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  1. #1
    Join Date
    Aug 2005
    Posts
    3
    Hello all,
    This is my 1st post. I've lurked at this site for awhile. Lots of knowledgeable folks! I'm about ready to sign a contract with one of the HVAC contractor for my yet to be built new home. I'm somewhat concerned about the size of the units the contractor spec'd out which I think is too large (in my inexperienced opinion!). I hope some of you won't mind giving inputs.
    Here's the brands and spec's (I spec'd out 2 stage units with 2 stage thermostats). Lennox G61MPV 110,000 BTU 1ST floor, 90,000 BTU 2nd floor; HSXA19 (probably mis-spelled in contract, should be HSX19?) 4 TON 1st floor, 3 TON 2nd floor. Lennox Signature thermostat and WB2-17 humidifier will be included. Here's the info on my new house: approx 3700 ft2, 2 stories with 2 story front foyer (100 ft2) and 1 1/2 story hearth room (224 ft2) at the rear opening up to 2 story breakfast room (200 ft2); 1st floor, 9' ceiling, 1950 ft2, front of house facing west with 108 ft2 of windows, back of house facing east with 234 ft2 of windows; 2nd floor, 8' ceiling, 1650 ft2, front house with 108 ft2 of windows, back of house with 80 ft2 of windows. House construction is wood framed with R19 walls (2x6 framing) and R38 vented attic. Windows are vinyl, low E, triple pane. The full basement will be unfinished. The house will be fairly air tight (I will try to seal all potential leaks and apply mastic to all ducts). Location is St.Louis, MO.
    I did asked for them to do a Manual J calc, but did not think about D calc. My wife got the heat/loss gain calc over the phone which from what I remember off my head (don't have #'s with me now) is around 90,000 1st floor/30,000 2nd floor. Is this about right? I cannot remember the calc for the A/C.
    Also, I'm debating on whether I need HRV or fresh air intake (I don't think I need fresh air since I will not have above 300 cfm kitchen vent and above 100 cfm bath vents). How do I know whether I need one of this system or not?
    Thanks!

    [Edited by miked2 on 08-03-2005 at 08:50 PM]

  2. #2
    Join Date
    Mar 2002
    Location
    Northeastern Illinois
    Posts
    611
    mike,you can do your own Man J on this web site. It does sound oversized to me. The heating AND the a/c. I think you should do your own calcs,if possible. The contractor,[hvac] that is doing your house,is he pretty reputible in your area? Have you ever used him before? Check him out. Ask other people he has worked for. This is YOUR house and the HVAC is the heart of it. Also,maybe look into infloor tubing in your basement floor for future use. Just a thought.LOL
    If it ain't broke don't fix it!!

  3. #3
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,471
    Summer Outdoor Design conditions for St. Louis MO
    94 dry bulb 75 wet bulb, appox. 37% relative Humidity (Low RH)
    The apparent low RH helps reduce the A/C load capacity required.

    Considering your triple window panes, good insulation levels, and very low air infiltration those tonnages appear high to me. That is merely an opinion; if possible, I would have your local power company do the heat load figures on your home.

    A super insulated home would have R-24 wall insulation, R-38 in ceilings, tight-fitting storm windows or double pane windows; low air infiltration and a well sealed vapor barrier.

    Don't spend the money until you know they have the figures right!

    http://www.udarrell.com/air-conditio...tent-heat.html

  4. #4
    Join Date
    Aug 2005
    Posts
    3
    Thanks for the responses. Hmm, where on this site can I perform manual J? Since, I ain't no expert (even an ameteur) I have no way of making sure that what I'm getting is correct.

  5. #5
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    mike

    Sounds to me like the contractor is installing systems for yur comfort, not for yur electric bill.
    What will you set yur thermostat for?? What temp do you want to maintain in your new home??
    Remembering that as yur home gets older, it will not be as tight as when new. As the unit gets older, it will not produce quite as much as new.
    The sizes seem right to me if you are like some folks who want 72-73 inside temps., year round, even when it is very hot.
    Otherwise, yu can reduce size of systems, if you want to maintain only 75 or higher inside, or you don't mind those hot summer days where you may not get below 78-80 during the daytime hours.
    So yu either pay alittle more each month to insure comfort, or yu pay a little less and deal with some days that may be uncomfortable, but cheaper.
    I always size for comfort.
    I've always felt that I did not want my customer to be able to come to me & complain ( even two years later ) that his system was not keeping up. And so far, after 14 years, Ive NEVER had a complaint about either not being comfortable, nor about high electric bills.

    Hope this helps,
    Richard

  6. #6
    Join Date
    Jun 2002
    Posts
    460
    My knee-jerk reaction is that they sized the A/C using the 500 ft/ton rule and that the downstairs A/C is too big because there isn't much cooling load through either the ceiling or the floor.

    I'm easily cooling 1900+ square feet downstairs -- including a 300+ sq foot great room with 18 foot ceilings -- over a crawlspace in Atlanta with 2.5 tons (1100 square feet upstairs), and it's a remod and the house isn't tight at all. I keep it on 73F all the time and it still doesn't run non-stop on a hot day and it holds the temp. No, it won't do a quick pulldown, but no trouble at all holding it.

    [Edited by JoeSix on 08-04-2005 at 11:14 AM]

  7. #7
    Join Date
    Aug 2005
    Posts
    3
    I like to keep the temp around 76 deg during the summer and 72 in the winter. My wife's another story. We fight over the setting until we compromise which is close to what I mentioned above.

  8. #8
    Join Date
    Feb 2005
    Location
    Dothan, Al
    Posts
    3,453

    Miike

    LMAO - We have the same problem here, only reversed. My wife would not even run the a/c most days, I, however like it about 73-74. Its a constant battle!!!

    So, if you can set for 76 and don't mind a few days where it may not keep 76 for awhile, then smaller systems will save, for the most part.
    However, if yur wife really 'wears the pants', yud better stay with the contractors choice. LOL

    Richard

  9. #9
    Join Date
    Jun 2002
    Posts
    460
    Originally posted by miked2
    My wife's another story. We fight over the setting until we compromise which is close to what I mentioned above.
    Easy fix. Zone one of the systems and put her in her own bedroom.

  10. #10
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    You sure?

    >>Summer Outdoor Design conditions for St. Louis MO
    >>94 dry bulb 75 wet bulb, appox. 37% relative Humidity (Low RH)
    >>The apparent low RH helps reduce the A/C load capacity required.

    Could this be true, that 37%RH is low humidity at 94 degrees? For one thing, that humidity content will be much higher when cooled to room temperature -- I don't have the psychrometric calculator handy but I think 60-70%. I once moved from Houston to St. Louis and the summers didn't feel very different between the two. And absolutely NOBODY was heard saying "yes but it's a DRY heat!" when I lived there<g>.

    Regards -- P.Student

  11. #11
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,471

    Re: You sure?

    Originally posted by perpetual_student
    >>Summer Outdoor Design conditions for St. Louis MO
    >>94 dry bulb 75 wet bulb, appox. 37% relative Humidity (Low RH)
    >>The apparent low RH helps reduce the A/C load capacity required.

    Could this be true, that 37%RH is low humidity at 94 degrees? For one thing, that humidity content will be much higher when cooled to room temperature -- I don't have the psychrometric calculator handy but I think 60-70%. I once moved from Houston to St. Louis and the summers didn't feel very different between the two. And absolutely NOBODY was heard saying "yes but it's a DRY heat!" when I lived there<g>.
    Regards -- P.Student
    You are right, as the air temp drops the percent of RH goes up.

    By comparison to the high humidity climate zones with high temperatures, that would be considered a relatively low latent heat load area. The big factor is how much air infiltration the home has.

    http://www.udarrell.com/air-conditio...t-removal.html


  12. #12
    Join Date
    Jun 2004
    Posts
    146
    Useful link:

    http://www.overgrow.com/tools/rhumidity.php

    37%RH at 94 degrees = 70%RH at 75 degrees.

  13. #13
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,994
    Originally posted by miked2
    Here's the brands and spec's (I spec'd out 2 stage units with 2 stage thermostats).

    Lennox G61MPV 110,000 BTU 1ST floor,
    90,000 BTU 2nd floor;
    HSXA19 (probably mis-spelled in contract, should be HSX19?)
    4 TON 1st floor,
    3 TON 2nd floor.
    Lennox Signature thermostat and
    WB2-17 humidifier will be included.

    Here's the info on my new house:
    approx 3700 ft2, 2 stories with
    2 story front foyer (100 ft2) and
    1 1/2 story hearth room (224 ft2) at the rear opening up to 2 story breakfast room (200 ft2);

    1st floor, 9' ceiling, 1950 ft2, front of house facing west with 108 ft2 of windows, back of house facing east with 234 ft2 of windows;

    2nd floor, 8' ceiling, 1650 ft2, front house with 108 ft2 of windows, back of house with 80 ft2 of windows.

    House construction is wood framed with R19 walls
    and R38 vented attic.
    Windows are vinyl, low E, triple pane.
    The full basement will be unfinished.
    The house will be fairly air tight
    (I will try to seal all potential leaks and apply mastic to all ducts).

    Location is St.Louis, MO.

    ... My wife got the heat/loss gain calc over the phone which from what I remember off my head (don't have #'s with me now) is around 90,000 1st floor/30,000 2nd floor. Is this about right?

    [Edited by miked2 on 08-03-2005 at 08:50 PM]
    Sounds appropriate to me with your HSXA-19 selection.

    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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