Sanity check my manual J - plz


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Old 08-21-07, 02:01 PM
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Sanity check my manual J - plz

I've done a manual J and heat loss calculation for my house and would like a sanity check.

The house: 2300 sqft built in 1923 plus 900 sqft added in 2007. The old part is 2x4 balloon wall construction with retro-fit blown in insulation in the walls and R-23 in the attic with 15 brand new dbl insulated windows. The new area is 2x4 construction with R-13 in the walls and R-23 in the attic and 29 new windows. The house is above a crawlspace and insulated with R-19 in the floor joists.

Location: Norfolk, VA

My heat loss calculation with two different programs came in at 78,000 btu/hr

My cooling requirement manual-J with one program came in at 50,000 btu/hr.

Do these "look about right"? I'm fairly confident in the heat loss, the manual-j however I'm less confident in.

My HVAC guy is trying to sell me a 3ton (downstairs) and 3.5ton (upstairs). Seems awfully large to me. I know he doesn't want a call-back if doesn't cool enough, however I would rather not pay the extra up-front for over-sized units and then prematurely wreck them due to short cycling.

Thanks.

Scott
 
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Old 08-21-07, 04:49 PM
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Loads

Your heating load sounds about right but the cooling sounds low. I have heard many good reports about a program available on the web for about $50. That program is available at: http://hvaccomputer.com
 
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Old 08-21-07, 05:05 PM
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The program Grady reccomends is a good one. Also did your HVAC guy do the calcs ??
 
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Old 08-22-07, 10:12 AM
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I did the manual-j from a free online program that seemed to ask some good questions...but left me a little uncomfortable, prompting the post.

Have heard of the $50 program but am so darn cheap that was hoping the freebie would suffice. Probably need to bite the bullet so i know for sure.

Have had several hvac guys out to the house. Only one did a load calculation and it appeared to me to be a less comprehensive program than the free one I utilized. I have the printout on file, but seem to remember that the calculation was very high (around 125,000 btuh). In general, I think that my local industry standard is to eyeball it, go a large percentage bigger and be done with it. I'd obviously like to get it right...

Thanks guys.
 
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Old 08-22-07, 01:22 PM
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Don't trust the free software, I've tried them and it's not as good as the $50 program. I've used it a few times, and been pleased with the results.

Spend the money so you know you WILL be comfortable... If you be cheap, you may end up paying for it in the long run.
 
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Old 08-22-07, 04:23 PM
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Thanks guys. You're advice is 100% on the money. I'll buy the software and get it right.
 
 

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