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Old 11-09-2008, 11:13 PM   #16
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Pre-charge on goodman straight cool


Quote:
Originally Posted by beenthere View Post
First.
How can you have the charge right without the duct work hooked?
Second.
16" round sheet metal is too small for 4 tons, let alone 16" flex duct.

Do you plan on using tee's, wye's, or what method for tapping in your supply branches to your trunk line.
Same question on return.


You charged that ac with no duct work hooked up? Either I missed that or you never mentioned that when i told you to make sure air flow was adequate.

Your probably over charged like hell with that fan running full bore.

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Old 11-09-2008, 11:22 PM   #17
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Did you add those numbers after your original post? I sure don't remeber seeing them 'til now.

You need seven inch flex to make up for friction loss. Six won't do it.

Google some flex sizing charts for your system.
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Old 11-09-2008, 11:49 PM   #18
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If the fan puts out 2000cfm and I have ten registers and assuming I can get the flow balanced, that would be 200cfm or less per 6" duct. 200cfm doesn't seem like much for 6" duct. I did look up the "ductulator" charts from quietflex. 200cfm works out to be 902 fpm for 6". Is that too fast? It just gives ranges. I'm confused about the friction. The fan gives output based friction. Friction is based on speed. Speed is based on cfm put out by fan. WTF!?!? It's a circle?
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Old 11-10-2008, 12:31 AM   #19
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This just an example of a duct lay out. That is a bit better at giving you air flow to your furtherest supplies.

You see the CFM's don't add up to the 1600 that, that 4 ton wants. But then again, there are no room sizes.
And you never said you did a load calc.
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Old 11-10-2008, 12:34 AM   #20
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You don't want to hear the noise that a 6" will have at 200 CFM.

Plus, you don't want a 4 ton system moving 500 CFM per ton.
Unless you live in a desert area, that you need to add humidity when you run the A/C.

How did you determine what size system you need?
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Old 11-10-2008, 08:52 AM   #21
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Plus you are assuming that the air will be evenly distributed to each register.

That ain't gonna happen. Some rooms may end up requiring more air than you thought and that will mean closing dampers to other rooms ect.

Or you just won't get the airflow strength you were hoping for after it passes thru all the bends and turns the fiter and the ac coil.
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Old 11-10-2008, 08:57 AM   #22
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Quote:
Originally Posted by protechplumbing View Post
If the fan puts out 2000cfm and I have ten registers and assuming I can get the flow balanced, that would be 200cfm or less per 6" duct. 200cfm doesn't seem like much for 6" duct. I did look up the "ductulator" charts from quietflex. 200cfm works out to be 902 fpm for 6". Is that too fast? It just gives ranges. I'm confused about the friction. The fan gives output based friction. Friction is based on speed. Speed is based on cfm put out by fan. WTF!?!? It's a circle?



It's as much as art as it is a science. That's why there are guys who make a living doing only flex and sheet metal duct systems.
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Old 11-10-2008, 09:00 AM   #23
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The houses to the left and to the right have the same square footage and very similar construction. They both have 4 tons and seem to work fine. Though both systems have undersized returns and bad temperature distribution they do keep the houses cool in the middle of summer.

Quote:
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You don't want to hear the noise that a 6" will have at 200 CFM.

Plus, you don't want a 4 ton system moving 500 CFM per ton.
Unless you live in a desert area, that you need to add humidity when you run the A/C.

How did you determine what size system you need?
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Old 11-10-2008, 09:23 AM   #24
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The houses to the left and to the right have the same square footage and very similar construction. They both have 4 tons and seem to work fine. Though both systems have undersized returns and bad temperature distribution they do keep the houses cool in the middle of summer.
Don't count on what other homes are like. Too many variables. Sun side exposure, color of roof shingles, ect. Since your stuck with the four ton might as well get the duct sized right.
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Old 11-10-2008, 10:57 AM   #25
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Ok, letís talk about the opposite extreme with the ducts then. What's the down side in over sizing other than cost? More heat gains in the ducts?
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Old 11-10-2008, 02:31 PM   #26
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Those other houses may be cooling.
But, are those systems dehumidifying.
Or, do those people have to set their thermostat set point lower, to keep the humidify in check?

An over sized duct system, in an unconditioned space, will have higher heat gain/loss.

But, even with an oversized trunk, the branches can be designed to have static regain, to get the velocity up for register throw.

A common misconception, is that a large duct won't throw the air as far as a smaller duct with higher pressure.

A standard PSC blower, can't deliver rated air flow, on undersized duct work.

With your design. You will have high static pressure. Loud registers. And still not get the proper air flow.
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Old 11-10-2008, 02:47 PM   #27
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A 6" duct moving 200 CFM, would have a Friction Rate of .32"(thats not static pressure, static will be much much higher), at a velocity 1050 FPM.

Add all the pressure drops of all the devices, and and the length of the run. You'll be over 1" of static.
No way that blower can move 1600 CFM against that static pressure.
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Old 11-10-2008, 04:48 PM   #28
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I remember the old 1600 sq ft homes from the early to late fifties.

Low boy 125kbtu furnace 1/5 or 1/4 belt drive motor 800 rpm, blower wheel 20x12.

The plenum was 20x18 and the trunk 28x8.

Moved a WHOLE bunch of air but so slow you never noticed it. The static was so low by today's standard it was laughable.

Now everything is high velocity thru ducts barely the right size or the static is so high the ducts scream.
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Old 11-10-2008, 05:40 PM   #29
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Those old units were set up for about 1000 to 1250 CFM, at 75 to 90į temp rise.

Had to use a 1/3 to 1/2 HP motor if you put more then a 3 ton A/C on it.
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Old 11-10-2008, 06:06 PM   #30
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max temp rise was 100*f on the ones I worked on. to get three ton 1/3 hp was all it needed.

Tried a half horse one time. Went back on a noise problem. Opened the the filter section panel (can you imagine! The smoke pipe was welded to the heat exchanger and exited thru the cold air stream to connect to the chimney!).

Anyway, as i was saying I opened the blower section (was making hell of a racket.) and the blower shaft let go and shot the blower wheel out the cold air opening like a bullet and bounced off the far wall.

Just too much juice with a half horse. 1/3 was plenty and the air just poured out of those wall heats. Had to use deflectors to get the air above waist.

Had a great one about twenty seven years ago. Lady called complaining
that her dog had not stopped shivering since we put the ac in.

went to check it out. Stupid helper didn't change the pulley and didn't add deflectors. All the air was pooling at floor level and much colder cause the
the blower was not moving fast enough.

AAhhh...the good old days

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