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  1. #1

    Confused Replacement Packaged Unit Sizing with some caveats

    Some background.
    We live in Fresno, CA in a 2000 sq ft ranch style home. The house was built in the 1970s and has fairly decent insulation for the age of the home. All the windows in the home are currently single pane. The current AC unit is a 4 ton, 1990 era Night and Day Gas/Electric dual pack unit mounted on the roof (packaged units very common around here). The AC and the return are on the far East side of the house.
    The previous owners converted the 300 sq ft garage, which is on the far west side of the home, into two bedrooms. They did place a 6x12" vent in each rooms and fed it by tapping into the main large flex duct that serves the main living area (also on the west side). The main living area is approx.. 800 sq ft and is cooled/heated by one large and very powerful 4 x 20" vent. The vents in the west end add-on rooms are approx 80' from where the A/C unit is located. Those rooms don't get much air and are much warmer or cooler than the rest of the house depending on the time of year.

    We're now getting quotes for a new unit as the Night/Day model we have has developed a slow coolant leak. We've gotten three quotes so far from reputable contractors in town. Two of the quotes are from larger outfits and one of them is from a smaller contractor who was recommended to me by a retired HVAC installer. The quotes are as follows...

    Contractor 1 (price lowest)
    -Install new 4 Ton, 14 SEER Bryant 577CNWA48090N unit on roof
    -Install new angle iron stand for unit
    -Add return to one of the add-on rooms to increase airflow to that side of house
    - Duct Test

    Contractor 2 (+1250 after $400 Rheem/Ruud rebate)
    -Install new 4 Ton Rheem 14 SEER unit on roof
    -Install new angle iron stand for unit
    -New Return for West end of the house (R6 Flex)
    -New Thermostat (not really needed as we already have a new T-Stat)

    Contractor 3 (+1200 after $400 Rheem/Ruud rebate)
    -Install new 5 ton Ruud 14 Seer Achiever Series
    -Install new angle iron stand for unit
    -Remove current sheet metal on roof connecting unit to register/return and create straight downshot from unit to roof for less heat/cooling loss.
    -Home run two new flex ducts from unit to add-on rooms. Cap off where previous ducts split off from main duct line.
    -Add second vent to main living area so current vent doesn't have so much air hitting one spot and feels less like a tornado.
    -Airtest ducts and seal as needed.

    I'm leaning toward contractor 3 because he seemed to take his time and really look at the current configuration and how to best optimize it. I'm a little worried though that a 5 ton unit is going to be too large. The current 4 ton unit doesn't seem to have a problem keeping up and can cool the house (except the add-on rooms) from 90+ to 78 in a couple of hours.

    Contractor 3's point is that because of the length of the duct we need the bigger unit to push enough air to sufficiently cool the add-on rooms. The first two contractors seemed to think that adding a return to one of those rooms would be okay with a new 4 ton unit. What do you think? I always read in these forums not to oversize the unit, so I'm a bit leery of a 5 ton unit. To further complicate matters we plan on changing to all dual pane high-e windows in the next year, so that really worries me a 5 ton is too big. Contractor 3 has said he will put in a 4 ton unit if I want; he’s just concerned I may call back in a year wanting to put fans in the ducts to push the air to the west rooms which would cost more than just getting the 5 ton unit up front.

    I am however tempted by Contractor 1 based on the price. If the additional return in one of the west add-on rooms works it just seems like a great deal. However I did mention that Contractor 1 had spec'ed a Bryant unit to Contractor 3 and he said he used to sell Carrier/Bryant units but had several develop coil leaks within only two years and doesn't feel as though they're as reliable as the Rheem/Ruud units. Is the reason that the Rheem/Ruud quotes are higher is that they're really better units. I definitely don't want to put some serious $$ down and have to pony up more 24 months later because my coil is leaking.

    Am I over analyzing this?

    Sorry for the long post. I just always seem to see responses on here saying they need more info.

    Thanks for any help/input.

  2. #2
    Join Date
    Oct 2004
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    So Cal
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    I might be biased- I'm a Rheem dealer.
    Looks like you answered your own question.
    I'm leaning toward contractor 3 because he seemed to take his time and really look at the current configuration and how to best optimize it.
    Looks like contractor #3 is also going bottom discharge & the others are staying w/ existing. I'd go with #3 & have them use the roof curb instead of the stand. I hate angle iron stands. If you re-roof in future- you will need to remove the unit & angle iron stand to do it right. Not so w/ a curb. I'd ask for a load calc to make sure they aren't oversizing the system.

    Here's a curb on one of our jobs.
    Old Carrier unit in background.


    New Rheem


    No exposed ducts, completely water tite. A re-roof won't affect the unit or curb. You'll need a roofer to patch it in but it's worth every penny.

  3. #3
    Thanks for the quick reply precision. Your install looks great! I can definitely see the benefits of having that versus the big angle iron stand that is currently on the roof. I think that is what contractor 3 had in mind. Right now the unit has a big swooping 90 degree piece of sheet metal ducting that comes out of the roof and into the side of the unit. Contractor 1 and 2 would definitely keep that as is and stated such. Contractor 3 wanted to get rid of that and just go a straight "downshot". I think by a downshot he meant what you're showing in the pictures but I'm definitely going to get clarification on that because a re-roof is probably in the cards before this unit gets replaced.

    What's involved in a heat load calc?

    Would an extra return on the west side of the house really help draw more cool air into those far rooms? Contractor 3 seemed to think it might help, but it probably wouldn't be sufficient. Contractors 1 and 2 didn't seem to think it would be an issue though. Is there any way to test this? Maybe by putting a box fan or something in one of the rooms and pushing the air into another part of the house? Or do you just have to rely on the contractor’s experience?

    Contractor 2 was a concerned about putting anything larger then a 4 ton in becuase of the return air volume. The current return is 25" x approx 14", does that sound like enough air volume to feed a 5 ton?

    Does anyone know if the Bryant unit quoted is actually less expensive than the Ruud/Rheem units? Contractor 1 has to be making his money somewhere. Either his unit is cheaper, his labor is cheaper/lower quality, or his profit is less. If his unit is cheaper is it because it's quality isn't as robust?

    Thanks again!

  4. #4
    Join Date
    Feb 2007
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    Las Vegas,NV
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    I'm afraid after growing up with old timers and doing many rooftop changeouts, I hate residential roof curbs! What happens now when someone comes along and has to replace that unit with a unit that is a completely different dimension? You wouldnt want a curb adapter, you would have to tear the whole mess out and start over.
    If you leave the elbow, you can come along and set any dimesnsion unit you want by simply changing the stand and having a simple transition made.
    It also appears that the adjustable roof curb limits the size of duct that you can attach to the bottom of it, I cant see where you could attach a 20" flex to the bottom of that base, it just doesnt look big enough.
    At least you slid that one under the shingles, most of the hacks around here set them on top of the roof and tar the crap out of em.
    Something else to remember is that by going downshot, the static pressure will run higher on some units. At least with an elbow, the air can come straight out the back of the unit and make a gradual downturn through the roof. Downshot, it smacks the back of the unit and has to force its way down.

  5. #5
    Join Date
    Jun 2005
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    Winston-Salem NC
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    1,133
    I can't recall a single rooftop package unit on a house. Had one that was on an ol house, big 2 story thing converted to a B&B, but even it was set on a flat roofed addition in the back.

    In NC that roof top unit would require railings around it to prevent a tech from falling backwards and off the roof. 42" or 48" high, extending IIRC 6' feet from either side of the unit.

    Nice looking install though. I like curbs better than stands myself on commercial applications.

  6. #6
    Join Date
    Oct 2009
    Location
    Kingsport, Tennessee
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    131
    As mentioned above, seems like you have this worked out.

    I only read through the post once, and dont have time to really look into what you have, but I just have one quick comment.

    In the long run, you will be more happy with the installer you really like, then the one that is just a little cheaper. Sometimes that "good feeling" is worth paying for.

    AND on the flip side, don't assume less price means less quality. There may be many factors that might make his price lower.

    I have a good friend in my area that drives one of the oldest vans around (not sure how its still running), BUT one of the best/cheapest techs around. I have called him several times with help.

  7. #7
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    On another note to precison havc, your install looks great and it appears that you do nice work. Always nice to see the stat wire protected, I get really annoyed when I see stat wire exposed all over the place.
    Thats the first adjustable resi curb I've ever seen where it was properly placed under the shingles. The hacks here will provide a five gallon bucket of Henrys with every resi curb install.

  8. #8
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    Quote Originally Posted by stonewallred View Post
    I can't recall a single rooftop package unit on a house. Had one that was on an ol house, big 2 story thing converted to a B&B, but even it was set on a flat roofed addition in the back.

    In NC that roof top unit would require railings around it to prevent a tech from falling backwards and off the roof. 42" or 48" high, extending IIRC 6' feet from either side of the unit.

    Nice looking install though. I like curbs better than stands myself on commercial applications.
    Funny you mention the railings, because I was born and raised in the Fresno area and most residential rooftop units over a certain pitch, had a catwalk mounted around them. You definately dont see that here in Vegas or in Phoenix.

  9. #9
    Join Date
    Oct 2004
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    So Cal
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    We don't mastic the you-know-what out of our installs. We have a roofer who works with us on all our jobs that require a roof modification.
    We only do a few of these ressy package units a year because most cities around here won't allow them anymore. Unsightly, ugly, roof load are terms that come to mind. On this install- permits were pulled & roofing was removed several feet back around perimeter of the curb. We replaced the roof sheeting under the unit & also added several 2x6 supports to the existing 2x6 rafters that rest on a wall top plate in attic to support the weight of the unit. These pre-fab roof curbs are adjustable from flat to a 5:12 pitch. Didn't require railings or walkways or screening.
    I didn't want to put a package unit in- I tried to talk them into a split system but he was dead set against it. It's his money but I sure would hate looking at it everyday.

    Only requirement from city was
    *Rafters must be supported mid span under weight of unit, subject to field inspection or submit engineering that says otherwise.
    *Unit can't be visible from the street. They don't care about sides or back yard.

    The old unit was sitting almost on the ridge. Even the angle iron stand was visible from the street! The new one- I tossed the curb on the roof, stood a piece of plywood on the top that was same height as unit & started sliding it down the roof until it couldn't be seen from across the street.

    This is after install from across the street.



    Like I said- we don't normally install package units on residential roofs but we will if customer insists & city allows.


    Now these- we install lots of these on commercial flat roofs. 12 on this roof.

    All on curbs.

  10. #10
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    What happens now when someone comes along and has to replace that unit with a unit that is a completely different dimension? You wouldnt want a curb adapter, you would have to tear the whole mess out and start over.
    You'd be doing similar with the angle iron stand if unit didn't fit- or you could do like the hacks do & toss a piece of plywood on the angle iron stand & let it hang over.
    How often are you planning to change the unit out? We do it once, we do it right.

    It also appears that the adjustable roof curb limits the size of duct that you can attach to the bottom of it, I cant see where you could attach a 20" flex to the bottom of that base, it just doesnt look big enough.
    That's an 18" supply & return. Unit is only a 2 tonner. You can get 20" into this curb & openings in unit are larger on the 3 1/2 ton & up. Even more if you run rectangular duct straight down.

    Something else to remember is that by going downshot, the static pressure will run higher on some units. At least with an elbow, the air can come straight out the back of the unit and make a gradual downturn through the roof. Downshot, it smacks the back of the unit and has to force its way down.
    Really.

  11. #11
    Join Date
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    downshot

    A curb is the only way to go, resi or commercial.

    Some Talk, Some Do
    "keeping condensing pressures low and evaporator pressures high"
    "Some customers are more interested in comfort than energy savings"
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  12. #12
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    Quote Originally Posted by precision hvac View Post
    You'd be doing similar with the angle iron stand if unit didn't fit- or you could do like the hacks do & toss a piece of plywood on the angle iron stand & let it hang over.
    How often are you planning to change the unit out? We do it once, we do it right.


    That's an 18" supply & return. Unit is only a 2 tonner. You can get 20" into this curb & openings in unit are larger on the 3 1/2 ton & up. Even more if you run rectangular duct straight down.


    Really.
    I understand doing it right, I wont do a job unless it's right. Eventually these units around here that have been put on adjustable curbs are going to need to be replaced, especially since most of them are hacked in. I would find it easier to set a new angle iron stand, than tear shingles apart trying to remove a curb.

    Something about pulling flex duct through the roof and attaching it to a curb, just doesnt grab me. I'm old school and I dont believe new and improved is always improved.

    As far as mounting downshot, most of the manuals I have seen, mention adjusting blower speed for static issues. The Goodman GPG is the worst I have seen for a downshot design.

    The curb issue is just my opinion, your install was awesome and I hope I didnt say anything to offend you.

  13. #13
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    Old school? I've been doing this since 1968
    Adapt or die.

    Not offended at all.

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