually just fine, and if you use the difference in money to reduce the load ~10-15%, you've bought some margin. And if you've boosted the operating efficiency at peak loads 25% by keeping it all inside of the thermal-insulation & pressure envelope of the house... (getting the picture?) Something like 1.5 tons less compressor + insulated/sealed attic maybe doesn't look so expensive up front after all, and the operating costs will surely be lower. Your AC contractor isn't in the biz of telling you where the cost/benefit breakdowns are- you're kinda on your own figuring it all out. Their job is to find the answer to, "can you cool this, as-is where-is?", so that's what they do.

The difference in compressor costs may be only $500-1000, but $1000 buys you ~ 250 square feet of half-pound foam installed @ R20 thickness (before tax credits or other subsidy), and typically 2-4x that where subsidized. But insulation is a lot cheaper to run than compressors, with a much longer life cycle to boot, eh? ;-) But estimating on the low side for compressor sizing ensures that it really runs at full efficiency, and it'll run longer cycles, keeping the air drier & more comfortable. Oversizing tends to make for short cycles & clammy feeling air, and higher power bills. When it doubt, drop a ton. If it doesn't keep up, lower the load by better insulating/shading/air-sealing- it'll be worth it in the long run. With heating & cooling equipment, larger than you actually need is never better- undersizing slightly is usually more efficient, and the equipment even lasts longer since it cycles less frequently.

BTW: Both the FSEC and Texas A & M did a lot of attic fan (& other cooling strategy studies) during the '80s & '90s. In most cases the power used by the fan equalled or exceeded the power seen by a central AC system to achieve the same cooling target temp without the attic fan. In many instances the attic was cooler because the attic fan was drawing cool conditioned-air up from the conditioned space (!), adding to the overall cooling load. Self-powered solar versions may buy you a handful of percent on the cooling bill if you have ultra-low infiltration between the conditioned space & attic though:

http://www.fsec.ucf.edu/en/publicati...SEC-GP-171-00/

But if you're committed to AC ducts & air handler in the attic, make it a conditioned attic- forget the whole attic fan concept entirely- insulate & seal, insulate & seal...

[bjferri]

Okay - that all makes sense but I have an attic fan and in the summer when it's not hot enough to run air, it's nice having the fan pull fresh air through the house acting like a whole house fan. Besides, I'm a fan of sprayed insulation (was actually watching a program today on it and all I thought was...What happens if you get a roof leak? How can you trace where the leak is coming from? How will you know you have one? Will the roof sheathing be rotted before you find out? All these things go through my head. I know I'm drifting from my main question but if I did proceed with spray insulation, I'd need answers to these questions... You have any smarty pants? lol

[Dana]

Okay - that all makes sense but I have an attic fan and in the summer when it's not hot enough to run air, it's nice having the fan pull fresh air through the house acting like a whole house fan. Besides, I'm a fan of sprayed insulation (was actually watching a program today on it and all I thought was...What happens if you get a roof leak? How can you trace where the leak is coming from? How will you know you have one? Will the roof sheathing be rotted before you find out? All these things go through my head. I know I'm drifting from my main question but if I did proceed with spray insulation, I'd need answers to these questions... You have any smarty pants? lol

Sure (I just make it up as I go along, can't you tell! ) I've got answers...

With half-pound foam, leaks are dead-easy to spot. It's open-cell, and you'll get a drip-drip-drip directly below the leak. The foam isn't hydrophillic- it won't soak up water like a sponge, but it lets gravity do it's thing with the water, and will dry all by itself after the leak is fixed, self-restoring to it's original insulative value.

The more rigid closed-cell 2lb foams are higher R-value per inch of depth, but it's waterproof and forms a vapor-barrier which can block both detection and drying of roof leaks. It's generally "safer" (not to mention quite a bit cheaper) to use the open cell half-pound stuff (unless you spend a lot of time thinking & designing the roof structure layup to guarantee drying paths calculating where the heating season dew-point lies, etc. etc. there's a whole boatload of discussion on this thread on another forum:

http://www.greenbuildingtalk.com/Forums/tabid/53/forumid/14/postid/55637/view/topic/Default.aspx

With all of this stuff, on some aspects it makes a difference what climate zone you're in- what's your zip code?