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  1. #1

    Variation in HVAC-Calc Fireplace Heat Loss?

    I bought the HVAC-Calc residential version I saw mentioned here because I wanted to get some idea of what variability might be in the load calculations results depending on things like ceiling insulation and so forth.

    But I was surprised at the program's BTUH range associated with fireplaces! The program provides 3 choices of fireplaces - Best (combustion air from outside), Average (glass doors, damper), and Poor (presumably a damper only - but not listed). The BTUH figures used by the program are 2543 (Best), 5086 (average), and 15,257(Poor). Since the difference is so large I have to wonder how much variability might there be in the Poor value? I have 2 "Poor" fireplaces (no glass doors) so a 5,000 BTU variance would be 10,000 BTU's out of my home's total 80,000 BTU.

    The other big variable seems to be the Infiltration value (I used the "normal" default value), which is about 18,000 BTU out of my 80,000 BTU total. That seems to be something of a guess since there are places to simply plug-in a different value. How much variance might there be in that number?

    I'm not knocking the program I'm just trying to get a handle on how much faith I should put in sizing the furnace accordingly when the two biggest contributors appear to have some considerable variability. BTW, the numbers I'm getting from contractors are lower than the program produces by about the difference in the Poor and Average fireplaces.

    I'd be interested in anyone's thoughts or guidance. Thanks.

  2. #2
    Join Date
    May 2004
    Posts
    497
    It's a good program, but what your missing is the value used in the calculation.

    I remember using the program long ago and tried each version like you did.

    But the calculations are correct. It really helps you understand where your money is going. I would look at fixing those issue with your BTU loss and your be better for it.

  3. #3
    Quote Originally Posted by r-290 View Post
    It's a good program, but what your missing is the value used in the calculation.
    I'm sorry, I don't understand what you mean. I'm missing "the value used in the calculation"? I listed the values for Best, Average, Poor in the calculation in my original post.

  4. #4
    Join Date
    May 2004
    Posts
    497
    Sorry, within all energy programs they use values to calculate the BTU loss or gain.

    There is some math behind the avg or poor, that's what I meant to point out. you just don't see those values.

    One tip would be to clear or delete the item and readd it, just to make sure its enter right.

  5. #5
    Quote Originally Posted by r-290 View Post
    One tip would be to clear or delete the item and readd it, just to make sure its enter right.
    Yeah, I did that several times because I was so shocked at the numbers.

    The fact that the program doesn't even ask for the fireplace size and there is this enormous difference in fireplace values is disconcerting. Then when you figure that the 2 fireplaces plus what seems to be just a rule of thumb guess of the Infiltration value make up 60.6% of my total house BTUH makes me start to question whether this load calculation stuff is any more accurate than just picking the size based on the house's total square feet. I already have contractors bidding different size furnaces after doing their own load calculations.

  6. #6
    Join Date
    Nov 2010
    Location
    Montreal, Quebec.
    Posts
    71
    I don't understand how a proper load calc can be done without a blower test door and agree with you, too much variance on this one item alone. I did my own load calcs with two different programs and then had it professionally done with a blower test door. Results are that I was 20kBtu high.

  7. #7
    Join Date
    May 2004
    Posts
    497
    Quote Originally Posted by Norm01 View Post
    I don't understand how a proper load calc can be done without a blower test door and agree with you, too much variance on this one item alone. I did my own load calcs with two different programs and then had it professionally done with a blower test door. Results are that I was 20kBtu high.

    Some guys use the machine to pressurize the building and then show you where the air is leaving the house. This is where the infiltration number comes into play. Wind has a huge factor on number of air changes per hour on your house. So if you live in say the mid west, the value will be higher than where I live for the same house. So you need to adjust for that. On homes that are 50+ years old its hard to say, as a lot of things could have been done to the house over the years, or nothing at all. This is where the blower door testing machine can help figure out the air changes per hour.

    If I remember correctly that program should have a report showing the BTU gain/loss. Maybe post that so we can see what your seeing.

  8. #8
    Join Date
    Mar 2010
    Posts
    195

    You ARE CORRECT, JUST a bunch of guesses

    Quote Originally Posted by Norm01 View Post
    I don't understand how a proper load calc can be done without a blower test door and agree with you, too much variance on this one item alone. I did my own load calcs with two different programs and then had it professionally done with a blower test door. Results are that I was 20kBtu high.
    Yo,

    Software only as good as inputs. But...On old houses You don't know the inputs. You just guess.

    Not just Fireplaces. But As you mention, your Infiltration rate Assumption is a Big Guess. With, as you also report, very wide tolerances. Another one is Duct loss/gains, Could be 7% could be 27%, you don't know till you test.

    What the guys do, is learn what guesses make the numbers come out the way they like, then they are smart, and the program is great.

    But wait,,what they really did was start with the answer. These facts are hard for some to accept.

    ACBD

  9. #9
    Quote Originally Posted by AC Bad Dog View Post
    Not just Fireplaces. But As you mention, your Infiltration rate Assumption is a Big Guess. With, as you also report, very wide tolerances. Another one is Duct loss/gains, Could be 7% could be 27%, you don't know till you test.
    Yeah, I was ignoring the Duct loss issue, which as you say again looks like nothing but a guess. At this point I have little faith in these calculations because the 2 fireplaces and the Infiltration rate assumptions make up 61.1% of my total heat loss. I started out a week ago thinking I was going to go with the contractor that convinced me they most closely used and followed the load loss calculations (i.e. the one I thought was the most technically up to date and scientifically savvy). Now I think I will go with the one that has the most experience and can explain why they don't JUST follow the load calculations.

  10. #10
    Join Date
    Mar 2010
    Posts
    195

    You Got It

    Quote Originally Posted by SeekingHelp View Post
    Yeah, I was ignoring the Duct loss issue, which as you say again looks like nothing but a guess. At this point I have little faith in these calculations because the 2 fireplaces and the Infiltration rate assumptions make up 61.1% of my total heat loss. I started out a week ago thinking I was going to go with the contractor that convinced me they most closely used and followed the load loss calculations (i.e. the one I thought was the most technically up to date and scientifically savvy). Now I think I will go with the one that has the most experience and can explain why they don't JUST follow the load calculations.
    Go with experience. Make sure they are looking at that Ductwork!

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