Load Calculations
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  1. #1
    Join Date
    May 2008
    Location
    East Bay, Northern California
    Posts
    10

    Load Calculations

    I am going to have a new furnace and all new duct work put in my house to replace the 35 year-old system I have. So far, I've had 3 HVAC guys come out to give an estimate. One took down info for a load calculation, and the other two did not. They just looked around. Is it common for HVAC estimators to skip the load calculation for small houses? I would think that, after a while, these guys have enough experience to more or less know what a house will need, but maybe I'm wrong.

    I haven't received the estimate from the guy who took all the load calc info. The other two guys recommended the following:

    Carrier 58MTB (96% efficient, two-stage, 38,000 - 128,000 BTUh) with alumaflex ducts
    Bryant 3350AAV (96% efficient, two-stage, 40,000 - 138,000 BTUh) with wireflex ducts

    But, I decided that I'm not going to have the wire flex installed b/c it is going in the crawlspace under the house.

    My house is a 1,450 sq. ft. Craftsman Bungalow from 1922. There is insulation in the attic, but not under the floor. I don't know if the walls are insulated. There are 21 windows in the house, and 18 are double paned. The tempurature rarely gets below 45 degrees where I live. Does the Carrier with 38,000 - 128,000 BTUh sound about right?

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,590
    Not really.
    What size furnace do you have now.
    And is it a 80% or 90% efficient furnace.
    The guys not doing load calcs are probably just puttin in the same size as you have now.
    Figuring that being 3 stage it will make up for over sizing.
    In reality, it will use more gas then it should.
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  3. #3
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    Jacob

    First off the 58MTB is not 96% AFUE it is 93% 2 stage with multi speed AC motor. Were is he getting these numbers from. Tell him to give you full model number with proper %. On smaller house you can get way without doing load calculations, but it is risky and like OP said you don`t want to over size. Efficiency does matter when sizing based on output. Why not 58 Infinity 96% with variable speed blower.(cheaper electrical bills and quieter operation)
    Do it right the first time.

  4. #4
    Join Date
    May 2008
    Location
    East Bay, Northern California
    Posts
    10
    The HVAC guy did not give me the 96% figure. I found that on the Internet. I may have been a bit off. The full model number on the estimate is 58MTB080-12. I got the rest of the info on http://www.furnacecompare.com/furnac...finity-96.html.

    I just looked at the Carrier website, and I guess the 58MTB080-12 model must be what they call the "Performance 93 Gas Furnace." I have no idea why he picked that one over the Infinity 96. I'll ask him. Quieter operation sounds good to me.


    I don't know what size furnace I have now. It has a logo on it that says "Day & Night," but it doesn't say anything else on it. When I bought the house a couple of months ago, I was told that the furnace was about 35 years old. The ducts were these big sheet metal ducts, insulated with asbestos and dirty old fiberglass wrapping. I had them taken away.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,590
    I don't think it gets cold enough in Cal that you need anywhere near a 120,000 BTU 90+% furnace.
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  6. #6
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    Not that Big

    Quote Originally Posted by beenthere View Post
    I don't think it gets cold enough in Cal that you need anywhere near a 120,000 BTU 90+% furnace.

    This model is 80 000 BTUs, But I still think it is over sized unless house is built pretty shotty.(NO Insulation)
    Do it right the first time.

  7. #7
    Join Date
    May 2008
    Location
    East Bay, Northern California
    Posts
    10
    Really good to know. Thanks, guys.

    I will definitely ask for a load calc and ask why he is suggesting a furnace of that size. I did tell him I wanted a high efficiency furnace. I said, "80 or 90% efficiency." I don't know if that has anything to do with why he picked this one...

  8. #8
    Join Date
    Dec 2005
    Location
    San Jose, CA
    Posts
    465
    no comparison between alumaflex and wireflex. Alumaflex is superior quality.

  9. #9

    your neighbor in San Jose

    People in the East don't realize that the wind blows right through
    homes built in California before the last few years.

    And we have crawlspaces that typically are highly vented. No wall
    insulation, etc.

    I have a 1700 ft^2 1960's ranch in San Jose with ceiling insulation
    and all double pane windows, but nothing in the walls, a living room
    with no ceiling insulation. It does get down to the lower 30's here
    on winter mornings. And it will take my 80% 100,000 BTU trane furnace
    a good hour to raise the temp 7 degrees on those winter mornings.

    I have been gradually reducing air infiltration here from can lights, and
    a myriad of cracks.

    While there may be people in Alaska who are happy with smaller furnaces
    than mine in really tight new houses of the same size, I don't feel that I'm oversized a bit.

    And you might not need as much furnace if you don't turn the temp down
    at night. But turning the temp down at night for a poorly insulated house
    here is a real savings. Particularly with our typically huge day and night
    temperature swings.

  10. #10
    Join Date
    Apr 2008
    Location
    Alberta Canada
    Posts
    2,246

    Why don't you add some.

    How they can ever get away with building a house with little to no Insulation should be a crime. Why don't you add some, exspecially in ceiling where 40% more heat is lost and gained.
    Do it right the first time.

  11. #11

    No Insulation

    My house was built in the 1960's a time when we were about
    to land on the moon. My grandfather's 1960's house had several
    hundred amps of electrical heating coils buried in the driveway to
    melt snow. People talked about how nuclear power would be too
    cheap to meter.

    I have added insulation to the areas that have attic.
    I've added dual pane windows. Mainly because the old
    aluminum ones were so ugly.

    I've discussed this on my thread thermostat issues.
    An admirer of modern style homes reacted violently there to what
    I think the most viable method of insulation my living room ceiling,
    which is exposed beams, planks and concrete shakes on top.

    Remember this is a very mild climate. If energy gets so bad that super
    expensive insulation jobs pay off for us, Canada will have to be
    abandoned.

    I remember when I was in college everyone thought a practical
    fusion reactor was just a few years out ...

  12. #12
    Join Date
    Jun 2008
    Posts
    80
    Quote Originally Posted by jacobsisland View Post
    Is it common for HVAC estimators to skip the load calculation for small houses? I would think that, after a while, these guys have enough experience to more or less know what a house will need, but maybe I'm wrong.
    Just had some work done by a new HVAC contractor because the last one was hit and miss. They sent a guy who's been doing this for almost 40 years, has a house similar to mine in size/volume, and he did a load calc. He wouldn't give any recommendations until he went back to the office, ran the calculations and then we sat down and discussed various options.

  13. #13
    Join Date
    Jun 2008
    Posts
    80
    Quote Originally Posted by bobRitchie View Post
    If energy gets so bad that super
    expensive insulation jobs pay off for us, Canada will have to be
    abandoned.
    I'm not sure what energy costs are like in your area, but in the northeast we're pretty high. People are adding extra insulation to attics, walls, etc. all over the place. Its a given you add it to the walls if not already there. Heck you can get DIY videos, go online for DIY, and rent the blowers and material yourself at home depot. Its just not that expensive here.

    Quote Originally Posted by bobRitchie View Post
    I remember when I was in college everyone thought a practical fusion reactor was just a few years out ...
    They're not! Oh man am I in trouble. What about cold fusion?

    But seriously, you are right. My house was originally built with just electric baseboard heaters, back when the nukes panacea were gonna make cheap, affordable, safe, reliable electricity. Well that didn't quite work out, but everything is relative.

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