Need to increase vent pressure


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Old 07-11-09, 04:12 PM
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Need to increase vent pressure

I have a 4,000 sq foot two-story house with two units. The downstairs unit barely moves any air at all when the furnace blower is running. You have to get your hand about 6 inches from a vent to tell air is coming out. The upstairs unit is fine, you can feel the air blowing from across a room. The furnaces are the same model and have about the same number of vents on each floor. The downstairs unit does have a small filter (12x36 compared to the standard sized 20x30 upstairs) and I am sure the ducts are longer with more turns coming down to the first level, but I am still very surprised at how drastic the difference in pressure is.

The low air flow is causes temperatures imbalances as much as 5 degrees from room to room during the hotter months when the A/C runs (I am in Houston so that is about 9 months out of the year). If I can do so without spending a fortune I would like to correct the problem. I was thinking about starting with a more powerful furnace blower. Or does it sound like I might have a clog or leak somewhere behind the drywall?
 
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Old 07-11-09, 05:13 PM
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Usually the downstair vent has stronger air pressure since the downstair A/C unit is bigger in most of the houses. Is this an old house ? Do you have this problem since day one ? or just started now ? Assume the original A/C and duct work design were OK when the house was built, then one of the following caused your problem. 1) blower fan is running slow for whatever the reason. 2) outward duct leak or Blocked by something. 3) Return duct or vent was blocked. 4)Do you have return vent in every room ? 5)Is the air temperature still cold at the outward vent ?? 6)remove the filter to see if that help ... try to answer these questions, and go from there....
 

Last edited by clocert; 07-11-09 at 06:25 PM.
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Old 07-12-09, 06:42 AM
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The downstairs compressor is actually bigger, 4.0 tons vs 2.0 tons. Yes, the downstairs vents blow cold air and the system manages to keep the space cool but it isn't circulating enough air to keep hot spots from forming. The house is about 9 years old but we have only owned it for 18 months. The previous owners say they never noticed the problem so can't give me an idea if it was original or developed over time. The downstairs has only one 12x36 return, which I think is too small. I have tried running the system without a filter for a few minutes and it doesn't seem to make a substantial air flow difference, still can't feel any air moving unless you get very close to a vent.

I figure the problem is one of three things:
1)The return is too small
2)The blower isn't powerful enough
3)The ducts are leaking or clogged (which would be very expensive to fix because this house has no crawl space between the floors)

Out of those replacing the blower is something cheap I could do myself, but wanted to get some advice before I spent the time and money.
 
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Old 07-12-09, 07:16 AM
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get a static pressure done to the duct system so to trouble shoot the problem. find someone certified by Natonal comfort institute
 
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Old 07-12-09, 08:44 AM
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You are getting there, now let's figure out if the return vent is big enough. Add all downstair vents sq footage, say you have 10, each (6x9) has 54 sq ft. total 540 sq ft. your return is only 432 (12x36), this may present a problem. Of course this number is not always accurate, duct work may alter the numbers. You can test this by open another return temporarily in the attic, disconnect the duct there or cut another hole, or remove a piece of metal from the box, etc. if you can figure out a easy/temporary way, just to try if bigger return will help. Remember you said the house was OK before per the previous owner, so if he/she was right, then this may not be the problem. Next thing is a stronger blower, how do you know the current one is bad ? (you may need to call a prefessional to check on that unless you can tell by just look at it) If you just want to put a new blower in yourself, you have to know the limit/size of your system, don't go over too much (bigger ones may not fit in there anyway). I don't know what else to suggest, very interesting and very common problem, most people just take the way it is. and blame on the builder day-in and day-out....
 
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Old 07-12-09, 09:14 AM
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Not sure if it was mentioned, I may have missed it. You could probably eliminate/confirm a return problem by taking the service panel off the side of the air handler. You may have to defeat the safety interlock to get it to run with the panel off.
 
 

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