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Old 07-27-2011, 06:55 AM   #1
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How TO Read Manufacturer's Fan Data


This is related to my other thread but thought it should stand alone. I have a 22 year old Bryant. The fan performance table shows cfm's for various static pressures with cfm's dropping off as static pressure increases. I assume this table is talking about fan discharge static pressure with the assumption that the fan inlet is not starved meaning the inlet pressure is basically atmospheric?

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Old 07-27-2011, 07:17 AM   #2
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How TO Read Manufacturer's Fan Data


That's total external static. Includes supply duct,return duct,evap coil, filter and registers with a furnace. An air handler would not count the coil since it internal.

To test return stick one probe between the filter and blower. To test supply remove the furnace limit switch and install the probe there. A wet 3.5 ton coil should be .25-.3 by itself.

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Last edited by Marty S.; 07-27-2011 at 07:24 AM.
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Old 07-27-2011, 08:22 AM   #3
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How TO Read Manufacturer's Fan Data


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Originally Posted by Marty S. View Post
That's total external static. Includes supply duct,return duct,evap coil, filter and registers with a furnace. An air handler would not count the coil since it internal.

To test return stick one probe between the filter and blower. To test supply remove the furnace limit switch and install the probe there. A wet 3.5 ton coil should be .25-.3 by itself.
This is what I am not understanding. Let's say I have two cases with the same fan with a total external static pressure of 0.5" wc. In Case A, all of the 0.5" occurs on the supply side across the evap coil and supply ducts/registers - iow, I have no restriction on the return side. In Case B, I have 0.25" wc on the supply and -0.25" wc on the return. The fan has a total external static pressure of 0.5"wc in both cases. But, in Case A, I have much more air flowing in the supply side which creates the higher supply static pressure. What am I missing here?
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Old 07-27-2011, 01:50 PM   #4
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How TO Read Manufacturer's Fan Data


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Originally Posted by artbuc View Post
This is what I am not understanding. Let's say I have two cases with the same fan with a total external static pressure of 0.5" wc. In Case A, all of the 0.5" occurs on the supply side across the evap coil and supply ducts/registers - iow, I have no restriction on the return side. In Case B, I have 0.25" wc on the supply and -0.25" wc on the return. The fan has a total external static pressure of 0.5"wc in both cases. But, in Case A, I have much more air flowing in the supply side which creates the higher supply static pressure. What am I missing here?
I think I have answered my own question. Having read several articles, I believe fan performance data is collected under standard conditions with a free inlet. This explains why I am getting so much more supply cfm when I remove my blower access door. It does make a big difference on how your total static pressure is distributed across your return and supply. If I am wrong, please let me know. Thanks.

PS On my Bryant Table, it says data include filter which I assume is the "rock catcher" which came with the furnace. Since its pressure drop is close to zero, the inlet is essentially free. So, if the chart says 1500 cfm at 0.5' wc, I have 0.5" wc for my evap coil, supply duct and registers. If actual static pressure is higher or lower that 0.5" wc, than my cfm's will be lower or higher according to the fan performance. If my return is restricted, eg -0.25' wc, my cfm's will be much less than 1500 cfm even if total static is 0.5" wc.

Last edited by artbuc; 07-27-2011 at 01:59 PM.
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Old 07-27-2011, 02:59 PM   #5
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How TO Read Manufacturer's Fan Data


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Originally Posted by artbuc View Post
I think I have answered my own question. Having read several articles, I believe fan performance data is collected under standard conditions with a free inlet. This explains why I am getting so much more supply cfm when I remove my blower access door. It does make a big difference on how your total static pressure is distributed across your return and supply. If I am wrong, please let me know. Thanks.p
Total static pressure is TOTAL. It doesn't matter how it's broken down (supply vs return). In a household situation, there is no such thing as a 'free inlet'. Do you have returns fashioned out of wall cavaties. If ducted, are there many twists and turns? Are the ducts sized properly? All of these situations contribute to the return static pressure.

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PS On my Bryant Table, it says data include filter which I assume is the "rock catcher" which came with the furnace. Since its pressure drop is close to zero, the inlet is essentially free. So, if the chart says 1500 cfm at 0.5' wc, I have 0.5" wc for my evap coil, supply duct and registers. If actual static pressure is higher or lower that 0.5" wc, than my cfm's will be lower or higher according to the fan performance. If my return is restricted, eg -0.25' wc, my cfm's will be much less than 1500 cfm even if total static is 0.5" wc.
See above. Assuming that your furnace/air handler has a PSC motor, when you crack open your blower door, you are in effect, adding a return; thus the increased airflow. You could use more return. Opening the door has lessened the return static.
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Old 07-27-2011, 05:27 PM   #6
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How TO Read Manufacturer's Fan Data


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Originally Posted by veesubotee View Post
Total static pressure is TOTAL. It doesn't matter how it's broken down (supply vs return). In a household situation, there is no such thing as a 'free inlet'. Do you have returns fashioned out of wall cavaties. If ducted, are there many twists and turns? Are the ducts sized properly? All of these situations contribute to the return static pressure.

See above. Assuming that your furnace/air handler has a PSC motor, when you crack open your blower door, you are in effect, adding a return; thus the increased airflow. You could use more return. Opening the door has lessened the return static.
Yes, I understand your point about total static pressure in your household system. I am talking about how to understand Bryant's fan performance data. When Bryant says my fan will produce 1500 cfm at 0.5" wc static pressure that means it will move 1500 cfm against 0.5" wc when you have a free inlet. When you start restricting the inlet by having pressure drop in your return, the fan will not move 1500 cfm with a total static pressure rise of 0.5" wc. I think we are saying the same thing.

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