Quick Check for Sizing Units to enough Airflow


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Old 07-03-09, 12:49 PM
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Quick Check for Sizing Units to enough Airflow

Quick Check for Sizing Units to enough 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 550-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
 
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Old 07-03-09, 02:14 PM
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Excellent post! I couldn't agree with you more. Air flow measurements should be part of every resi service call with questionable airfow. My experience has been that blower charts are rarely available, so velocity and area have to used to get a ballpark idea of airflow, which if you carefully take readings will be close. How many times have you seen overcharged units where the morons, er.. technicians, add refrigerant to fixed orifice systems to keep the evaporator from freezing when the real problem is insufficient indoor air flow? I lost count years ago.
 
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Old 07-03-09, 03:02 PM
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Installation & setup has to deliver close to its Nominal BTUH Rating

Right-on DaddyJohn,
A lot of A/C's are only delivering around 55 to 60% of their nominal BTUH Ratings.

You can't properly size a system if the installation & setup does not deliver close to its nominal BTUH Rating!

Air conditioning users don't have to check pressures which is illegal, all they need are the right low cost thermometers & a way to check airflow velocity.

Checking the condenser temp-split is also important.
The mfg'ers should list the condenser splits along with the indoor temp-splits on their performance data charts.

The new high SEER units don't have a very high split.
Do some EER & SEER Google searches, you'll be surprised what you might find, ha...

We need "broadcast media" help to Reduce the wasteful use of our energy resources. I'm work Tweeting on that on the most recent popular social media.
 
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Old 07-04-09, 03:10 PM
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Originally Posted by HVAC RETIRED
Quick Check for Sizing Units to enough 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
I had to correct an error.
I re-figured the top one at 600-FPM but left the 550-FPM there!

I thought someone wold notice it, but nobody checked my figures.

Working these equations can be very useful to any Tech.
(I corrected it)
 
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Old 07-04-09, 06:48 PM
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I know that the new higher efficency condensing units operate at a lower split. But what about the split across the evaporator, is that lower as well?
 
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Old 07-04-09, 11:07 PM
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There's a company that sells a line of instruments called Fieldpiece that includes an anemometer (and thermometer) cheap enough for DIY - $75 or less for a new one on fleabay.
 
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Old 07-05-09, 06:05 AM
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Higher 13-SEER Indoor Split

Originally Posted by theapprentice
I know that the new higher efficency condensing units operate at a lower split. But what about the split across the evaporator, is that lower as well?
The indoor airflow temp-split is a little lower but nothing like the lower outdoor temp-split.

A Goodman 13-SEER 1.5-Ton, at 75-IDB, 63-IWB, about 50%RH; 400-CFM per-ton airflow; it'll get around a 20-F split from 65-F to 105-F outdoors. It only drops 1-degree to a 19-F split at 115-F outdoors.

A very high humidity 71-F IWB will drop that indoor split to 12-F; & down to 11-F from 105 to 115-F outdoors.
 
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Old 07-05-09, 07:22 AM
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Might be Good enough for Govt Work - just need BallPark data

Originally Posted by PoorJohn
There's a company that sells a line of instruments called Fieldpiece that includes an anemometer (and thermometer) cheap enough for DIY - $75 or less for a new one on fleabay.
Poorjohn, I didn't find that one. Where's it at?
I found this one for $98.00.
Fieldpiece AAV3 Air Velocity with Temperature Head

Looks like it might be good enough for govt work.

Anyone could buy it & offer to check airflow, (for say $10), on anyone's system that appeared to have low airflow & not cooling good enough; 10 friends & relatives & it's paid for.

Put a proper worded small AD in a local paper & you might be able to charge $20 or a little more. Real low airflow would be costing them plenty on their utility bills.

HVAC Techs' have to learn to get the proper test instruments & get the airflow right, -before they do anything else!

You can buy a Dryer Magnehelic gauge for 60 to $70 dollars to test Static Pressures & compare to mfg'ers blower data.


I know the lower cost things to do first, that will usually correct those airflow problems.
 
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Old 07-05-09, 01:16 PM
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Originally Posted by HVAC RETIRED
Poorjohn, I didn't find that one. Where's it at?
I found this one for $98.00.
Fieldpiece AAV3 Air Velocity with Temperature Head

Looks like it might be good enough for govt work.
Not expensive enough for govt work! You're right about current offerings - in the last month four were sold (Advanced search/completed listings) for $59.98 to $66, plus $14.95 for the meter unit. So anyone who wants one should email the present sellers and point that out, maybe get a price break, or just sign up for email notification as they come on sale.

The parts I have so far - base and thermometer/anemometer work great, so I've got my eye open for more accessories. I did buy the data logger which should be really slick for recording temps over a period of time.
 
 

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