HVAC-Calc Guidance - http://www.hvacmechanic.com/ Forums
Original message
| Street
 |  "HVAC-Calc Guidance"
, posted Tue 3 Jun 15:49  
I have put in my home information in HVAC-Calc and sized the unit but I am having trouble understanding the CFM calcuation and duct sizing. I understand that 400 cfm/ton is typically used but when I select all the rooms for main return and supply trunks, it gives me just over 2600. What CFM should be entered? Also what should be selected for the speed of the supply and return duct systems.
Street
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Replies:
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| Freon
| "Re(1):HVAC-Calc Guidance" , posted Thu 5 Jun 17:37
The HVAC calc should give you the total cooling needs for your house. Start there. Then worry about how you distribute that cooling ability next.
If you need 60,000 BTU/hr of cooling then you will need a 5 ton system. At 400 cfm/ton, that's 2,000 cfm of air flow.
So the diameters of all the supply trunks from the air handler plenum have to be able to handle the 2,000 cfm, as do all the return lines to the air handler. As you run the supply main duct and you add taps to bedrooms etc, you decrease the available air flow and can reduce the diameter of the supply duct accordingly. That's the idea... it's an art, not science. Add dampers to all supply taps off the main duct so you can adjust later.
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| | dwcaveney
| "Re(2):HVAC-Calc Guidance" , posted Thu 5 Jun 23:04:
Hello Greenwood MS.
Basically the issue is you don't know what size blower you need until you size your ducts. So Freon is saying that you start by dividing cooling btu by 12000. xample 78000btu/12000=6.5 "ton" AH will move 2600CFM @.5"Wg.
I know by your question, you are at the point where you really are designing the system, everyone gets confused here. Most guys just say "screw it I'll put in a 5 ton system and some duct work"
It takes 288,000 BTUs to turn one ton of ice into one ton of water in 24 hours.
That rate of heat in BTUs applied to a ton of ice per hour, will result in the whole ton melting into water in 24 hours.
Refer to one ton of cooling, as a per hour rate. 12000 btu cooling, but refer to air flow as a minute rate 400cfm (clear as mud, right?)
288,000/24hrs = 12,000 BTUhour =200 BTU minute = 1 TON Cooling.
So you blow away 200BTU per minute with 400CF of air per minute through the evaporator coil.
The 400cfm = "ton" is actually a misnomer, but we all use it.
In California PG&E is telling us to use 480cfm to blow away 200BTUm through the coil.
Here's a little curve for you, packaged units are not the way to size air conditioning and heat. Choice of Coil, Compressor & Condenser, Furnace & Air Handler are all part of sizing process.
ie: 5 ton C&C, 4 ton coil, 80k btu furnace, 2000cfm blower.
Don't take it from me, I don't give a hoot (not a wise old owl)
I'm a freshman in HVAC.(which I'm sure you can tell from my prior posts)
If it gets any more complicated I'll buy a window unit for the family room.
Hang in there Street
DW
[this message was edited by dwcaveney on Fri 6 Jun 12:13] |
| dwcaveney
| "Re(1):HVAC-Calc Guidance" , posted Thu 5 Jun 13:27:
Sorry Street,
Sizing is a lot of work, hope I didn't mix you up telling you to size it right. Your bidders are the best of the best?
Some guys like to know what they are getting, you will.
"When it's all said and done, there'll be a lot more said than done."
If I understand your question correctly:
2600 CFM (high speed rated @.5"Wg) 6-1/2 ton?
example: Manufacture X 5 ton .1"Wg=2335cfm, .2=1970, .3=2110, .4=2030, .5=1950 (2000=5ton)
Trunk and Branch Velocities
Supply side T=600fpm, B=600fpm
Return side T=600fpm, B=400fpm
External Static Pressure minus Device Pressure Losses equals Available Static Pressure, Jojo refers to. (i think?)
Normally you put in TEL Total Effective Length (similar to DPL) of Duct System, includes total duct length, coil, fittings, then size ducts for air delivery.
Did you buy a one house HVAC-calc license? Checked that whole house fan and gable fan suggestion, lot of guys say won't work in your climate.
Check this out http://www.trane.com/Commercial/DNA/View.aspx?i=1250
Good luck
DW
[this message was edited by dwcaveney on Thu 5 Jun 14:26] |
| Jojo98
| "Re(1):HVAC-Calc Guidance" , posted Tue 3 Jun 21:52
You need to know how much static press. your A/C should operate at first.
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