I am planning to build a new home in the next 6-12 months in Texas, but have have been in the thinking mode for quite some time. I am wanting to put he AHUs (2-zones) in the attic, but I am not too sold on the returned air located in the ceiling just below the AHU. This is somewhat unsightly, but not too sure what else to do. I have thought about some sort of wall chase R/A ,but do not know what to do with the top plate of the stud wall. Just thinking out loud here. Any advise will be greatly appreciated. WW—57
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Please do yourself a favor and make the attic conditioned space.
I am a believer in a return in every major room. And up high for AC so that it can suck the hot air out of the room. But I really don't have any experience in your area.
As to your question I don't know why you can't do the same thing as is commonly done where the duck work runs under the floor.
Run a return duck and where need branch off and run the the stud bay and cut the bottom plate (top) and use the duct (or less idea pan it) up (down) to the inlet vent.
William the Geezer, the sequel to Billy the Kid - Shoe
We are getting away from the attic AHU in Florida. You just can't keep a reasonable efficiency.
Build a closet for it in conditioned space.
I have to agree with greftwell.
Put the unit(s) in a closet where you can get to them. This keeps them in an already conditioned space and makes servicing them them a lot easier. You will still run your duct work through the attic, but if installed and insulated correctly it will never need any service work.
Putting the unit in an unconditioned space will decrease its' efficiencies. Conditioning the space for the unit will add cost to your construction that will add nothing to the occupancy comfort.
Routine maintenance on a attic install is a PITA for the tech, so the service call price reflects it. It is also such a PITA for the HO that even simple filter changes get neglected, leading to those expensive service calls.
You're in Texas man. Do you really want to go into an attic that may reach 120 to 130 degrees to change a filter, much less work up there for an hour or so?
Find a good HVAC contractor and include them in your design phase. They can help you so much at that stage that you will save a ton of $$ on the installation and maintenance cost. Most people find a design they like and then have the mechanical contractors make all their stuff fit into that design. That leads to halfazz solutions to problems that didn't need to exist in the first place, adds cost to the installation, and compromises the overall performance of the system.
Good points. Put another way ... Would you go into that hot attic? Would you want to? Would you want your HVAC equipment to be there ALL THE TIME. If you REALLY care about your HVAC ... reallys this is self serving ... you would give him a nice place to efficiently and comfortably work his whole life to providing you comfort. Need to have a mind shift.
If you put it in the attic consider the 'new' approach like the other poster indicated. Make the attic part of the thermal envelope. Put return ducts where they need to go. Avoid flex duct as much as possible!! Good supply AND return make for good comfort and efficiency. There is a PIER study on doing this (enclosing the attic in the envelope).
The alternative is to consume some floor space and put it in it's own room. Better access for maintenance. Costs you floor space, though. But enclosing the attic also will cost.
>>If you put it in the attic consider the 'new' approach like the other poster indicated. Make the attic part of the thermal envelope. Put return ducts where they need to go. Avoid flex duct as much as possible!! Good supply AND return make for good comfort and efficiency. There is a PIER study on doing this (enclosing the attic in the envelope).
Good points, but I wonder at the cost return on investment.
I know all the arguements for hot roofs and putting the HVAC system in a conditioned space. I'm just tight azzed enough to want those $$ spent for my comfort, not just a piece of equipment. Why condition over a thousand cubic feet of space in the attic when you can add less than 100 sq.ft. to the living space for less than $1000 ?
I had an archy and a energy consultant working on my plans at the same time. I then had my HVAC contractor go over the plans and see if there were any barriers that would screw up his best possible install. We beat that horse to death, but I think we all benefited from the experience and I got a better house out of it.
I hear people pound on the HVAC companies because thier systems don't perform as well as expected. Many times it is warrented because of poor installation. Other times it is because the installation methods used were the only option left to the them.
How many times have you seen a two story home with under sized supplies and returns on the second floor, and the system in a basement or first floor closet? Betcha there was no chase for either planned for on the print. The HVAC contractor has to cram his stuff in whatever space is left or he anyplace he can find. To many turns, to small ducts, and everthing leaks cause he couldn't seal it up. All that stuff is additive when it come to performance.
That is why I now encourage anyone thinking of building to bring an HVAC consultant while they are still in the design phase. Energy efficient homes don't just happen,..... they are well planned!
Builders do things like this because they are hitting a price point and will not be paying the electric bills. Is this your house?
Yes.
We did the design process about 14 years ago. Based on $0.06/kwh back then and the passive solar/geothermal design our annualized heat/cool cost was estimated to be around $500./yr., for a 2400sf ranch with a 2200 sf walk out basement.
This was going to be our retirement home, so was an "empty nester" design. However the "nest" got refilled and the retirement option got pushed way out. Cost went nuts since the original design, so I'm making some modification for cost control as I move along building it. I don't expect to hit the estimated heat/cool figure with the changes I am making, but I should still be way under the norm for our area.
I'm not a true HVAC guy. I've just done a ton of work on commercial systems and been through several courses. I also have a few mechanical engineers here at work that we consult with when neccessary.
I went through eight HVAC contractors before I found one that actually did a load calculation that agreed with the original system recommendations. When I went out and looked at his installations, they were top shelf jobs. After that we sat down together and sketched in his proposed duct work design and I had the floor trusses designed with chases to accomodate it.
The up front design cost and consultations may have added a couple of thousand dollars to the cost, but I'll recover that over the long term with a more efficient system.
I would like to respectfully suggest that a 14 YO design is dated. We (the building industry) has learned a lot in the last 14 years.
We (the building industry) has learned a lot in the last 14 years.
So have I.
Materials and equipment have gotten better I'll agree.
Craftmanship and workmanship are timeless.
I've been here for almost that long.
I aint got my head in the sand.
"Craftmanship and workmanship are timeless." Like common sense?
Good HVAC design is critical.
Although few jurisdictions actually enforce it, the IRC now requires load calcs and duct design to be done according to ACCA manuals (Manual J, 8th edition for loads; Manual D for ducts). Manual J, 8th edition, has you figure room-by-room loads and airflow requirements. This is far superior to to simple "block load" calcs.
I have reviewed many, many hundreds of such designs in my job as mechanical plans reviewer for my jurisdiction, and I can tell you that, even with top-of-the-line computer programs (Wrightsoft, Elite, etc.), very few HVAC contractors will come up with an accurate load calc, or an adequate duct design.
Even the most efficient, properly-sized equipment will be defeated by poor duct design and unbalanced system airflow. I see this as the biggest single problem in the profession.
Most HVAC contractors use a slide-rule type duct calculator to size ducts, and that's good--IF they work from an accurate friction rate. But friction rate is a result of a couple of separate calculations that most contractors have never learned to calculate--or never bothered to calculate. Instead, they pick a number out of the air, like 0.08 or 0.1 and apply that to every duct run. Big mistake.
I know this post paints HVAC contractors with a broad brush . But I know from experience that it's generally true, and the best HVAC contractors out there have experienced the frustration resulting from trying to compete with these guys. Too bad, because I believe the HVAC system is the most complex system in any house, and deserves quality design and installation to go along with the excellent equipment that is available today.
One of the master HVAC guys I worked with was certified to do air balancing. I worked with him a few times as a helper. He would get a list of supply air specs for a building and we would take days to test and adjust every area.I was lost most of the time.
What surprized me was how much it helped the comfort level and reduced the load on the system. It impressed me so much that I included it in the specs for the system going into my house.
I'm still learning though.
Now you've given me another area to research and talk over with my contractor.
Thanks.
Great comment about air balancing. Getting into commercial work, balancing is everything. Without it you may as well shoot from the hip on the design.
Air balancing is everything you said ... and more ... it gets into energy savings, too.
You review designs, but have never created one, correct?
I don't deal with residential HVAC on a regular basis, but I design, review and troubleshoot commercial systems daily. I disagree with some of your statements.
Designing duct via equal friction is not a big mistake. It is a very common, long accepted and tried and true method of duct design. I have designed many commercial systems by that method and seen them in operation. All of them are practically self-balancing, and work very well. What is the big mistake? Please explain.
Equal friction duct systems are fine, and, as you point out, they are commonly used in commercial applications.
What I was referring to was the mistaken practice, common among residential contractors, of "choosing" a friction rate to use with the duct calculator without first calculating the available static pressure, which is what's left over after subtracting pressure losses from grilles, registers, dampers, cooling coil, filter, etc. from the external static pressure.
After you know the available static pressure (ASP), the next step is to calculate total effective length (TEL) of the duct system--which is the measured length of the longest air path together with the fitting equivalent lengths. (Many commonly-used fittings have an equivalent length of 60 to 120 feet as regards the pressure losses they create.)
When you know the ASP and the TEL, only then can you calculate the lowest friction rate (LFR).
ASP divided by TEL x 100 = friction rate/100 ft. of duct.
Equal friction rate systems work in commercial applications because the engineer/designer uses a different approach--he uses the load calc to decide what airflow he wants, then he designs the duct system to a particular friction rate (0.1 iwc is common), then he sizes the duct system. Lastly, he has the luxury of choosing from a wide variety of blowers to provide the ESP that will give the performance he's designed for.
Residential systems are designed by first choosing the equipment (sized to the load calcs), THEN designing the duct system as explained above, and this is the process required by ACCA manuals.
That is why good DESIGN is critical as you said. The design will nail it down. The contractor needs to then follow the design. Don't let the contractor be the designer.
I totally agree ... more often than not, unless they are handed a design, they will schlep it together (design and materials) using rules of thumb and shortcuts.
If you are a good contractor who thinks they do well ... then prove it with a submitted document showing design loads and sizing calcs. Hand it to your client/BO. Do it right and stand proud that you did a good job.
Yup.
I think of it as being a little like a doctor that follows AMA guidelines compared to a doctor that doesn't. If it comes to a lawsuit, the one who has deviated from published standards has no defense.
Now that ACCA standards (which are certified by ANSI as American National Standards) have become required in the IRC, it provides a baseline for judging whether or not the system was designed and commissioned right. Your documentation becomes your defense.
But more importantly, it helps to avoid needing a defense--customers are far more likely to be happy with a properly-designed, professionally-installed and balanced system.
Good points all the way. Good business = good documents supporting your actions. I'm glad to hear the IRC requires calcs/design. Too many rules of thumb thrown around willy nilly and the result is significantly compromised systems that often don't perform well ... and people who have little recourse/knowledge to reject the work.
You aren't really 'conditioning' the CF space in the attice; it's more of a buffer zone. The PIER study probably outlines both the relative cost and the savings if you wanted to get into it.
The average cost per sqft of living space is more like $100-$200/sqft in my experience. 100 sqft would cost you $10K+. Realizing that a closet doesn't cost much, but if you expand your house to accomodate that closet, you are adding foundation, roof, exterior walls, etc. Personally I'm for the extra space/mechanical room approach.
My guess is the improved attic will cost less than the mech room and foster far better maintenance than putting it into an unconditioned attic. Plus the efficiency is higher.
I would suggest spending the money on foam insulation on the envelope of the house and keeping the AHU in the attic. Put returns in the major rooms. What people need to get away from is unconditioned attics and just seal up the house.
Where in West Texas are you? I have an HVAC contractor who services Midland/Odessa if you are interested.
Regards
Bruce
I live in Midland, Where do you live? WW---57
Austin
It can depend upon the structure being built--a two-story, unshaded, plan-book house with a 10/12 roof may not have a good answer.
Getting a vertical AHU into a closet-sized space (aim for around 34 x 28 frame-to-frame) inside the house is handy. No excuses for not changing/cleaning the filters at all.
However, you are left with the supply ductwork in the very hot attic with a whopping R6 or R8 blanket wrap.
For those of you not familiar, in TX, supplies are run to the ceiling or high on the wall, so that the cool air flows down through the warmer air--returns are usually low wall or under-door. Ask the hvac guys "ever'body knows that" <sigh>.
As this will likely be on a slab, getting ductwork under the floor will likely not be possible. Which leaves you with gaps under the doors. or louvered panels through to the hallways.
Now, Midland gets a lot of sun. That sun heats the entire roof surface for better than 12 hours a day, equinox to equinox. The roof surface then heats the deck, which heats the framing, which then heats your attic up. Would not be surprised that attic temps are nearer 130 than 120 over there near the permian basin.
I'd like specify a semi-conditioned attic, get 3" of iso-board on the deck, and aim for only enough a/c duct "leakage" to get the attic to 88-86 at mid afternoon. I'd spec a metal pan roof on 1x2x4' staggered (to keep the metal roof guys happy by venting).
But, that's me. It's 101º DP 61º for HI of 102º; Wunderground shows a much nicer Midland at 92º 57ºDP and HI of 91º--that 2400' of elevation difference is handy.
Thanks for your reply. Yes it does get hot in the summers in west Texas in the attic. I took my wifes kitchen thermometer in the attic once just to see what the temp was on that day----135. Your insight is valuable to me. Thanks alot. BTW, where do you live? WW---57
BTW, where do you live?
A mere 400 miles off to the ESE in Brazos County, Bryan, specifically.
Living between two rivers does keep it damp around here.Occupational hazard of my occupation not being around (sorry Bubba)
Since you have not yet started the framing, you can incorporate many of the excellent suggestions previously posted, which discourage attic-located equipment. You can figure at least a 20% energy penalty.
What I'd suggest is to have your trusses built with an HVAC chase running down the center. This is just a bumped-up section with straight sides similar to vaulted ceiling trusses, only the chase is smaller. Not a big cost increase.
Next, locate your equipment in closets, not the attic, and run supply trunks in the chase, which is part of the conditioned space. Run supply branches to the sides for high sidewall supplies in adjoining rooms. Use a large central return for each AHU, but be sure to provide an unrestricted return air pathway from each room with a closeable door to allow returns to reach the central return grille. This could be hi-lo grilles thru the stud bays to reduce noise.
The whole idea of building your own house is that you don't have to do cheap and unwise things that volume builders do. So design the house so that you can run all the duct where you want without putting anything in the attic.
Why not put it in the basement?
He is in west Texas.
Capt'n Mac can explain that one to you.
Capt'n Mac can explain that one to you.
LoL!
After somebody 'splains it to me <g>
"Frost Depth" in the Midland area is around 6-9" (IIRC), and a 24" slab turndown accommodates that depth nicely. So, digging a hole and walling it up is an additional expense.
The "where" in the geologic geography of the state matters, too.
Blackland prairie is chock full of montromorilite, an under-nice, highly-expansive mineral makes stick awful stuff to dig into, and then wants to try and crush anything in it as it cycles from wet to dray and back again.
Off in the Piney woods, the digging is bad as it's full of tree roots and high water tables.
Down to the coastal areas, you get very high water tables, and sandier soils that are awful digging. By the time you get the basement waterproof, you have to figure out how to keep it from floating out of the ground.
Head off to the SW into the mountains, and there's places with almost no soil, you could make a "basement" but your neighbors would just call it "the first floor--howcom it ain't got no winnoes?"
Then up the Edwards plateau and up the Panhandle and the ground is baked hard, and could be dug ok, but all most need is an 8'x8'x8' storm shelter, not an entire basement.
But, mostly, basements don't get build as a slab is faster, and because nobody thinks to do it any differently.Occupational hazard of my occupation not being around (sorry Bubba)
I completely agree with your first sentence, but as Captnmac stated, most duct work in Texas is run in the attic, but I will work with my good friend in Austin who is a mech. engr. to help design the duct system. Thanks to all for the input. It is very helpfull. WW---57