Central Air return duct

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  #1  
Old 06-16-03, 05:08 PM
John28356
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Central Air return duct

Sorry if the answer already exists, but this seems to be "unit setup specific".
Location: South central NC, temp been around 90 during day.
A/C: 3 ton Goodman package A/C, MOD # PCK036-1
Mobile home, 784 SF, (well insulated).
Temp diff between cold inlet/return grill: 18-19 degrees.
Cold air inlet duct: 14"
Return duct: 12"
Return grill size: 12 x 20.
Thermostat: Hunter programmable (set at 79).
A/C is 2 spd. Currently wired for low spd, ran high last year, same problem.

The A/C comes on for a few min, then compressor kicks in, runs for too long a time, IMO, (approx 15-20 min), compressor kicks off, fan continues to run for 5 min or so, unit shuts off. I had a 2 1/2 ton previously which did not run nearly as long. When I moved this mobile home recently, there had been a code change requiring 14" cold air duct (was 12" previously).

My questions:
1. Should my return air grill/duct be larger? Chart in manual doesn't make sense to me.

2. Should this unit be run at the high speed or low speed wiring configuration?

I am receiving conflicting info. Sorry this post is so long, but I wanted to include all the info I thought you might need. Thanks in advance for any help or info you can give me. Read a lot of the old posts, and this seems like a great help site.
 
  #2  
Old 06-17-03, 06:04 AM
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John28356:

The operating conditions you describe sound like the unit is operating fine.
I'm not sure of the humidity conditions where you are but the run time you describe helps reduce humidity in the home.
The life of the equipment is extended by not having to start and stop more frequently.

The bigger question is the comfort level in the house. Get a digital humidity indicator to see what your run time is doing to the humidity.
 
  #3  
Old 06-17-03, 07:27 PM
John28356
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GregH--- Thanks a lot for the response. Don't know where I can borrow a meter, but I'll try. Not into HVAC, so loath to buy special tools.

Can you give me some advice on the high/low speed? Which should the unit be running at?

Again, thanks for the help.
 
  #4  
Old 06-17-03, 08:47 PM
H
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humidity

low speed for dehumidification, high speed for normal, Return duct is way too small. Should be 15" round or min of 10X18 prefered 10X20. Supply duct is the same...for 1200CFM (3 ton)
Oh, do you mean cold air OUTLET (not inlet)? We call it the supply? Now there is the possibility you mean the outside air damper??? either way, inlet and outlet are too small...
 
  #5  
Old 06-22-03, 02:35 PM
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Unhappy 3 tons for 784 SF ??

I have 900 sq ft POORLY INSULATED (none in walls/floor, 3" in attic, house 80years old) and can cool it with a 10,000 BTU window unit.
If you need 36,000 BTU for 784 sqft someone lied to you about how well insualted that place is ...
36,000 BTU will cool some of the newer 2,500 sq ft houses with no problem.
Now that's what I call well insualted
 
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Old 06-22-03, 02:53 PM
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it does

sound oversized, which would cause short cycling, i show 17 inch for .05 static (recommended) what is the supply size? 14"? if so duct is undersized, is this a mobile home...is the return grille in the floor? this will make a difference.
 
  #7  
Old 06-22-03, 05:08 PM
John28356
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Thanks for the replies. 54regcab, yeah, I know it's oversized. I was replacing my 2 1/2 ton last year, there was going to be a 2 day delay getting it, so the dealer gave me the 3 ton at the 2 ton price. This place is as well insulated as I can it get with 2x4 wall and roof (R17). Re-insulated just over a year ago.

hvac54-- yes, it's a mobile home. Return is on the floor. The supply and return are both 14". When the 2 1/2 ton was hooked up, it ran for about 7 - 8 minutes and cut off. I moved the house last year not long before getting the 3 ton, and had to put in 14" supply due to code change. I had 12" previously. I never realized the larger A/C size would make so much difference in required duct size. That's what happens when DIY'ers mess where only professionals should tread, I guess. Looking for a pro now, but they're all pretty busy installing new units and servicing regular customers.

hvac4u-- if you have additional advice for a mobile home, would appreciate getting it.

By the way, the mobile home came with the 2 1/2.

Again, thanks for the replies.
 
  #8  
Old 06-22-03, 05:53 PM
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Lightbulb 1.5 tons

Should be plenty if the House has decent insualtion.
10,000 BTU cools my 900sqft mostly uninsulated house !!
I'll bet your power bills are in the $200.00 range !
 
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Old 06-22-03, 06:55 PM
John28356
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54regcab-- Correction on my insulation R value. I just went out and checked my left over pack--- it is R15. Sorry about that. As for my power bill, the last one was 2 June, $40.00 and some change. My July and August 2002 bills were about $75.00 which is about $10.00 higher than they were with the 2 1/2 ton, but this 3 ton is running a lot longer. However, my power bill is really off topic, isn't it?
 
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Old 06-22-03, 07:27 PM
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Lightbulb Power bill and run time of A/C

If your power bill is that low the unit isn't running very much, seems like your house has 1/2 decent insualtion but the unit is just way too big.
You probably are experiencing the "freeze thaw" common to oversized A/C's meaning when the unit is running it's freezing in the house, then when it shuts off it feels hot because the unit hasn't removed the humidity.
Replacing with a smaller unit is the only permanent solution, and your power bill indicates you don't have much heat gain or really low electric rates !
How many KWH's are you using ?
Your unit uses about 3.5 KWH for every hour of run time.
You can do the math to figure out how much it runs. (use winter as a base unless you have electric heat)
On the hottest days you should have at least 8hrs run time, 12-16 would be best for high humidity removal.
 
  #11  
Old 06-23-03, 04:15 PM
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air across the coil

Your going to have to increase the air across the coil...speeding up the coil will only remove even less moisture. Sounds to me like you have an oversized unit also but if it has an outside air intake (fresh air makeup) and alot of glass it could be the difference...even still, three tons is a bit much!!! You'll find it cold and clammy...fix the duct sizing...
 
  #12  
Old 06-23-03, 06:02 PM
John28356
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54regcab and hvac4u: Glad to get the advice. Had a Sears installer come out just an hour ago and look at my setup. Since he was through for the day, he hung around, drank a beer, and gave me some free advice. ( He thought I had a Sears unit, that's why he came by). He showed me a national code book for mobile homes in this area, and it calls for a 2 1/2 ton in this size mobile home, so according to that I'm not too oversized. He recommended I put in another 14x20" floor return with 14" duct to a Y at the unit. This will double my return air volume. He said I had a good air supply coming in, but recommended I check my connections at the main duct again to ensure I have no leaks there. He also recommended I run the unit at the low speed connection to better dehumidify. He said my 14" supply duct is the standard size here. (I knew that was code). Does this make sense to you guys? I'm going to do it on Thursday. I've been trying to find someone to do it for almost a week, and I've given up. I'll post the results if you're interested. Thanks once more.

hvac4u-- I wanted to add that I think you are correct. When the unit is running it feels good, but when it shuts off it does feel warm immediately even though the temp stays down for quite a period. Too much humidity, huh. Going to lower speed right now before it gets dark.
 
  #13  
Old 06-23-03, 06:12 PM
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Lightbulb 2 1/2 tons CODE ?

They must be assuming some VERY POORLY insulated (older mobile homes)
If you power bill is $75.00 there is no way you need 2.5 tons of cooling unless your electric rates are dirt cheap.
Just because something is common practice doesn't make it right.
Are they using 1 ton per 350 sq ft ?
 
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Old 06-23-03, 06:24 PM
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Wink john

our ability to diagnose your problem is in direct proportion to the amount of info provided...kudos to you for accurately describing your problem.
 
  #15  
Old 06-23-03, 06:56 PM
John28356
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54regcab -- electric usage for July 02 and July 01:

Month of July 02 (with 3 ton): 805kWh

Month of July 01 (with 2 1/2 ton): 738 kWh

Both months were before I re-insulated. Does that usage tell you anything? Only thing I know is that July's are HOT in s. central NC.


So hvac4u-- I tried to supply all the data a tech might need. Do you think the recommendations I received today will help to alleviate my problem? Again, thanks.

BTW, I noticed your "Semper Fi". I'm a retired Army 1SG myself, but never had a problem with the Marines. Had a good friend in the Marine 2d Division G2 at LeJeune about 30 years ago. He was a guy I had known in various Army intelligence schools and in VietNam.
 
  #16  
Old 06-23-03, 10:00 PM
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Try it...

What have you got to loose? Increase the return and the supply Might get a bit noisy. Its rather like a water hose if you choke down the diameter of the hose you can't pump the same amount of water through it. The amount of energy used to move it is less, its moving less air than it is capable. Your temperature difference will be greater also, without the proper heat load tyhe coil will tend to freeze up.
Fill out a 2404 and do a good PMCS
 
 

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