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I keep reading that you should keep HVAC ducts in conditioned space for maximum efficiency. We will be building in the spring, and I’m wondering about some if/thens.
Heated area: 34×48 (1632 sf); slab floor; 10′ perimeter walls, vaulted ceilings to 13′ under clearspan trusses; central wall slightly off center running length of house, could be 6″.
The few HVAC contractors I’ve talked to want to run ducts up in trusses, individual supply ducts (S) out as far as possible and through ceiling. Return ducts(R): either one high, one low, or both high. I guess this is just “standard”.
In my infinite wisdom, it “seems” to me that creating two chases at top of central wall, below finished ceiling, one side for S, one for R, with individual S & Rs each room coming down central wall, spread apart as much as room allows. Maybe high and low registers for S, to be changed w/ season. (I’ve worked out the esthetics, no problem there.)
I “think” that having ducts in conditioned space would more than offset less-than-ideal register placement. Could also use metal ducts, which I understand are far more efficient than flex.
But I would like to hear from wiser people!
Replies
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You might want to think about insulating the slab and putting the source ducts in the slab.
That is basically the way that my house it built.
*Ummmm, I'm not a big fan of ducts in or under the slab, myself. High likelihood of mold growth.
*Yes, I am not hearing good things about ducts in slab. But I ran across this article, most of which was lifted from a NAHB study.http://www.buildersresourceguide.com/emerging_technologies/hvac_space/hvac_space.htmlHere is part of it:"One benefit of placing HVAC equipment and ductwork within conditioned space is improved energy efficiency. Reduced utility costs may be on the order of 20 to 35 percent, or as much as $300 per year. Improved comfort due to higher (or lower) air delivery temperatures, especially with heat pumps, is likely. Health may be improved as potentially irritating or harmful items such as dust, pollen, moisture, and radon are not pulled into the house through leaky ducts, especially those that would have been in crawlspaces. Lower initial equipment costs may be achieved as required system heating and/or cooling capacity, and ducts themselves, can be downsized. Material and installation costs for duct insulation can be avoided. In some cases, costs for air sealing ducts can be reduced or avoided when complete duct sealing would previously have been done. (Note that some, including the EPA, suggest air sealing ductwork regardless of location to ensure adequate air delivery to specific rooms, among other reasons). "Luckily, we are at the planning stage, and our vaulted ceiling lends itself to an attractive chase solution. I'm unsure about the kind of unit(s) to get, and their placement. I just need to find an HVAC contractor who is willing to work with me; unfortunately, the ones I've talked to so far just want business as usual.
*JimI build my house in 1979 and lots of details where not done as well as they could have been.But I think that in slab ducts and work well if the details are taken care of.Probably the saving thing in my case is that the house is on a hill side so that that mositure "usually" drains away.I did have a problem about every 2 years when the ducts would fill about half way with water. We dug down and found that there was drain pipe from a higher level (uphigh is a garage with a basement) that terminated at the level of the slab. When we extended the drain line out to daylight everything has been fine.I know what you mean about the HVAC contractors. My house is not that big (1600 sq ft, 800 sq ft on a floor). But with the hill side the first floor has a 12ft foundation wall on the back, 4' for 1/2 way on the sides. The other half of the sides and the front are at ground level. The is the living room, dining room, and kitchen area, the main living space. All of the HVAC's wanted to treat it as a "walk out basement" with a couple of ducts in the ceiling (AND A COLD FLOOR). I finally found someone that would put in two systems, with one in the floor. The only problem is that they did not size it correctly and the furance is about 2-3 times oversized.If I had to do it over again I would get radiant heat in the floor.
*Bill,I could be wrong, but I would think that ducts in slab would be an inefficient way to transmit heated or cooled air.........though probably better (at least in summer) than in a 140 degree attic.We will probably go with RFH when we retire to the Oregon coast in 10-12 years, because unlike here in central CA we won't need AC.
*JimI have not looked at a complete analysis and there is more information available now then when I did this.But I don't see the the losses would be any more than a radaint system and probably less. You are not trying to heat the slab, just transfer the heated air. The slab does not become warm, but it does take the chill off it; ie, with bare feet it feels cool, but not cold enough that you "dance".
*Jim,Installing ductwork in conditioned spaces is always preferable to ducts in non-condtioned spaces. If you cannot do that, make sure the ducts are sealed and insulated. Ductwork in conditioned spaced needs to be sealed as well.As far as locations of the registers, it is always best to have one high (return) and one low (supply). Having a high and low supply and changing from season to season is not something you want to do.Use only sheet metal duct except where flex is necessary to make a connection to a register and keep the length to under 3 feet.Underfloor duct is fine and has been used in the HVAC industry for the past 50 years. Mold and other biological growth is the result of several factors, though, not including duct location.If you know how to, or know someone that can help, run loads for your house to get the right size of equipment. Oversized AC systems never work well and creak a clamy, humid environment. Err to the small side if possible. Except for less than 1% percent of the time your AC (if sized properly) will never see design conditions. A smaller unit will run more efficiently and more often, more effictively dehumidifying the air in the house (summertime).
*Tim,Thanks for taking the time to respond.I'm wondering about your statement: "Having a high and low supply and changing from season to season is not something you want to do."Do you assume I wouldn't want the hassle, or is there some other reason?I live in California. I didn't think I wanted the hassle of opening/closing windows every night/morning to utilize the whole house fan, until the energy crunch this past summer. When I saw the dramatic reduction in KWH, the "hassle" seemed minor. Doing things seasonally won't even register, so to speak.
*Jim,Having and high and low set of supply registers (or return registers) will not be a problem. However, I do not believe that you will get any benefit from this, and will be wasted money. Better to spend that money having the duct work sealed and having a good system of returns installed. If a room has a well placed supply and return, you will always get good air flow regardless of which is high or low.A common fault in residential systems is lack of returns and poor register locations. If your supply is from the floor, try to take return from the wall close to the ceiling or from the ceiling and vice versa.A rule of thumb for a "better than average" house: (By that I mean good double pane insulated windows, walls with R-19 or better, attics with R-30 or better(and ventilated)- 1 ton of air conditioning is good for 800-1000 square feet. If the ducts run uninsulated and unsealed in unconditioned spaces, that changes, down to 400-600 sf/ton.Standard residential furnaces (even the very expensive ones) only come with 0.50 " w.c. static pressure. In terms of zip, that's not much, so use of flex duct and under-sized duct really kills you in residential systems. Make sure the ductwork is big enough. If your contractor cannot tell how much air is going through a duct and at what velocity, find another one because he doesn't know what he's doing.
*As a slight aside, is a non-vented crawlspace, poly on the dirt, insulation on the foundation walls only and with the minimum supply of conditioned air as dictated by the codes, considered a "conditioned space" in terms of HVAC? Thanks. David
*Tim,Can't document a sourse right off, but I've read that the ideal duct system would have register for heated air at or near floor level, since the air will rise naturally; the reverse for cool air. It makes sense, intuitively.If my attachments work, I hope you'll be able to see my proposal, and how (I don't thin) it would cost much extra to provide high/low register for supply.
*And 3-D
*"Having and high and low set of supply registers (or return registers) will not be a problem. However, I do not believe that you will get any benefit from this, and will be wasted money. Better to spend that money having the duct work sealed and having a good system of returns installed. If a room has a well placed supply and return, you will always get good air flow regardless of which is high or low"I expect that there's no absolute right or wrong on this, but I have been taught that high/low return registers (high in summer to pull warmer air from the room and low in winter to pull the cooler) are the best approach.FWIW, as a home inspector, I only see high/lows once or twice a year, and they are always in older, then "high end" homes with lots of nice detailing throughout."Underfloor duct is fine and has been used in the HVAC industry for the past 50 years. Mold and other biological growth is the result of several factors, though, not including duct location."Not sure what he's referring to here: under a slab, sorry, I don't like it: if everything's fine its ok, but how many times is everything fine?If he means boxed in chases between joists, its low quality work IMHO. Very leaky approach.Time said: "If your contractor cannot tell how much air is going through a duct and at what velocity, find another one because he doesn't know what he's doing"YES!
*I got my first HVAC bid back yesterday; when I requested the bid, a month ago, I hadn't formulated my duct scheme, so didn't mention it.Conditioned space for the house 1632sf; R-19 walls (10ft), R-38 ceiling (vaulted to 13.6); modest amount of dual pane/low e windows (117 sf). This is central valley CA, w/ mild winters, rather hot summers.They call for a 3 1/2 ton unit. Isn't that rather too large?We have to submit "energy calcs" for a building permit here. I've had it done, and wonder if, buried somewhere in the numbers, there isn't sufficient information for HVAC sizing. Anyone know?The guy who did the calcs used the point system, and we came out 6.73 points to the good. Here are some numbers: 19,094 cu ft; energy use (kBtu/ft2-yr), heating: 15.67, cooling: 7.71. I see he only specified 10.84 SEER, and we will get at least 12. AFUE: .80. I hear mixed reports on higher efficiency furnaces.PS Hope most people are able to open my attachments. I tried to convert the pict files to jpg, but Photoshop for some reason wouldn't do it. (Original design done w/ Minicad 7; maybe if I'd saved as eps instead of pict......)
*Bobi FWIW, as a home inspector, I only see high/lows once or twice a year, and they are always in older, then "high end" homes with lots of nice detailing throughout. My first floor, the one with the supply ducts in the floor, have hi/low returns. But the 2nd floor only has high returns.While it does have a few extra's for it's time (79) is very far from hi end.Bill
*Jim, what about the surface area of the walls and ceilings at a given R-value? Linear feet of crack (door and window edges)? I've only done heat calcs on my own house, but I used the model in the ASHRAE handbook. It factored in those things, plus a bunch more, with nary a mention that I can remember of volume. I don't think that's anywhere close to as important as surface area.Andy
*Andy,That's pretty easy. Ceiling: 1612sf, R-38; walls:1429sf, R-19; "cracks": 211lf.The ASHRAE handbook you mention.....I'll look around for it.Up above, Tim mentioned that 1 ton per 800-1,000sf for "better than average" house was considered about right. If so, the 3.5 tons called for in our first bid is WAY over.
*Holy Moly, Andy.......that book (amazon) is $134 new, $80 used. Hope my lie-berry has it!
*It is often said that cooling systems are usually oversized: an HVAC contractor isn't going to get a call back for putting in too large a unit (and will make a bit more profit) but folks will scream like bloody murder if the unit's too small.The problems with oversizing, as I understand them are (i) inadequate dehumidification and (ii) shortened life expetancy from frequent cysling of the unit.If I recall correctly, there is a "formula J" wich is the standard for sizing A/Cs.I'm not familiar with your area, but for a house that size and age in NW Ohio I would expect to see a 1 1/2 - 2 ton unit. NW Ohio is a tad different from CA, though.
*David,As far as heat transfer into and out of an uninsluated sheet metal duct, the crawl space you described would be considered an unconditioned space. However, most crawl spaces are relatively mild environments compared to attics.
*Jim,I have read that, too, about the high supply for cooling and the low supply for heating. Don't recall where. I would tend believe that is an attempt to fix the problem of poor air flow by getting a little help from natural convection.BTW, those numbers (800-1000 sf/ton) were for my area (northern IL) and by no means a hard and fast rule. 3 tons is reasonable for your house, 5 tons would not be. If someone has done energy calculations on your house, I would go with their recommendations. As far as SEER/EER numbers go, that is independent of what the loads are on your house. I would recommend a SEER of 11 - 12. More efficient units are available, but the pay back is not reasonable. Get a 90% plus AFUE rated furnace, if possible. I never heard anything bad about higher efficiency furnaces.ASHRAE is the American Society of Heating, Refrigeration and Air-Conditioning Engineers. A benefit of membership is the Handbooks. Membership is $140/year, the books are "free".
*Tim and Bob,There is software called HVAC-CALC 4.0 which seems to be Manual J for computers. They offer a one-time use for $39 for homeowners. I wonder if you or anyone else here is familiar with it.On the other hand, would I be out of line to ask any HVAC contractor who bids if he has used Manual J, or the like?PS Kinda wish someone--esp. an HVAC professional--would comment, one way or the other on my duct layout scheme above!
*Jim,Sorry, but I can't seem to open those attachments. I am not familiar with that software either.I would think that asking a contractor how they sized the equipment for your house is a reasonable question. Some manufacturers provide load calculating software to installers and retailers of their products.
*OK, I figured out a way to save images as jpg; maybe this will work for you.
*And....
*Jim,That worked. Looks pretty good. The only problem I see is that you have potential to short circuit along the central wall and not have much of an effect on the outer wall. Contradicting what I said about running ducts in conditioned spaces, I would run supply branches (insulated) from the main trunk line up into the joist space and put the air at the outer wall, preferably at windows and doors, and bring it back across the room to the central return. This way you get air flow across the entire space. The high and low returns should work fine as they are. Be sure to locate the thermostat near a return, on an interior wall.BTW, I checked design info for some CA cities: Bakersfield-104, Barstow-107. Those are the 0.4% design summer dry bulb temperatures. What that means in English is that 99.6% of the time, the outside air will be at or below the temperatures listed. Quite a bit higher than the average. The 3 1/2 tons sounds like a good selection.
*Jim, I don't know about the computer program. To my way of thinking, spending $80 or even $140 to properly size an HVAC system is money in the bank. If you'd like, please email me and I'll see if I can't get you a copy of formula J.Andy
*Tim,Yeah, summers out here are pretty hot, though low humidity.And I was worried, too, about whether conditioned air would get to outer walls. I keep reading, though, that temperature differences in different areas of a room w/ good insulation is minimal. I suppose it's guesstimation......Andy, thanks, and I will.
*Jim, Try this to size your system...If that's not accurate enough for you, HVAC-Calc is a pretty good piece of software.
*Gee, thanks, Mongo; I knew there HAD to be a simple way to do it!
*Jim,Any luck with the calculations?
*You know, I put them on hold because a builder we like (haven't decided for sure) is trying to talk us into using radiant floor heat! His bid for the house is so much lower than the first guy that we're thinking, hey, maybe cake and eat it, too. He further claims that we could, given our location, and good whole house fan, do without AC. I'm a bit leery of no AC, and wondering about options.
*Jim,Radiant floor heat is a wonderful heating system. Air conditioning is optional anywhere, though I don't know exactly what "your area" is. It is a matter of comfort and cost, but you already know that. If your budget doesn't allow for a boiler and a furnace/AC, you have a decision to make if you want the in-floor heating. Generally, its easy to heat a space well, cooling and dehumidifying (well) are more difficult. Good, luck!
*Tim,One thing that occurred to me: if we go with RFH, then perhaps high registers, for AC only, would be even more plausible. We'll just have to see what an AC-only system would cost. I keep wondering, also, about just putting a largish through-the-wall unit somewhere, up high in its own opening, for the occasional times when needed.Several years ago, when I built a small painting studio for my wife, I included a simple, home-made RFH heating system. That studio is the nicest place to be now in the winter !
*Jim,I think that you'll find the cost of an AC only (central) very close to the cost of the furnace/AC combo. The condensing unit, the duct work and the fan coil/blower will all be sized to move cooling air, and the controls will be close to the same. The burner, vents and gas lines are not s major portion of the cost of a central system. Where an entire 3 1/2 ton basic system installed might cost from $7000 - $10,000, you might save 700 - 1000.
*That through-the-wall unit sounds better and better, if I could just figure out a place to put it that wouldn't look bad
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I keep reading that you should keep HVAC ducts in conditioned space for maximum efficiency. We will be building in the spring, and I'm wondering about some if/thens.
Heated area: 34x48 (1632 sf); slab floor; 10' perimeter walls, vaulted ceilings to 13' under clearspan trusses; central wall slightly off center running length of house, could be 6".
The few HVAC contractors I've talked to want to run ducts up in trusses, individual supply ducts (S) out as far as possible and through ceiling. Return ducts(R): either one high, one low, or both high. I guess this is just "standard".
In my infinite wisdom, it "seems" to me that creating two chases at top of central wall, below finished ceiling, one side for S, one for R, with individual S & Rs each room coming down central wall, spread apart as much as room allows. Maybe high and low registers for S, to be changed w/ season. (I've worked out the esthetics, no problem there.)
I "think" that having ducts in conditioned space would more than offset less-than-ideal register placement. Could also use metal ducts, which I understand are far more efficient than flex.
But I would like to hear from wiser people!