Upflow vs. downflow...am I crazy?
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  1. #1
    Join Date
    Aug 2006
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    8
    I recently bought a home. Actually an older doublewide in need of some repair and upgrades, but the price was too right to turn down with the land it came on.

    So here's the thing. We knew it needed a new roof when we bought it. With that in mind, I decided to do a full, tied-in proper roof over with a steeper pitch. Not only is it an old doublewide with a shallow roof pitch, better than half of the 2400sf house has vaulted ceiling inside - absolutely no "attic" space and very little insulation in that. (My last elect bill was $450!) I am convinced that most of my problems are located here. My new roof will be a metal standing seam with a thermal barier underneath. And, of course, more insulation. In addition, all of my fiber floor ducting needs to be replaced. I've patched up a lot of leaks, but it's just shot...

    My HVAC question is this: I was told by a local HVAC guy that great gains would be made by converting from a downflow to an upflow furnace and running my new ducting overhead. It wouldn't be a huge issue to do it while I'm building the new roof, but is there any truth to this "benefit"? Am I better off just replacing the floor ducting? (I'm also redoing all the floors to a wood laminate, covering up the floor vents is also not a big issue.) A benefit to the overhead ducting is that I can have everything run while I'm building the roof, then swap out the furnace. The tie in would be quick with minimal downtime. About the only negative I can see is the cost of a new furnace, since everything is getting replaced either way. Even at that, the furnace would have to be replaced eventually...

    Thoughts? All things being equal, what would you do in your house?

  2. #2
    Join Date
    Aug 2006
    Location
    San Diego,CA
    Posts
    9
    Maybe this will help, my sister lives in Maricopa county,Arizona. She has a single level modular home with vaulted ceilings and like you ,has no attic space just a little insulation (or maybe alot) and a roof between her and about an average daily temp of about 104 - 114 degress for about 3 -4 months out of the year. Now she has central air consisting of 5 ton upflow unit (vertical standing )located in a hallway closet, there are both floor registers & sidewall supply registers. I think you should utilize existing floor registers and since your talking about an open roof you can probably easily add a duct run or two without severe adverse effects. The reason I say using floor supply as in my sisters house is that most of the supply ducts are running underneath sub floor giving the duct a more stable environment than running it all overhead, being so close to the heat generated in attic space or lack of is just one more task that your A/C equipment will have to compensate for.I've been in attics in relatively pleasant outside temp of ,say, 80 -82 degrees with low humidity only to see temps of 125-140 degrees in attic space. I am also installing wood lamenant floor in my sisters place as soon as it cools off a bit, about 850 sq ft. with about 6 floor registers to cut in. P.S. the subfloors in modulars like mobile homes is absolute CRAP. good luck

  3. #3
    Join Date
    Feb 2005
    Location
    Kentucky
    Posts
    57
    If the ductwork under the house is in good shape and insulated I would probably go back with a downflow unit. Since it is a doublewide it probally has a central return air. I would ad a few retuns in the ceiling, going through the new attic of the 2400 sqft home. Personally I dont like supply and return vents both in ceiling.

  4. #4
    Join Date
    Sep 2005
    Location
    Texas
    Posts
    2,793

    You can answer that question

    by asking yourself this question. Can ducts convey cool air more efficiently while underneath a semicool floor or in a HOT attic?
    Having said all that, I doubt that is the cause of your 450 electric bill

  5. #5
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    allow the space to put in LOTS of insulation! R30+ into the "attic"
    if ducts are in the attic, how will you access them later?

    insulate the "skirt" with Styrofoam. & walls when residing.

    install storms -- separate at least 0.75" with wood strips.

    apply recflective film -- & sun shading screens
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  6. #6
    Join Date
    Aug 2006
    Posts
    8
    OK, thanks for all the replies. Tinman, I'm a little confused. Do you mean run ducting from the floor to the ceiling just to add a couple more ducts? I'd have to find a closet or something to do that and wouldn't my air handler need to be resized??

    Pipefitter (hey, me too!), the old duct is already shot and needs to be replaced anyway. Part of the reason I was questioning going overhead, since I was replacing it. I did think about the individual returns. I may do that.

    Air2space and Tinman, I questioned that myself (running ducts in the hotter attic space). This guy was just so adamant about it, I thought maybe I misunderstood something? The main thing, I am convinced, that will knock my electricity bill is the insulated roof over. That, and replacing or covering all the single pane windows. This place is full of windows all the way upinto the peak of the vaulted ceilings in both the family room and master BR(which we love) but this ain't helping my bill any, either.

    cem-bsee, I hadn't thought about Styrofoam. I plan on redoing with log siding and rock, though (bringing the rock up higher than conventional underpenning to get even further away from the "trailer" look). Will added styrofoam give any benefit to that?

    Thanks for all the feedback, guys! One piece at a time, I'll get there...

  7. #7
    Join Date
    Nov 2005
    Location
    Northern VA
    Posts
    512
    Keep in mind that normal residential HVAC equipment is often NOT approved for installation in a "manufactured home", which a doublewide would be. They're fine for installation in a "modular home" however.

    I am a bit confused by your roof comment - is this a manufactured or modular home?

  8. #8
    Join Date
    Jun 2001
    Location
    DFW
    Posts
    684
    If the new duct work ends up being in conditioned space, and if it will be possible to get to it to work on it, then it could be a good idea. The problem with the duct work in manufactured housing is that it is quite restrictive and requires special blowers. Being in the crawl space, it is also a pain to work on and the old fiber stuff is subject to rat infestation and other critter problems.

  9. #9
    Join Date
    Dec 2003
    Posts
    709
    Or you could get a packaged heat pump or a/c and run new ducts under the trailer. I have done this with good success, at the very least get a bid to have this done for comparison, I will give you some pros / cons

    Pros, package unit outside

    All fan noise and other unit noise outside
    Drain outside, never have water issues in closets etc.
    Easy service, all service done outside on ground level
    Good, even airflow especially with properly sized ductwork
    No need to vent combustion air through roof or sidewall
    Ease of adding fresh air intake or economizer if desired

    Cons, package unit outside

    Large unit, yorks are roughly 48" square
    Must cut in new return register, because old downflow usually pulls return right from the closet louvered door
    ????

    Cons downflow or upflow

    All unit noise is inside
    Possible drain and water problems with closet installation on MDF wood flooring
    Service can be difficult
    poor airflow is usually the case especially with poor duct design, airflow can be good with the right design

    Pros

    Higher efficiencies available in split systems, like 16-21 seer
    ????

  10. #10
    Join Date
    Aug 2006
    Posts
    8
    OK, I'm doing my best to soak all this in. Learning a lot as I go! Realistically, it's looking more and more like I'll just replace the existing ducts underneath.

    Hivacer, right now I've got about a 10 year old Rheem heat pump outside. It's an 11 SEER. Sure, I could do better, but I'm not sure it's worth the expense right now. It seems to work fine, and I figure my money is better spent on the roof, insulation, windows, etc. I'm sure I will upgrade one day... Question, though, if I run the appropriate sized ductwork now with the old downflow, is there any reason the same duct couldn't be tied into if I decide to install a package unit later?

    Perel, I'm completely confused by your post. I've heard the "manufactured vs. modular" thing before. I just always figured "modular" was a more hi-falutin' sounding word for folks who didn't want to admit they lived in a trailer! (Now I know that there are some modular homes being built in recent years that don't fit that description.) What are you saying is the difference? Why is residential HVAC equipment not approved for installation?

    I am saying I live in a doublewide. The idea with the roof is a freestanding 6/12 roofover, as I'm afraid the existing roof would not support the load. I will anchor 6X6 posts in concrete hugging the house tightly. I will then run probably 3X12 Glulam beams and prefabbed trusses, covering with a thermal foil barrier, plywood, and metal standing seam and insulate the new "attic". The gable ends will be closed. In fact, I am installing log siding all the way up into the gable ends and between the posts, so it will look like one integrated unit. Does that kinda explain it?

    Thanks again for everybody's help and patience here. Looking forward to more feedback!

  11. #11
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Originally posted by hivacer
    Pros, package unit outside

    All fan noise and other unit noise outside
    Drain outside, never have water issues in closets etc.
    Easy service, all service done outside on ground level
    Good, even airflow especially with properly sized ductwork
    No need to vent combustion air through roof or sidewall
    Ease of adding fresh air intake or economizer if desired
    You forgot one big one. If he goes with the package unit, he gains a closet.

    Oh, and you can get 16 SEER 2 stage cooling package units in gas, electric and heat pump varieties.
    If more government is the answer, then it's a really stupid question.

  12. #12
    Join Date
    Nov 2005
    Location
    Northern VA
    Posts
    512
    There is a good description of the differences between modular, manufactured, and site-built here:
    http://homebuying.about.com/cs/modul...dularhomes.htm

    And how to tell which you have here:
    http://homebuying.about.com/cs/manuf...realestate.htm

    I'm not sure exactly why normal residential units aren't allowed in manufactured (mobile) homes; the install instructions specifically say not to do so.

  13. #13
    Join Date
    Aug 2006
    Posts
    8
    Yeah, I was pretty sure you would have to call this manufactured. The confusion comes from seeing too many people (and companies) call their doublewides modular homes.

    Hmmm...so I wonder how long it would take to recoup the installation cost of a 16 SEER vs my 11 SEER?

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