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# Is my system too small??

#1
07-27-14, 03:58 PM
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Is my system too small??

Had this system installed last year in the fall.

ARI is 3657138

24ACC624A003 (outside unit)
FV4CN5002T00

2 ton system. The area its cooling is about 1330 square feet. Its a 2 ton unit. Delta T is 21 degrees. I was told heat loads were calculated. Im near Houston and the system really can't keep the temp below 78. Its currently 98 here outside. One thing I know is that the room gains heat REALLY quick. If I get it to 78 and turn off the AC, then I can be back at 82 degrees in about 30 minutes not problem. My insulation is really lacking in the ceiling and the knee walls need rigid foam behind them.

Currently there is R13 in the kneewall and R11 in the ceiling.

Is there a way to calculate a heat load somehow kind of to determine if my system is sized right? When the sun goes down, it seems to do OK.

If I were to add insulation to R60 and put rigid foam board behind my kneewall would that solve this problem?

#2
07-27-14, 04:14 PM
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Here is a link to an A/C calculator. It allows you to take into account the amount of insulation, so you should be able to see how much of a difference increasing it would make.

#3
07-27-14, 04:16 PM
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Yeah, it is too small.
Run that load calculation to prove it.

#4
07-27-14, 06:43 PM
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How do I get to tonnage based on it? I ran it and got a bunch of numbers. I don't have 'outside walls' on the second floor. The north and south is a roofline. This area to cool has kneewalls, a short angled ceiling and a flat ceiling. So, im no longer convinced the system is undersized......well, if the insulation was to minimum code. Im not sure how to properly fill out that loadcalc for this room application.....ideas?

Last edited by agdodge4x4; 07-27-14 at 07:15 PM.
#5
07-27-14, 07:49 PM
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If you only need two tons (instead of 2.5 tons) and the system is not cooling I'd verify that you are getting the correct airflow from that air handler.

A manometer is required for this test.

#6
07-28-14, 07:43 AM
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Join Date: May 2008
Location: Wisconsin USA
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If you're getting an indoor delta-T of 21°F then it must have very low airflow through the evaporator coil or the duct system is leaking a lot of air, & if in unconditioned space, perhaps not well insulated.

The blower may only be delivering around 600-CFM or less instead of 800-CFM; 600-CFM is only 300-CFM per/ton.

At 21°F * 1.1 is 23.1 * 600-CFM is 13,860-Btuh sensible.

What if only 250-CFM per ton is actually getting to the rooms:
23.1 * 500-CFM is only 11,550-Btuh sensible being removed to the outdoors.

At 800-CFM * 23.1 is 18,480-Btuh sensible.

At MAX 900-CFM * 23.1 is 20,790-Btuh sensible.

Of course at that CFM unless the humidity was low it wouldn't be getting a 21°F indoor delta-T.

#7
07-28-14, 11:34 AM
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Just to be clear, an 002 unit and 024 outdoor unit is only designed to deliver 700 CFM on 'normal' model. This is in my install manual. You can set it to 'hi' or 'low' on the board. High will deliver 15% more than normal and low will deliver 10% less than normal.

So, as designed, its only supposed to do 700 CFM. If I want to set it to 030 (which is not technically correct) then it will deliver 875cfm.

Thats not an opinion..700 is what its supposed to do.

So, how can I expect 900 cfm on a unit that will not properly cool that type of airflow? I CAN set it to do that, but it won't be as cool, and it certainly will be humid.

I have checked the ducts. They are new, but I checked anyway, they are continuous runs and haev mastic everything. I did not feel any leaks.

Also, all ducts are insulated....not sure if well, but insulated.

#8
07-28-14, 12:13 PM
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OK....im lost. How do I use this chart and a manometer to verify my airflow? I don't have a manometer, and unless its about 15 bucks, Im buying one. But...if they are cheap, how would I use this chart and where do I measure.

Last edited by agdodge4x4; 07-28-14 at 12:34 PM.
#9
07-28-14, 12:27 PM
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..................................

#10
07-28-14, 03:06 PM
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Location: Wisconsin USA
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New clean filter clean blower wheel blades & evaporator...

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