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Old 12-11-2009, 09:33 PM   #1
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Manual J


Can anyone give me a link to get a Manual J

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Old 12-11-2009, 09:55 PM   #2
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My local library was able to borrow the 1985 version.

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Old 12-11-2009, 09:57 PM   #3
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Quote:
Originally Posted by Yoyizit View Post
My local library was able to borrow the 1985 version.
I meant just a website to fill in information
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Old 12-11-2009, 10:17 PM   #4
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It'll cost ya'. . .I've seen several. Probably ACCA runs one, maybe ASHRAE, another.
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Old 12-12-2009, 06:08 AM   #5
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Here is a link to one. It has a 49 dollar fee. But will let you do what ifs. As far as seeing if an improvement to your home will reduce your heating and cooling needs. Its worth the money.

HVAC Calc
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Old 12-12-2009, 11:13 AM   #6
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Quote:
Originally Posted by beenthere View Post
Here is a link to one. It has a 49 dollar fee.
"In decision theory, the expected value of perfect information (EVPI) is the price that one would be willing to pay in order to gain access to perfect information.[1]"

So today, in the US, $49 is the expected value of 'slightly imperfect' information regarding a $5K to $10K purchase.

If your furnace is oversized you waste money and lose comfort for years and
if it is undersized you lose comfort for years.

This might not be a bad price, at all.
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Old 12-18-2009, 12:46 PM   #7
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Manual J


Right sizing, or confirming Manual J
This won't work with a variable output furnace.

Toff = 65F, when you don't need heat.

Tper is the 97.5 percentile cold temp, depending on your area, when your furnace is running all the time.

The Slope = (Tper-65)/100

Tout = the average of 5 or 10 outside nighttime temps. Taking the average cancels out the effect of rain, snow, wind, clear weather, etc.

The furnace Duty Cycle in percent at Tout = 100(on time/(off time + on time)) = (Tout-Toff)/slope
Average this over a few cycles.

E.g.
Tper = 5F
so the slope is (5-65)/100 = -0.6

Tout = 29F
so the DC is 100(29-65)/(-0.6) = 60%
If your furnace has this duty cycle at this outside temp. it is right sized.

Last edited by Yoyizit; 12-18-2009 at 10:34 PM.
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Old 12-18-2009, 09:43 PM   #8
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I used the one beenthere suggested and it was great. I was really surprised at the tonnage changes as I removed or added insulation, added shade, house orientation, etc.
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Old 12-18-2009, 10:47 PM   #9
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Quote:
Originally Posted by 7echo View Post
I used the one beenthere suggested and it was great. I was really surprised at the tonnage changes as I removed or added insulation, added shade, house orientation, etc.
Good.
I hope you do better than my house/furnace match.

For an avg. January temp of 36 degrees and my 169,000 input BTU/hr furnace being on 1/4th of the time [from my gas bills] in that same January, my furnace/house is good down to -51F. You can use this averaging method with single stage or mult-stage furnaces.

I doubt this is the 97.5 percentile cold temp. for the District of Columbia area.

Last edited by Yoyizit; 12-18-2009 at 11:22 PM.
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Old 12-18-2009, 10:51 PM   #10
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D.C. Design temp is 14F.
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Old 12-18-2009, 10:54 PM   #11
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Quote:
Originally Posted by beenthere View Post
D.C. Design temp is 14F.
The more I know, the less happy I am.
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Old 12-18-2009, 11:07 PM   #12
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Quote:
Originally Posted by Yoyizit View Post
The more I know, the less happy I am.

Time for that new properly sized furnace.
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Old 12-18-2009, 11:37 PM   #13
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Quote:
Originally Posted by beenthere View Post
Time for that new properly sized furnace.
The independent consultant I hired, Mr. C. O. Detector, hasn't yet given me the go-ahead on that project.

And I use smoke detectors for cooking timers.

From what I've read on several forums, my gas bills won't change with a new unit, which kinda' pushes the payback period for this 10 kilobuck purchase out to infinity.

Is that smoke I smell???
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Old 12-19-2009, 05:43 AM   #14
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Quote:
Originally Posted by Yoyizit View Post

From what I've read on several forums, my gas bills won't change with a new unit, which kinda' pushes the payback period for this 10 kilobuck purchase out to infinity.
Not 100% true.

Short on times mean its not getting to its full efficiency on most calls.
Undersized duct work means you have higher duct losses.
High discharge air temps mean more heat is going out the flue then need be.

Proper sized furnaces on more appropriately sized duct work gives even heat through out the house. Making it more comfortable. And often allowing people to set their thermostat to a lower temp(even 1 saves money on the heating bill. If you believe in statistics ).
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Old 12-19-2009, 12:13 PM   #15
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Quote:
Originally Posted by beenthere View Post
Here is a link to one. It has a 49 dollar fee. But will let you do what ifs. As far as seeing if an improvement to your home will reduce your heating and cooling needs. Its worth the money.

HVAC Calc

Shill and conflict of interest!

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