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Old 01-29-2009, 12:47 PM   #1
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heat loss (sensible and latent)


i am trying to size a furnace for a house. i am a little confused on how to obtain a figure for the design latent heat loss, and how knowing the latent and sensible heat loss figures are used in making a blower selection. i just want to install a furnace, no a/c.

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Old 01-29-2009, 01:26 PM   #2
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heat loss (sensible and latent)


Latent, is for cooling.

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Old 01-29-2009, 01:30 PM   #3
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heat loss (sensible and latent)


A rough way to figure which does not take into account window or door sizing or which direction there is exposure would be square footage of house X height of ceilings X number of exposed surfaces. For example a ranch of 1000 sq ft with 8 foot walls would require approx 1000 X 8(ceiling height) X 5(number of exposed sufaces) on a slab would be around 40,000 btus. This example on a crawl space would be 48,000 because of 6 exposed surfaces. This calculation would need to be done for each floor and added to total. Again this is a very rough figure to work from.
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Old 01-29-2009, 04:44 PM   #4
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heat loss (sensible and latent)


i would like to do a more in depth calculation for my own sake. i dont want to spend 389$ for the program, so i am trying to decide between buying manual J from acca for $60, or paying the $50 for a one time use of their program. im leaning towards the former because i would like to have something tangible to you use in the future.
is it as accurate to do these calculations by hand? which would you do?
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Old 01-29-2009, 04:53 PM   #5
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heat loss (sensible and latent)


Yea, by hand is just as accurate.
Either method is GIGO.
So no matter if you use a program, or long form, its only as accurate as the data you enter.

Program is faster.
Allows you to do what ifs. In just a minute.

What if you added insulation here.
What if you sealed these windows.
What if you added a fireplace.
etc.
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Old 01-29-2009, 05:40 PM   #6
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heat loss (sensible and latent)


that reminds me, i do have a fireplace and its drafty as h***. there is not flap door in it. what should i do about that?
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Old 01-29-2009, 05:42 PM   #7
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heat loss (sensible and latent)


Get a damper installed ASAP.

Its letting most of your heat go up the chimney.

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