Knowing & testing for the correct, adequate duct system airflow is the first essential toward efficient A/C performance.

Low cost test instruments:

Get a low cost Testo Tester & along with CFM ballpark figure actual BTUH & EER - the information on it:

http://www.amazon.com/Testo-605-H2-H.../dp/B000774B6A

Everyone, very low cost anemometer to get airflow FPM Velocities, get it:

http://www.amazon.com/Crosse-Technol.../dp/B0002WZRKE

This should be helpful: (Edited 4/5/10)

Duct system CFM X* 4.5 @sea-level, or use X* 4.25 if 1000' above sea-level, X* change in enthalpy = BTUH (Ballpark) Operating Performance.

"U Must Right Click Link & open in New Tab," look-up wet bulb enthalpy figures on chart," & figure enthalpy change.

Wet Bulb Enthalpy Chart

http://www.udarrell.com/wet_bulb_enthalpy_chart.pdf

Then have a manual "J" heat-gain/heat-loss load calc done, followed by a manual "S" for equipment sizing & then a manual "D" for proper ductwork sizing.

All registers, grilles, filter racks, & diffusers must be located & sized for optimal efficient performance of the system. Order Turning Vanes for long radius 90-ells; use long radius 45's.

Identifying your registers/diffusers & their (Ak) sq.ft. area, so you can multiply the FPM Velocity times the Ak to get the (CFM) Cubic Feet per Minute airflow from that register.

Have or do a manual J heat-gain calc for each room. If a room calls for 3,500-BTUH; first divide 12,000-BTUH by the CFM PER TON you want to use.

I.E., Wet coil, 12,000/400=30-BTUH per each CFM; Wet coil 12,000/425=28.235294; 3000/28.235-= 106.25-CFM; Dry coil, 12,000/450= 26.6666-BTUH; 3500 / 26.6666= 131.25-CFM

If register/diffuser has the same (Ak) free-airflow-area, as the duct run!

Room calls for 3,500-BTUH, using 450-cfm per/ton dry coil or 26.6666-BTUH per CFM= 131.25-CFM.

I.E., 6" rd duct .6*6=36*.7854=28.2744sq.ins/144=0.19635-sq.ft.; 131.25-cfm / 0.19635-sq.ft= or 668-fpm velocity.

Use this page for the easy math formulas & links to use:

http://www.udarrell.com/sizing_ac_duct.htm

Here is a link that might be useful: Duct System Airflow Formulas & Links