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Old 05-25-2014, 06:40 PM   #1
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Blower Volume -- CFM/RPM etc


So everything is now hooked up and running fine...
except that it *feels* like the speed/CFM is too high.

The FIVE speed tap choices are:
Black @1665 cfm <-- existing factory pre-set
Yellow @1525 cfm
Orange @1375 cfm
Blue @ 1045 cfm <-- default for heat (and cont fan setting)
Red @ 880 cfm

Questions:
1) Is there a way to correlate CFM to RPM?
2) How low is *too* low to run the AC on?
3) What is most common?

Thanks.

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Last edited by TarheelTerp; 05-25-2014 at 06:42 PM.
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Old 05-25-2014, 07:01 PM   #2
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Blower Volume -- CFM/RPM etc


Blower for what?

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Old 05-25-2014, 07:07 PM   #3
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Blower Volume -- CFM/RPM etc


What size is your a/c? Model # would help
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Old 05-25-2014, 10:12 PM   #4
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Blower Volume -- CFM/RPM etc


400 CFM per ton
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Old 05-25-2014, 11:00 PM   #5
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Blower Volume -- CFM/RPM etc


Quote:
Originally Posted by hvac instructor View Post
400 CFM per ton
2.5T coil... x 400 = 1000cfm
= running a LOT slower than the preset
Thanks.

What RPM do the several speeds of an AC Blower motor operate at?
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Old 05-26-2014, 01:17 AM   #6
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Blower Volume -- CFM/RPM etc


Quote:
Originally Posted by TarheelTerp View Post
2.5T coil... x 400 = 1000cfm
= running a LOT slower than the preset
Thanks.

What RPM do the several speeds of an AC Blower motor operate at?
What size is the outdoor unit. If its only 2 ton, then you only want 800 CFM. Unless your in a dry climate.

There is no true correlation between speed tap and RPM, as RPM will be effected by air flow volume, which will be effected by static pressure.
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Old 05-26-2014, 06:44 AM   #7
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Blower Volume -- CFM/RPM etc


Quote:
Originally Posted by beenthere View Post
There is no true correlation between speed tap and RPM...
I understand the physics (I think) but I'm used to seeing the several RPMs of multi-speed motors on their label. I can't see this (PSC) motor's label without removing it (not doing that) and the literature doesn't say anything I can find. So I ask.

Quote:
What size is the outdoor unit.
If its only 2 ton, then you only want 800 CFM.
Both the coil and Condensing Unit are 2.5T.
The 95% 60K BTU furnace will handle up to 3.5T

The instructions say to adjust the speed to match install etc... The instructions also say the lowest (880CFM) is too low for heat. So I ask.

How low is *too* low to run the AC on?
and, what is most common?
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Old 05-26-2014, 07:01 AM   #8
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Blower Volume -- CFM/RPM etc


First. None of those CFM listings are accurate. CFM is depended on how much static the blower is working against. In the manual it should list a static pressure rating that those CFM listings are correct at.

320 CFM per ton is usually the lowest you can go on an A/C, which would be 800 CFM on a 2.5 ton A/C. Generally most people try to run a 2.5 ton A/C at 1000 CFM.

The motor won't have multiple RPM listing on it, only it rated high speed RPM. Which is probably 1075. And that is only true when its working at its rated horse power. While we call them speed taps, they are not. They are actually horse power taps. By decreasing horse power, the motor slows down. The motor only has 1 run winding. Each tap is placed at a different point on the run winding to cause it to lose horse power. Lower horse power means less RPM.
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Old 05-26-2014, 07:54 AM   #9
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Blower Volume -- CFM/RPM etc


Quote:
Originally Posted by beenthere View Post
First. None of those CFM listings are accurate. In the manual it should list a static pressure rating that those CFM listings are correct at.
Right. It does... I just used the first column @ 0.1 SP..

Quote:
320 CFM per ton is usually the lowest you can go on an A/C Generally most people try to run a 2.5 ton A/C at 1000 CFM.
That's the answer.

fwiw... when I run the continuous only (no AC) that speed/volume level (1045 CFM) *feels* about right.

Quote:
The motor ...is probably 1075.
While we call them speed taps, they are not. They are actually horse power taps. By decreasing horse power, the motor slows down.
There ya go... that's the missing bit of detail needed.

Thanks.

PS: The install instructions are a bit schizo. On some things they go down to a kindergarten level of explanation detail and on other things they completely gloss over or ignore useful information.
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Old 05-26-2014, 08:26 AM   #10
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Blower Volume -- CFM/RPM etc


Even a cheap fiberglass air filter, will have a pressure drop of .07". Your A/C coil dry will have .1"(.13" min when wet), your return grille will be about .03"PD, and your supply register will also be about .03"PD. So without including the duct work itself yet, your already at .23" with a dry coil. Add in your duct work, and you be at .8" Total External Static Pressure. Recheck air flow CFMs at .7 and .8", and see what it says.
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Old 05-26-2014, 09:00 AM   #11
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Blower Volume -- CFM/RPM etc


Quote:
Originally Posted by beenthere View Post
Even a cheap fiberglass air filter, will have a pressure drop...
Recheck air flow CFMs at .7 and .8", and see what it says.
I have. But until I get a real tester to see what the actual running numbers are this is still all about learning where the foul lines are. You've been a big help.

Last Q's on this:
The three "extra" motor leads are all landed on a "spare" terminal on the circuit board and the two other two on the black module with the red thing. (see picture below).

1) What is that black module the other two leads connect to?
2) What is the red thing on it?
3) Do the "spare" terminals connect to anything?

Thanks.
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Old 05-26-2014, 10:19 AM   #12
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Blower Volume -- CFM/RPM etc


The black module is the relay that energizes either the heating, or cooling terminal.

The spare terminals are just landing terminals so that the extra speed tap wires don't dangle and short out.

Red thing is just a protective cover.

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