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  1. #1
    Hello.

    I'm renovating a two-unit house and pursuing an Energy Star rating on the units. My HVAC guy usually applies a 'rule of thumb' so he's a nervous about the sizing numbers HVAC Calc determined. I think he's bought into the heat loss calc but he's wondering if we'll have the proper CFM for the system. As a developer I'm not an HVAC expert but I'd like to start paying more attention to doing it right.

    Anyhow, I'd like some expert opinion to these to see if these numbers seem reasonable, specifically the blower sizing.

    Unit One (1st fl) is approx. 900 sq ft. Heat gain is calculated at about 14k BTUH, Heat Loss about 30k. For reasonable a margin of safety, I anticipate using a 1.5 ton AC with a 40k BTUH gas forced air furnace. My HVAC guy is assuming a 800 CFM blower for this unit.

    Unit Two (2nd & 3rd fl) is approx. 1700 sq. ft. Heat gain: 22k, Loss: 37K. Equipment sizing: 2.0 ton and 60k BTUH furnace. My HVAC guys is assuming a 1200 CFM blower for this unit.

    Question: Do you anticipate any problems with the blower sizing for these units, specifically since my heating needs are much greater than my cooling needs?

    Thanks in advance for your help and opinions.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,399
    You need to set the blowers appropriate to the size of the A/C. On the upstairs, the 3 ton blower probably will have to be set on low to move 800 CFM for the 2 ton. Downstairs, medium or medium low for 600 CFM for the 1.5 ton. Excess air means high humidity and few installers bother to change from factory settings. Otherwise, things look good. Small homes are so commonly oversized.

  3. #3
    Thanks for your reply.

  4. #4
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    are you going the extra mile such that the ductwork can be in the conditioned space? or, will the old ductwork be ok -- better be SURE.
    read LOTS here --
    be sure the ductwork & unit has all seams & joints sealed. if all takeoffs are the same size, dampers probably are needed -0-
    getting 3" tk insulation on ducts if not in conditioned space?
    got lots of returns? one out of ea bdrm?

    getting 5" filters?


  5. #5
    Join Date
    Dec 2000
    Posts
    44
    Hey Somerville, I am happy to see a builder interested in the HVAC system beyond just the price. I stopped doing builder work because guys working out of their trunks were cutting my bids by at least half. PLEASE realize the importance of a room by room ACCA Manual-J load calculation on EVERY job. I can't tell you how many new homes have equipment two or three sizes too big. I have to believe an educated consumer in today's fuel climate, would pay extra for a professionally installed and warranteed comfort system. I now do all my work word of mouth and musc of it is correcting poor installs in new construction. What would a homebuyer say to a 19 SEER A/C warranteed by the MANUFACTURER for the next ten years including PARTS AND LABOR? OK anyway...off my soap box. Good luck with the Energy Star.
    PS- couldn't your HVAC contractor provide the "professional" opinions you are looking for?
    RULE OF THUMB IS FOR HITCHHIKING.

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