CFM Confusion-equations and calculations
CFM Confusion-equations and calculations
(OP)
I am new to the forum, and have little to no experience with designing HVAC systems. I have searched these forums and other websites for help on the subject. It seems that each equation I find for certain calculations, I get a different answer than what another equation for the same topic gave me.
Anyways, my first basic problem is finding the required CFM for a heating system.
The area that needs to be heated is approx. 10ft X 4.25ft X 7 ft. I need to figure out the required CFM for this volume. I used an airflow equation I found on engineeringtoolbox.com, and calculating the BTU with the equation (exposed sq. ft*(max inside temp-min oustide temp) * heat loss value) [aka 1/R-value]=BTU. That equation could be wrong, I found that on a greenhouse website. Using other calculators I came up with smaller values.
In any case, the value I came up with was 2247.7 BTU. Seems low? Using the Airflow equation from engineeringtoolbox.com (h ttp://www. engineerin gtoolbox.c om/design- ventilatio n-systems- d_121.html) I came up with about 0.036821905 m^3/s or 78 ft^3/min. once again, seems very low.
Finding the necessary CFM is just the beginning of my problem, but if anyone can lead me in the right direction, I will attempt to take it from there. Thanks!
Anyways, my first basic problem is finding the required CFM for a heating system.
The area that needs to be heated is approx. 10ft X 4.25ft X 7 ft. I need to figure out the required CFM for this volume. I used an airflow equation I found on engineeringtoolbox.com, and calculating the BTU with the equation (exposed sq. ft*(max inside temp-min oustide temp) * heat loss value) [aka 1/R-value]=BTU. That equation could be wrong, I found that on a greenhouse website. Using other calculators I came up with smaller values.
In any case, the value I came up with was 2247.7 BTU. Seems low? Using the Airflow equation from engineeringtoolbox.com (h
Finding the necessary CFM is just the beginning of my problem, but if anyone can lead me in the right direction, I will attempt to take it from there. Thanks!





RE: CFM Confusion-equations and calculations
I notice you're a student at Purdue. Suggest you perhaps join AHSHRRAE and then either pick up a copy of their "Principles of Heating, Ventilating, and Air Conditioning" which is a sort of textbook which summarizes much info in their Fundamentals handbook, or "HVAC Simplified". Books appear to have bargain pricing and can be found at ashrae.org.
RE: CFM Confusion-equations and calculations
RE: CFM Confusion-equations and calculations
If you're not air conditioning an occupied space, usually the minimum ventilation required is 6 air changes and hour. Heating that air flow up to a comfortable temperature to supply to the space is where your heating load comes from. Q=1.08*CFM*delta T. The discharge temperature can then be set to offset the heat loss of the room.