Duct sizes contributing to high humidity?

[teddy bear]

Addressing your point above in bold, you're on the right track. If your existing return air inlet is undersized for your unit, increasing the inlet size will reduce the velocity of the air entering the unit. Will it increase the volume of air entering the unit (which is what you're really after, since effective dehumidification and cooling of a building depends on sufficient turnover of the volume of air contained in the building)? That depends. If your supply ducts are restrictive and poorly installed, any increase in return air size, while netting perhaps a decrease in velocity, may not yet yield sufficient volume.

Why? Imagine this. You have a section of garden hose about four feet long. You stick your mouth over one end and blow air through the hose. You feel air exit the opposite side. You then think "I wonder if I can get more air through the hose if I make the end I'm blowing air into larger?". So you do it. You make a funnel that you can still fit your mouth around, stick it into one end of the hose (without actually reducing the area of that end's opening), and blow into it. You then find it's just as hard to blow into the hose as before, and you don't get any more air out of it than before. You may feel like less air is rushing out of your mouth and into the hose, but all you have done is reduced the speed of the air exiting your lungs and entering the hose, not the volume. The resistance the four feet of hose is still the same. Decrease the resistance by making the hose larger in diameter, and it suddenly becomes easier to move air through it. And you can ditch the funnel.

All that said, if your supply ducts are sized properly (iffy), then increasing the return size by itself may bring you some relief. However, if your system is oversized for the heat load of your house, you will still not be happy with your comfort, I believe.

Sorry, I must dissagree with you on your air velocity story. Restricting the air return slows the air flow through the air handler and through the cooling coil. He is not changing the demonsion of the coil or the supply duct around it. If the coil was in the return opening, there could be something to the velocity thing. The amount of moisture removed by a given coil with a specific cooling capacity is dependent on the amount of air flowing through the coil. Any enlargement in the ducts, return or supply will increase the air flow. An increase in air flow will raise the coiling coil temperature. Raised coil temps remove more sensible heat and less latent heat. Make sense??
Regards TB

[backpacker]

What type of duct material does the problematic system have?

The existing (and planned upgrade) return is flex duct 16". It seems to be stretched pretty well. The supply side is 16" round sheet metal. Most techs really say they like it; it tapers every so often and toward the capped end it's down from 16" to 6" or 8". Everything's sealed with mastic.

The Amana package unit has 16x16 openings (supply and return). The installer measured the flow at 1187 after slowing the blower down by one setting. We've slowed it down 2 more settings (I believe) since installation.

Restrictions/kinks seem to be in good shape. There's a slight return duct restriction where it exits the brick wall, but it's not major. The supply runs to the registers have some very minor kinks due to copper and PVC pipes, but they seem reasonable. I get great air flow out of every register. There are dampers for the runs and they have been adjusted and roughly balanced.

Kind regards -- look forward to the comments!
Backpacker

[tipsrfine]

Package unit?, as in "outside"? I would check to make sure there are no air leaks in that unit; it may be pulling in outside air. That bonus room over a garage? Good source of air leakage there also.

[SBKold]

What has been missing in all this discussion so far is any person in your home performing a Total External Static Pressure (TESP) test. This test can find more airflow problems in a shorter amount of time than any "guess and check"

Yes shophound!

I am familiar with these style M chassis package unit from goodman and I do believe it is a 2.3ecm blower.

So a return restriction will definitely increase velocity thru coil because motor will run full tilt boogie.

I just at the same time still agree with TB that decreasing static will certainly be a great project but unlikely to actually lower humidity.

Sidenote :: I have had 2 customers complain about humidity after we installed said m chassis goodmans.

I wonder if someone can compare Ari data and see if there is a reason these units are not dehumidifying well.

[Shophound]

Sorry, I must dissagree with you on your air velocity story. Restricting the air return slows the air flow through the air handler and through the cooling coil. He is not changing the demonsion of the coil or the supply duct around it. If the coil was in the return opening, there could be something to the velocity thing. The amount of moisture removed by a given coil with a specific cooling capacity is dependent on the amount of air flowing through the coil. Any enlargement in the ducts, return or supply will increase the air flow. An increase in air flow will raise the coiling coil temperature. Raised coil temps remove more sensible heat and less latent heat. Make sense??
Regards TB

You are correct that the air handling section as well as the evap coil do not change in size when the return ducting size is changed.

What I sought to illustrate was a restrictive return decreases overall air handling volume as compared to what the system could move in terms of volume if the return (and supply) ducting were not overly restrictive. I think you would agree with this point.

Recently our regional association of RSES held a business meeting in a hotel banquet room in Waco, TX. We quickly noticed that every time the a/c kicked on, the room filled with a horrendous noise of air rushing through a grill. It did not take long to find the source; it was an 8 x 8 return grill located dead center in the room attempting to return all of the air for a 1,600 square foot banquet room! I can only imagine the feet per minute velocity across that grill, but I also know full well that system, probably three tons or more, is severely starved for air volume and its a wonder the compressor hasn't died, yet!

I do agree with you, and in hindsight should have worked this into my post to the OP, that his system will not dehumidify well if it is starved for return air. It is true that if the system is starved for return air, it will run a colder coil/lower dew point. It will not, however, be a better dehumidifier for the house, because the latent gains to the house outrun the ability of the system to move enough air over the coil to get rid of it.

My caution to the OP is that increasing the return size may or may not solve the problem, as that would depend on how well the supply side delivers air without undue restrictions or leaks. Tips brought up a good point about the system being a package unit located outside. I would definitely advise the OP to check the unit's blower door, and to check if it has a fresh air intake. I also mentioned in my very first reply if the mechanical side of the system has been checked for problems. Not sure OP ever responded to that question.

[backpacker]

Tips brought up a good point about the system being a package unit located outside. I would definitely advise the OP to check the unit's blower door, and to check if it has a fresh air intake. I also mentioned in my very first reply if the mechanical side of the system has been checked for problems. Not sure OP ever responded to that question.

Shophound, I responded that they've done a thorough visual inspection, but not TESP test. I'll get w/ my tech to check the blower door, fresh air intake, and run the TESP test. I'll probably ask for all that after we do the return side upgrade. After all, he's the expert. I certainly don't get the impression he needs more work like doing the duct jobs just for fun or profit.

Yes, Tips, I've heard some whistling sounds outside at the package unit. I'll mention it - hopefully he'll have a method to seal it up if it's a leak.

Thanks,
Backpacker

[tedkidd]

Shophound, I responded that they've done a thorough visual inspection, but not TESP test. I'll get w/ my tech to check the blower door, fresh air intake, and run the TESP test. I'll probably ask for all that after we do the return side upgrade.

After all, he's the expert. I certainly don't get the impression he needs more work like doing the duct jobs just for fun or profit.

"it's been my experience you have a brain tumor, let's schedule surgery. No no, this isn't science, it's magic, trust me..."

After the brain surgery, biopsy the tissue removed and rejoice at the money saved not testing BEFORE going under the knife? Doesn't any of this sound out of order?

Certainly he's not coming to your home expecting to profit...(?)