HVAC Energy Conservation

This page does not explain everything you need to know about proper duct sizing a system for optimal comfort, but provides some general guidelines and concepts.

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*You could ballpark the CFM using the static test & the air handler’s graph. You could measure the CFM delivered to each room with a hood Alnor Balometer, it’s usually the best instrument to use, but not cheap. Anemometers are reasonably priced & should be used for checking velocity from diffusers.

Measuring the air velocity from diffuser’s is a bit tricky because you should use the diffuser mfg’ers data which you should always have with you.You can usually get the diffuser mfrs data, say its a 1.5-Ton system that already has 6″rd branch duct runs, to achieve enough CFM airflow, you need close to 700-FPM velocity in the ducts.

I would want to use a diffuser with a little more free open sq.ft area than the 6″ duct  which area is 0.19635-sq.ft., say middle of the room in the ceiling; Hart & Cooley 2-way curved blade 12×6 has Ak .235-sq.ft. Free-Air-Area delivering 140-CFM at 600-FPM - diffuser face velocity.

This would help lower the velocity of the duct through the diffuser & reduce air noise.Throw is 7.5-ft toward each wall. Terminal velocity at the occupant level is 75-FPM.

If one diffuser face-velocity were to yield 135-CFM times 5 outlet runs - yields 675-CFM *X’s 26.666-BTUH per CFM = <18,000-BTUH, - that’s 450-CFM per-ton of cooling means less BTUH per one CFM.

Whatever CFM you need for that room or area, divide the Sq.Ft. free-air-area into the requires CFM to get the FPM velocity.Say we’re using 450-cfm/ton of airflow; 12,000-BTUH / 450= 26.6666-BTUH per CFM.

You need 3600-BTUH for that room, 3600 / 26.666 that’s 135-CFM / by .235-sq.ft. diffuser area is only 574-fpm face velocity.  Using 400-cfm per/ton / 12000-BTUH is 30-BTUH per CFM.Below is an example of this problem with a (Thermo Pride OL 11 Low Boy Oil Furnace)Scan of My Oil Furnace Blower Curve While the image loads switch back & read this pageOIL HEAT FUEL COST COMPARISONS New!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

http://www.boyertownfurnace.com/ProductDocuments/index.aspx

Download the installation & service manuals

http://www.boyertownfurnace.com/ProductDocuments/HallmarkONLYManual042909.pdf 
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

 Taking the manifold gage head pressure & gage condensing temperature is very important data. Coupled with a condenser air discharge temp-reading, if the condenser gage pres/temp is too high compared to the TH reading, there may be non-condensibles in the system. 

Also, there is a legitimate formula I use to determine the operating BTUH it is delivering at all the data taken.

All the mfg’ers ought to list the condenser temp-split (it varies with EER & SEER) just like they list the indoor split, it is valuable trouble shooting info.

You can also use the condenser temp-split (it contains both Latent & sensible heat) combined with the indoor data to plot the indoor CFM. I was never good at math, but those equations have to balance, & they do work!

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Quick Check for Sizing Units & Ductwork to Achieve Adequate Airflow 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.Never sell units requiring more airflow than the duct system & Blower will support!- udarrell  >Darrell Cynergy Home HVAC Energy Raters Listen While Reading