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Old 08-15-2013, 08:22 PM   #1
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Improper HVAC sizing and insufficient return?


We've recently invested in an estate in Central Florida with 6,050+ sq ft house on it. It has 3 A/C Units. A 3-ton that services about 1,200 sq ft in the guest suite. And 2 5-ton units that service the remaining 4,800+ sq ft. 2-story building. While that seems like enough, it's split such that one of them covers about 3,500 sq ft while the other covers the nearly 1,350 remaining. From what I understand, each ton covers 500 sq ft maxing a 5-ton unit at 2,500 sq ft.

Question 1: Is it possible for a 5-ton unit unit to adequately cool 3,500 sq ft? Especially here in Florida?

Also, it seems there is only 1 return to it. It's in the ceiling of the bottom floor at one end of the building. It's high up but I'd guess it's about 20"x20". I've read a lot online and it seems there should be 150-200 sq" per ton for return ducting. It isn't loud so I'm guessing it isn't starved for air, but it sure doesn't seem like enough. It's only about 1/3 of the return of the other 5-ton unit.

Question 2: Should I add more return air?

Because all of the return ducting is in the walls and not in the attic space, the only place I could add return is in the base of the air handler. There is a box below the air handler that the single return duct goes into. I could cut the drywall and board and add a return grill there.

Question 3: Should I cut a return below the air handler? How large?

These past couple of weeks the unit hasn't been able to keep up with the heat and I'm trying to see what I can do to make the upstairs more bearable during the summer. With it set at 76, the upstairs gets over 80 in the middle of the day. TIA!

If you want to see pictures of the attic ducting, you can find them @ http://thegardenchateau.com/downloads/list.php

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Old 08-15-2013, 08:41 PM   #2
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Improper HVAC sizing and insufficient return?


Add more return to it. A 20X20 isn't nearly enough.

A 20X24 would be ok as a second return size.

Does your zone system have a bypass damper. If so, it may be stuck open. That will cause poor cooling.

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Old 08-15-2013, 09:49 PM   #3
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Improper HVAC sizing and insufficient return?


The zone system was cut away when we bought the place. The A/C guy that we hired to put the compressors in (the compressors were missing when we bought the place but the air handlers were still in tact) wasn't familiar with wiring the dampers back up. Three of the dampers still work fine but one needs to be replaced. I'm not sure what a bypass damper would be. Is that what is in the picture http://thegardenchateau.com/downloads/P1000560.JPG? There are no electrical connections to it. The setup is similar to http://www.zoningnews.net/wp-content...-Picture-1.jpg but without the wiring to it. It comes out of the top of the supply box on top of the handler and spreads out to several rooms to what appear to be return grills (non-adjustable grills).

Should I put a 20x24 return grill directly below the Air Handler? Would that hinder it from pulling air in from the current return since it is so much closer? I stuck my phone up and took a picture of the under-side of the coils and am amazed at how filthy it is! I cleaned the top side that is accessible and it wasn't that bad but I'm wondering if the previous owners even used a filter! I'm going to try and get it cleaned out this week. I hope that makes a difference as well.

Last edited by crazyhandpuppet; 08-15-2013 at 10:02 PM.
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Old 08-15-2013, 10:02 PM   #4
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Improper HVAC sizing and insufficient return?


Can't say for sure if that is a bypass damper. or just another zone damper that had its motor removed.

The blower will still pull air from the other return.

The coil is probably still dirty. Clean it again, from the underside.
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Old 08-16-2013, 05:53 PM   #5
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Improper HVAC sizing and insufficient return?


I'm pretty sure it isn't a zone damper. It goes in the right side and out the left but from there branches off to a half-dozen other places. I've ordered a 70cfm duster to be able to blow out the coils. Are some cleaners better than others? Lowes has some no-rinse stuff that costs like $5 a can. Should I be using anything in particular?
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Old 08-16-2013, 06:13 PM   #6
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Improper HVAC sizing and insufficient return?


No rinse evap cleaner in a can is ok to use.
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Old 08-20-2013, 12:02 AM   #7
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Improper HVAC sizing and insufficient return?


I've cut the largest hole I can feasibly get in there at 12x20. I then crawled up and cleaned most of the inside of the coils. I'll try and blow it out tomorrow and spray some cleaner on it. I can feel a noticeable increase in air flow at the vents so I'm feeling good about the changes so far. But once the air handler cranks all the way up there is no way I can get the 1" filter to not crumple and get sucked up into the coils. After fighting and struggling with 2 of them, I figured I might as well make a few modifications and upgrade to a 4" filter. I can get a 20x20x4 to fit although it would be smaller than the 22x22x1 that would currently go in there.

Any advice on what kind of filter to use? Some of the ones I'm seeing say 3 month, some say 12 month. Do the 12 month ones actually last 12 months? Or would I benefit from using a higher MERV rating and changing it out every 6 months? What's a good brand? Nordic Pure, Filtrete, Honeywell, something else?

Thanks!
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Old 08-20-2013, 05:35 AM   #8
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Improper HVAC sizing and insufficient return?


Honeywell.

How long they last depends on the house, and how often the system runs.
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Old 08-28-2013, 04:42 PM   #9
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Improper HVAC sizing and insufficient return?


It's been a bit but I wanted to follow up. Once I cut it all open and started making the modifications I started cleaning out the return duct and realized it didn't bend the way I expected it to. So I snaked a camera into it and saw that it went straight up into the attic! That's not how I was expecting it to go so I went up and started tracing the maze of flex again. Lo and behold, I was completely wrong about the returns!

That box is a passive bypass damper. Since it was connected on the supply side I assumed the flex connected out from it was supply even though the vents all looked like return. After seeing the flex coming from the return going up into the attic, I realized that one side of that damper is supply and the other is return. I guess it opens if the supply pressure gets too high? That means that all the vents it connects to are return and each bedroom has a return in it. I guess it was made right after all! I suppose that makes it all good news, but doesn't explain why it was so bad at cooling the house unless the zone dampers are required for proper functioning.

I'm now curious as to why the airflow picked up so much after adding the new return area at the bottom of the air handler even though there is so much return. To me, that would mean there wasn't enough return air coming in. But if I feel the air at the returns at the far end of the structure I feel no movement at all. Sticking a hair up to them doesn't result in the hair sticking. It just falls back down because of the lack of suction. Does that mean the runs are too long or the flex is too big for it to pull air that far?

While it turns out I was wrong on the return sizes I'm glad I've figured out how all the spaghetti maze of tubing works and can now use better 4" filters over the 1" filters. Is there anything I can do to increase the return at the far reaches of the system? Thanks!
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Old 08-28-2013, 05:53 PM   #10
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Improper HVAC sizing and insufficient return?


Use a tissue, a hair is too thin.

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