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Old 07-31-2009, 04:06 PM   #16
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HVAC flow rates and pressures. Cunning install ideas?


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Old 07-31-2009, 04:59 PM   #17
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HVAC flow rates and pressures. Cunning install ideas?


I didn't see a response to your question re: why heat calcs are based on sq footage, rather than cubic footage.

Simply put, you are thinking of your home as having a volume. While this is true, the gain or loss of heat is determined by the 'envelope'.

Such factors as construction, insulation, orientation to the sun, geographical location, etc., determine that value. So, a house with a given volume could be designed as a structure with long/low walls, but greater floor area, and different orientations to the sun and insulation values. Each, while having the same volume, would show different heat/loss values.

You can download and try HVAC-calc (hvac-calc.com) to get the idea.

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Old 07-31-2009, 05:07 PM   #18
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HVAC flow rates and pressures. Cunning install ideas?


Not really disagreeing with you but it would seem a simple sf to volume ratio would allow a more accurate calc. Though I have not looked at the calc you recommended, I could sure see a huge difference between a house with 7ft ceilings and a house with 10ft ceilings in which the sf would remain constant.

I realize that heat load has most to do with the surface area of the exterior walls and such. I know when I have discussed with a good friend and HVAC designer, he looks at the surface area exposed closely.

Yes, I can certainly see your point that volume may not tell the whole story on heat loads but I sure think it could help in the accuracy department.
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Old 07-31-2009, 08:33 PM   #19
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HVAC flow rates and pressures. Cunning install ideas?


Quote:
Originally Posted by viper View Post
Basic numbers that I have right now are:
6" - 80-100cfm
7" - 115-150cfm
8" - 175-225cfm

These all assume flex duct, .1in H2O ballpark, and no more than a 50ft run. Does this seem in line? My biggest problem is that damn flex is hard to calculate due to the increased turbulence in the line. The stuff I want has the foil inner wall and not just the bare insulation.


Also, to maintain max efficiency, the calculated flow of the RA ducting should be about 20% larger than the output to minimize depression and noise at the RA ducts.

Am I in the ballpark with this stuff?

Also, does anyone have some files on Manual J calcs? I would like to learn a bit more on heat loss/gain factors commonly used. My goal is to be able to calculate ROI of different approaches to climate control whether it be added insulation, white exterior paints, etc.

As noted, we are going to be doing several things in our other new building including closed loop floor heat/cool. I will run a a supplemental HVAC system on that but would like the floor system to handle plenty. I want to design a wood or oil fed boiler for the water heating in the floor. Heating 12K sf can get expensive!

Code in KS limits flex to only 14 ft run
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Old 07-31-2009, 08:55 PM   #20
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HVAC flow rates and pressures. Cunning install ideas?


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a simple sf to volume ratio would allow a more accurate calc.
Volume would be more accurate but also more trouble to measure.
Evidently sf is 'close enough'.
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Old 07-31-2009, 09:00 PM   #21
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HVAC flow rates and pressures. Cunning install ideas?


Quote:
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Volume would be more accurate but also more trouble to measure.
Evidently sf is 'close enough'.
Infiltration is a biggy too

You want t load calc program HVAC-CALC
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Old 07-31-2009, 09:31 PM   #22
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HVAC flow rates and pressures. Cunning install ideas?


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Code in KS limits flex to only 14 ft run
I am trying to think of a good reason to do that???? Lack of control of radii in turns? Too much turbulence and variables in flex? I could certainly see a wide margin of flow rates depending on how it was installed. I have seen 100% flex installs in KS and even an HVAC installer was recommending 100% flex in my installation.

Just asking
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Old 07-31-2009, 09:50 PM   #23
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HVAC flow rates and pressures. Cunning install ideas?


Quote:
Originally Posted by viper View Post
I am trying to think of a good reason to do that???? Lack of control of radii in turns? Too much turbulence and variables in flex? I could certainly see a wide margin of flow rates depending on how it was installed. I have seen 100% flex installs in KS and even an HVAC installer was recommending 100% flex in my installation.

Just asking

Flex makes guys lazy and it makes the worst duct system when misapplied.

It's easier to just throw a long run of flex down and go on to the next one than it is to run rigid metal.
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Old 07-31-2009, 09:58 PM   #24
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HVAC flow rates and pressures. Cunning install ideas?


A real load calc does take volume into consideration.

Only hack short cuts/guesses don't.
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Old 08-09-2009, 01:22 AM   #25
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HVAC flow rates and pressures. Cunning install ideas?


Viper,

Are you from up by Paradice?

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