Thread: Starving the supply

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Hello,
You won't believe this one. A 1850 sq ft house on a slab. House is 14 months old and customer cannot get summer temp in house under 79 degrees inside. Unit has been looked at several times and all is well they say. Customer might have found the problem. The dust pattern on his filter is about 8x15. Who ever installed this unit did not cut the hole big enough.

How much CFM is the unit producing with that small of a hole. How large should the hole have been? Customer has a 3 ton A/C.

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If you checked the static just above the filter,and the supply side also to get a total ESP,you could read the cfms on the fan chart(read all foot notes)..

We find this same thing very often here in Florida,i can tell you is often enough capacity loss to prevent the system for cooling the home,and/or raise the electric bill.

Usually happens on new construction jobs.

3. 8X15 I get approximately 675 CFM 3 ton = 1200 cfm

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Originally posted by millerman
8X15 I get approximately 675 CFM 3 ton = 1200 cfm

The above appears to be using a ductolator with a Friction Rate of .1.

FR is for 100 feet or equivalent feet of duct,in this case the 8X15 is likely an inch or two at the most,so the cfms are likely much higher,but a restriction ,such as this ,certainly needs to be removed.

It should be as large as the opening in the airhandler,or very close.

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Originally posted by millerman
8X15 I get approximately 675 CFM 3 ton = 1200 cfm
If you look at your Ductulator again and look at .06 static, you will notice the 8 X 15 hole will only flow 500 CFM. A correctly designed system would have a TESP of .5. If you look at a fan curve chart you will notice as your TESP rises, your air flow decreases. With this being the case, your incorrectly designed system has a larger TESP than .5 and it is probably more likely around 1.0 causing your 8 X 15 hole to flow much less than 500 CFM.

As your TESP rises, your CFM decreases.

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We have seen this many times,and it's likely moving 800 cfm or more.

The ductolator is of little to no value here,test the ESP,and read the cfm on the fan chart if you want to know what it is.We all know it's less than required ,although I'd check the ESP after the hole is enlarged,to insure proper cfm.

1.0 ESP is off the fan chart for a PSC motor,which this likely is,but a good 3 ton air handler can deliver 1190 at .6 ESP.

At 500 or 675 cfm cfms.I'd think it would be icing up,not just unable to go below 79°.

[Edited by dash on 05-10-2005 at 04:10 PM]

7. What size is the return duct if the hole was only cut to 8x15? Were they just to lazy to cut the hole larger or did they have what they believed to be a good reason to cut it this size? Seems like there is a missing piece to this puzzle.

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The return is larger. I think the filter was 21x10.

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from what little i know about air flow, the return should be about the same size as the supply and at leat the dimentions in the opening of the furnace. i might sugjust purchasing small return grille capable of holding a filter. measure the square inches on the hole in the furnace (size of filter or slightly smaller). then find the square inches on the opening provided. that was like 8 by 15. the difference between the two is what size filtered return grille you need put in. i dont know if this would work but it's just an idea.

10. 3 TON UNIT = 20 X 30 Filter = 20 X 30 Hole behind filter

Larger filter size is OK.
Smaller filter size OR smaller hole (i.e. 8 x 15)
immediatly behind the filter will increase air
speed through filter beyond filters limits
This will also cause filter grille to 'sing'

Since the return was not properly installed in the first
place... I would venture to guess the rest of the duct
system would need an inpection.
First place I'd start is place plastic over return grille
with fan in ON position (system switch OFF)
See if any air is blowing out of the supply vents.
This will let you know if there are any return air leaks
real fast. You will also be able to hear any leaks
faster when inspecting the duct.

After that check for proper airflow through system.
I'd normally check refrigerant charge and total capacity.

If airflow looks good and charge looks good but capacity is off...
I'd check for stuck electric heaters

If all that looks good. I'd check house for probs. causing
unit not to cool properly. i.e. not enough insulation, air leaks in structure, dryer vent leaking or run under house, attic ventilation, etc.

This is just quick stuff off the top of my head.
Its not my 'full' diagnostic procedure.
Didn't want anyone to shoot me for missing something hehe.

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i'd check out the thermostat.

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I want to see docholiday check the air flow with his garbage bag setup he mentioned on another thread.

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Is this a tract home with similar houses in the neighborhood?
If so check out your neighbors see if they are having problems? If not make a comparison...
Just a thought....

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