Figuring CFM Requirements For Heating
Rockford
| Posted in Energy, Heating & Insulation on
Hello All,
I’m getting ready to install our existing 75,000 Btu forced air furnace (heat only- we don’t need AC) into our new house, but need a little help in figuring what CFM each room will need.
I have a “Ductulator” slide rule to help determine duct sizes and after a little research, it looks like .05 is the desired frictional coefficient to use for flexible duct.
Any quick, or “rule-of-thumb” methods for calculating individual room CFM demands?
Thanks!
Jim
Replies
You first need to do a load calculation in BTUs for each room because factors like exposure, window size, and local heat sources all make a difference. Once you have the load calculated for each room, then you can worry about the air volume needed to deliver it.
This is the formula for cfm:
Qw, Btu/hr / (60 min/hr * pw lbs/ft3 * cp Btu/lb deg F * delta t deg F) = CFM
<!----><!----> <!---->
For standard air,
Qw, sensible heat loss
pw, density = 0.075 lb/ft3
cp, specific heat = 0.24 Btu/lb·deg F
Delta t is the temperature difference (rise in heating) between return air (a nominal 68 deg F) and supply air which can vary over a wide range and is influenced by heat exchanger temperature and air flow rates. A nominal 50 to 60 deg F rise is common but can higher or lower. Check with the manufacturer.
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The same formula for water can be used where the output is USgpm
For 100% water, density at 68 °F = 8.34 lb/USgal and specific heat = 1.0 Btu/lb·°F
This link will take you to an Excel spreadsheet which you can play with to compare CFM's based on delta t etc.
http://www.healthyheating.com/tools/Water%20to%20Air%20Flow%20Load%20Comparison%20Spreadsheet.xls<!----><!---->
RBean
web: http://www.healthyheating.com
blog: http://wonderfulwombs.typepad.com
Edited 11/27/2005 11:48 pm ET by RBean