While researching for air pressure gage on Monday, I came across your blog and post regarding HVAC Energy Conservation ” Blog Archive ” Reply to Carmen which looked very interesting to me. I am very impressed with your site. The quality of the design…

Actually, even on service calls where there are cooling problems the ductwork should have a quick Manual D performed.

Then take the ESP static pressure & compare to blower graph or chart, also take the FPM duct velocity.

Then do a quick estimate of airflow per equipment tonnage.To find area of a round duct; Duct diam is 7″; 7″X7″= 49-sq.ins., X’s .7854 = 38.04845-sq.ins divided/ by 144= 0.2672541-sq.ft. area X’s FPM Velocity 600-FPM = 160.35246-CFM X30 = 4,810.5738 each 7″ run X’s 6 branch runs = 28,863-BTUH, or airflow for 2.4-ton.

That would also be good for 2-ton; at 550-FPM velocity X’s 0.2672541= 147-CFM X 30 = 4,410-BTUH each run X 6-runs = airflow for 26,460-BTUH.
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The Return-Air & Return-Air Filter area are critical to proper airflow & system performance. The Return-Air filter area should be sized for 300-FPM velocity; quick ballpark to use is 2-CFM airflow per sq.in. of filter area. Remember even the filter grille’s free-air-area is only ballpark 75% of the listed size! I.E., 1000-CFM / 2= 500-sq.ins., X’s .75= only 375-sq.ins.

375-sq.ins., / 144= 2.6-sq.ft. | 1000-CFM / 2.6= 384-FPM or, above 300-FPM & that is without the filter! You cannot have too much filter area!

The design of the some OIL furnaces’ with their ultra large heat exchanger coming to near the top of the furnace and the blower set to the side at the bottom of the furnace, can be an engineered airflow problem. The blower set to the side blows against the heat exchanger and the back of the furnace which blocks directional airflow velocity thus generating high initial velocity & static back pressures against the blower.

Most installers set the A-Coil directly on top of an oil furnace with no transitions resulting in another restriction, between the huge HEAT-Exchanger resulting in blocking of directional velocity airflow, killing velocity flow & causing a huge leap in (ESP) static pressure.

A Major “Oil Furnace” Airflow Problem Fix
Regal & Hallmark & nearly all Oil Furnaces - Installation manuals

Download the installation & service manuals

If the oil furnace is used in connection with summer air conditioning the evaporator coil must be installed at least 6” above the oil furnace for proper airflow. Distances less than 6” will result in decreased airflow. Make sure outlet supply takeoffs are NOT blocked by the coil. In all cases, refer to the manufacturers’ data for static pressure losses to ensure the total system static pressure does not exceed 0.5” WC.
MOLO Plumbing & Heating sets the A-Coil at least 6″ above a Thermo Pride OL 11 oil furnace. They know the importance of unrestricted airflow! http://www.molocompanies.com/plumbingandheating/index.html

Never sell units requiring more airflow than the duct system & Blower will support! - udarrell - Darrell Udelhoven >Darrell Cynergy Home HVAC Energy Raters Listen While ReadingDarrell Udelhoven

Reply: It might lower it a hair, but very little. - udarrell

It depends on whether you are going to use the lower 850-CFM setting all the time.

If the lower setting, then charge it to that 850-CFM & at a little above the normal load conditions it will usually be operating at.

The higher the indoor humidity the higher the load through the evaporator.

Use both Superheat & subcooling to check for appropriate operation of a TXV metering device.

Use just Superheat for a fixed piston device.
Use the diagnostics in all of my Web pages:
http://www.udarrell.com/ac-trouble-shooting-superheat-subcooling.html

- udarrell

If you are planning on lowering the CFM across the evaporator to remove additional humidity, should you first charge the system with the nominal air flow suggested by the manufacturer ie. 1000CFM with 2.5Tons or lower the speed first ie. 850CFM and then charge.

Respectfully yours,

Practice energy conservation and learn all you can about HVAC.

http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html

- udarrell

Learn all you can, we need to conserve on energy use & lower our utility bills!

http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html

- udarrell

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- udarrell