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  1. #1
    Join Date
    Oct 2010
    Posts
    18

    Help with sizing?

    I have a dual unit setup for my home, which is 3200 sq ft, ~2000 down, ~1200 up. The downstairs unit failed a couple of weeks ago. The downstairs is 10' throughout, except the entryway which is 19' for a 15' x 10' area, and a vaulted ceiling to about 12' in the master bedroom.

    I currently have a 5 ton Carrier for the downstairs, but curiously the house was quoted in all the builder's paperwork as having a 3.5 ton downstairs. (The 5 ton is the original unit, so that's the disparity.) I've had a feeling for a while that the unit was oversized. I've read several threads in here, and I've seen several setups similar to mine, even southern setups (I'm in Texas) that I've never see sized to 5 tons.

    Well now that unit has failed, and I'm starting to get quotes. The problem is, I'm getting a WIDE disparity, based on assumptions of what has to be done to the duct work. Nobody has done a full load calculation - not that they won't, but the initial sales guy won't.

    I did a map of all the duct work, and noticed some peculiar runs (I'm not an HVAC guy, so who knows). Can I tell from the size of the ducts whether the number of supplies and returns matches the 5 ton unit? I realize you cannot tell if 5 ton is the right size without performing a load calculation, but I'd at least like to know if the current duct work is properly supporting a 5 ton unit.

    For supply, I have 1x10, 2x9, 3x8, and 1x6. For return I have 3x14.

    Thanks,

    swendler

  2. #2
    Join Date
    May 2008
    Location
    Portland, OR
    Posts
    236
    The return is only large enough for a 3.5 ton system.

    For anything more than that, you need a full load calculation (Manual J) and duct analysis (Manual D).

    There are a lot of great companies in Texas that will do this correctly, just keep calling around until you find one that will perform the Manual J DURING the initial sales call.

    If you're between Austin and San Antonio, my dad can help you out.

  3. #3
    Join Date
    Oct 2010
    Location
    Delaware
    Posts
    36
    You want them to do a load calc. When they built the house it was probably 5 ton single system or if you opted for two systems then prob would of been 2.5 upstairs, 3.5 downstairs and contractor put in wrong unit. What kind of system are you looking at anyway. Single or multi-staged?

  4. #4
    Join Date
    Sep 2003
    Location
    Western PA
    Posts
    1,360
    Keep calling around until you find someone to do a load calc during their first visit.
    Local 449?

  5. #5
    Join Date
    Oct 2010
    Posts
    18
    Quote Originally Posted by 06Corvette View Post
    You want them to do a load calc. When they built the house it was probably 5 ton single system or if you opted for two systems then prob would of been 2.5 upstairs, 3.5 downstairs and contractor put in wrong unit. What kind of system are you looking at anyway. Single or multi-staged?
    The system was originally spec'd as a 3.5 ton downstairs and a 3 ton upstairs. However, we made some major changes to the floor plan. The most significant was that we converted a great room with 19 foot ceilings to upstairs (8') and downstairs (10') rooms. I'm not sure how the two story great room would have been zoned between the two units, but we ended up with a 3 ton and 5 ton.

    The other change we made was upgrading from 10 SEER units to 12 SEER units, but that shouldn't have changed the BTUs. But maybe they had to go to different sizes in the 12 SEER.

    I really don't know, but I do believe the unit is oversized. I'll have to find someone that will do the load calculation off the bat, as it will bring some sanity to the wide range of quotes I've had.

    BTW- I did some energy saving steps when building the house as well. We wrapped the house with insulating foam board outside the frame, and then wrapped that with Tyvek. The foam was to increase the wall rating from R-13 to R-19. We have a radiant barrier on the roofing, with continuous ridge and soffit vents for good heat removal. We had a sill sealer applied at the foundation. (I had to fight the builder to do a few of these.) So will all of this will impact the load calculation?

  6. #6
    Join Date
    Sep 2003
    Location
    Western PA
    Posts
    1,360
    BTW- I did some energy saving steps when building the house as well. We wrapped the house with insulating foam board outside the frame, and then wrapped that with Tyvek. The foam was to increase the wall rating from R-13 to R-19. We have a radiant barrier on the roofing, with continuous ridge and soffit vents for good heat removal. We had a sill sealer applied at the foundation. (I had to fight the builder to do a few of these.) So will all of this will impact the load calculation?[/QUOTE]

    Absolutley.
    Local 449?

  7. #7
    Join Date
    Oct 2010
    Posts
    18
    UPDATE: Well, I had two contractors out this week. Both were recommended to me by reputable sources.

    Both noted that my duct sizing was below par, on the order of 3.5 to 4 tons, versus my unit size of 5, but only one did a load calculation to see what I needed. His calculation was a tad above 3.5 tons, but he recommended that I go to a 4 ton system. The other guy quoted me a price to upgrade my supplies and returns to 5 tons, and priced me a 5 ton system. BUT, he said if I call him back he'll do the load calculation and reprice the system smaller if needed. This was a a little disappointing, as he had the most convincing presentation with regard to their quality, materiels, and thoroughness (of the install, not of the sales call).

    So I'm closer to sanity, but still have a variety of quotes, with one now for a 4 ton system - albeit close to what I think I need, but different from all my other quotes.

    So let me ask, is there any rule of thumb about how the BTUs of a system relates to cost? I.e., is a 4 ton system of the same product line roughly 10% less than a 5 ton? Or is that trying to oversimplify it?

    While I'm at it, I seem to be gravitating to either Trane or Carrier. My target was to get just high enough to get into tax credit territory, but both companies have rebates going on for units a little higher up. So as I look at these systems, I'm curious as to the pros and cons of these competing systems:

    Carrier Performance + Infinity Furnace, 4 ton
    24APA748A003, CNPHP6124ATA, 58CVA110-1-20

    Trane XL16i, 4 ton
    4TTX6048E1000A, 4TXFH063CZ3HHA, TUD2C080B9V4VA

    I believe both are two-stage, but I was confused as to whether the Trane was possibly two-stage in the 5 ton only. As quoted, the Carrier is running ~$ more.

    Oh, and are HVAC surge protectors just for milking more margin out of the customer, or do they have actual value?
    Last edited by beenthere; 11-04-2010 at 06:33 AM. Reason: price

  8. #8
    Join Date
    Jan 2004
    Location
    Burleson, Texas
    Posts
    1,741
    what part of Texas, im in south Ft Worth...

  9. #9
    Join Date
    Oct 2010
    Location
    Delaware
    Posts
    36
    Quote Originally Posted by swendler View Post
    UPDATE: Well, I had two contractors out this week. Both were recommended to me by reputable sources.

    Both noted that my duct sizing was below par, on the order of 3.5 to 4 tons, versus my unit size of 5, but only one did a load calculation to see what I needed. His calculation was a tad above 3.5 tons, but he recommended that I go to a 4 ton system. The other guy quoted me a price to upgrade my supplies and returns to 5 tons, and priced me a 5 ton system. BUT, he said if I call him back he'll do the load calculation and reprice the system smaller if needed. This was a a little disappointing, as he had the most convincing presentation with regard to their quality, materiels, and thoroughness (of the install, not of the sales call).

    So I'm closer to sanity, but still have a variety of quotes, with one now for a 4 ton system - albeit close to what I think I need, but different from all my other quotes.

    So let me ask, is there any rule of thumb about how the BTUs of a system relates to cost? I.e., is a 4 ton system of the same product line roughly 10% less than a 5 ton? Or is that trying to oversimplify it?

    While I'm at it, I seem to be gravitating to either Trane or Carrier. My target was to get just high enough to get into tax credit territory, but both companies have rebates going on for units a little higher up. So as I look at these systems, I'm curious as to the pros and cons of these competing systems:

    Carrier Performance + Infinity Furnace, 4 ton
    24APA748A003, CNPHP6124ATA, 58CVA110-1-20

    Trane XL16i, 4 ton
    4TTX6048E1000A, 4TXFH063CZ3HHA, TUD2C080B9V4VA

    I believe both are two-stage, but I was confused as to whether the Trane was possibly two-stage in the 5 ton only. As quoted, the Carrier is running ~$ more.

    Oh, and are HVAC surge protectors just for milking more margin out of the customer, or do they have actual value?
    First is this a horizontal system or vertical. Also looks like the train is a 90% furnace since they are going with a 80000 btu furnace and the carrier is a 80% 110,000. Also if going with 17 seer model i rec going with the 21 seer unit-- cost is usually few hundred more but big difference-- 17 seer isnt bad but was cheapened up alittle on design and get complaints of noise. Also i dont believe that the 4 ton units in the 17 and 21 seer qualify for federal but if go for the 58mvc 3stage carrier furnace then you will get federal and manufacturer rebate of 1000 with both 17 or 21

  10. #10
    Join Date
    Jul 2007
    Location
    Delaware
    Posts
    3,824
    3 14" returns?
    Always here

  11. #11
    Join Date
    Oct 2010
    Posts
    18
    Quote Originally Posted by energy star View Post
    3 14" returns?
    Umm...yeah. That's what's entering the plenum. Two are vented through a 14"x25" grill. I believe the other is either the same, or a 20"x20", One guy thought the return duct sizing was okay there. Whereas another wasn't so sure.

    Quote Originally Posted by 06Corvette View Post
    First is this a horizontal system or vertical. Also looks like the train is a 90% furnace since they are going with a 80000 btu furnace and the carrier is a 80% 110,000. Also if going with 17 seer model i rec going with the 21 seer unit-- cost is usually few hundred more but big difference-- 17 seer isnt bad but was cheapened up alittle on design and get complaints of noise. Also i dont believe that the 4 ton units in the 17 and 21 seer qualify for federal but if go for the 58mvc 3stage carrier furnace then you will get federal and manufacturer rebate of 1000 with both 17 or 21
    These are horizontal units.

    The quote says an 80% 80,000 furnace for the Trane. But you're right about that 110,000 sticking out like a sore thumb. Even the 5 ton systems quoted were 80% 100,000. I'm guessing that was a mistake and will check with him.

    I did look up the AHRI tables for everything that's been quoted me, and everything so far is listed as eligible for the tax credits. Of course, I haven't received quotes for the 21 yet, but will pursue that.

    Quote Originally Posted by andserco View Post
    what part of Texas, im in south Ft Worth...
    I'm in Flower Mound, just north of DFW. If you have someone you'd care to recommend or something, please PM me. I'm still wide open.


    One other thing I noticed today is that the Trane evaporator quoted is a flat coil instead of an A-coil. The other quotes I've received were for their A-coil. As I understand it, the flat coils are still copper versus the all-aluminum A-coils. He probably said that as he was putting in the numbers, but I must have missed it. He did say something about the coil offering better airflow. But by the numbers it seems that the flat coil has poorer efficiency. I haven't found a thread that really discusses the pros and cons there. Some say that flat coils leak less than A-coils. But what about performance?

  12. #12
    Join Date
    Oct 2010
    Location
    Delaware
    Posts
    36
    Reason he did 110,000 btu furnace with the 61 coil to get the higher seer rating instead of going with the 90000btu and 4 ton coil. The 21 wont qualify for the federal in 90,000 or 110000 btu. Rec mvc furnace with 17 or 21 to qualify for federal from furnace instead. 110000 btu furnace sounds a good 20-30 thousand btus too big and yes it is two stage but u have good chance of shortening heater life. Otherwise go with 90000 with 17 seer with 4ton coil--diff is .5 eer and 1seer.

  13. #13
    Join Date
    Oct 2010
    Posts
    18
    Quote Originally Posted by 06Corvette View Post
    Reason he did 110,000 btu furnace with the 61 coil to get the higher seer rating instead of going with the 90000btu and 4 ton coil. The 21 wont qualify for the federal in 90,000 or 110000 btu. Rec mvc furnace with 17 or 21 to qualify for federal from furnace instead. 110000 btu furnace sounds a good 20-30 thousand btus too big and yes it is two stage but u have good chance of shortening heater life. Otherwise go with 90000 with 17 seer with 4ton coil--diff is .5 eer and 1seer.
    Yep. I was combing through the AHRI site tonight and discovered that Carrier's units list the 5 ton for efficiency quite a bit.

    What is the hit to go to the MVC furnace? It looks like that's a premier unit, and quite frankly, nobody around here has been quoting anything over 80% AFUE. The story you get down here in Texas is put the money in cooling, not heating.

    However, I did find one combination that seemed to fit. Is this the one you were referring to with the 4 ton coil and 90K furnace? I'm not an HVAC guy.

    Carrier
    Performance 17, 4 ton, 24APA748A**31
    Performance series 4 ton coil, CNPH*4821A**+UI
    Infinity 80, 90,000 BTU furnace, 58CV(A,X)090-16

    This combination comes up at 16 SEER, 13 EER. And it seems to match 4 tons all around.

    But you're right. There was nothing much else available. The only other option I saw that was interesting was this:

    Carrier
    Performance 17, 4 ton, 24APA748A**31
    Performance series 5 ton coil, CNPH*6124A**
    Performance Boost 80, 90,000 BTU furnace, 58PH*090-16

    Thie combination is 17/13.4, but I don't think the furnace supports two-stage - not that it's essential.

    I will inquire on the 21, but I don't think I can swing that if I can't get the tax credit on a well matched system.

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