Duct sizes contributing to high humidity?

[backpacker]

My relatively new Amana APH1536 first floor package unit is not keeping my RH below 70%, particularly at night. My second floor fancy 20 SEER Am Std split system just barely gets it below 60% upstairs. Here are my house/HVAC specs:

First floor 1850 sq ft
Package unit: Amana 3 ton HP
CFM: 1100 (dipswitch setting B)
Return: 16" round flex, about 25' long, sealed with mastic
Grille: 20"x25", centrally located in home with CLEAN MERV 6 filter
Supply: 12 equal runs, nicely tapered toward far end from HP, sealed with mastic
Drain pan and condensation pipe in great shape w/ P-trap
T-stat is digital touch screen but no RH sensor
T-stat set at 75 degrees constantly
Fan is in Auto mode
Encapsulated crawlspace (CleanSpace)
Crawlspace Santa Fe Advance Dehu set at 50%
Energy star windows
Humidity range: 75% at 6AM 64% at 6PM
Temp range: 73 at 6am 75 at 6PM

Based on recommendations from my tech, I am considering adding a second 12" return duct and filter grille from my foyer area and then joining it to the existing return box. Then he would install a new 18" or 20" return duct from the existing return box to the package unit. Will this help my RH? Other suggestions???

[Shophound]

Aside from duct leakage or building leakage, the other common reason for poor humidity control in cooling mode is a problem with the refrigeration side of the unit. Has that beeen thoroughly investigated?

[beenthere]

What size is he second floor 20i.

[teddy bear]

The delivered %RH in the home is dependent on the a/c cooling coil temperature while cooling. The coil must be 45-50^F to provide 75^F, <50%RH during 1-2 hour cooling runs. During low/no cooling load conditions and outdoor dew points of +50^F, supplemental dehumidification are required. This is typical wet cool weather that is common in green grass climates.
After you get your a/c setup right, reducting the Santa Fe Advance to provide supplemental dehumidification for the whole house. Ducting the air from the mainfloor of the home to the dehu with a supply back to the home and a minor supply to the crawlspace will provide <50%RH throughout the home. Or you can add an Ultra-Aire 70H to the main floors of the home.
Get the a/c setup correctly first. I gather you are not concerned about fresh air change during calm weather?
Regards TB

[udarrell]

There are a number of the A/C system problematic causes of lack of humidity control:

1. A refrigerant overcharge
2. Refrigerant undercharge plus icing-up
3. liquid line restriction
4. low evaporator airflow
5. inefficient compressor superheat & suction may appear normal under less than a full heatload
6. non-condensibles in refrigerant system
7. TXV metering device: bulb loose;poorly insulated bulb; bulb lost charge; non-condensibles, poor to normal with a TXV

I'm not going to list how you check for each problem on the list, however, a good Tech will know how...

Heat pumps have accumulators which makes any adjustments in the charge a bit tricky. A packaged HP unit would come with the correct charge...most of the problems listed above occur with split-systems.

[hcong]

Does your A/C cycle on and off with the "on" cycle being short? If this is true, your unit might be oversized. The return flex could be bigger and there is a good chance our return airflow is too restrictive. Did you have a load calculation done before you put in your new units?

[motoguy128]

What is hte location? What size is hte upstairs unit?

That severe of a humidity problem makes me thing oversizing. If it's a well insulated home 3 tons can still be pretty large for even 1800sqft.

I also wonder if the unit is drawing in outdoor air somehow. TO have RH that high, you must be in a very humid climate or have a very tight home and do lots of showering and cooking without ventilation.

[backpacker]

Awesome posts, everyone! Answers to several questions on the replies are:

1) Location of home - North Alabama. RH issue is by far worse at night.
2) Upstairs is 1400 sq ft
3) Upstairs unit is a 3 ton split system Am Std, 20 SEER Communicating. It almost never needs to run at "full blast"
4) Upstairs has a bonus room over a garage with heavy heating/cooling load. A single 1-ton commercial-style supply and register are dedicated to that room.
5) Upstairs has no return duct, just a box - unit is in a closet right beside the grille.
6) A load calc was done on the 1800 sq ft downstairs at the time of unit installation. Result was 3.5 tons. I talked the tech down to a 3.0 ton based my known RH problems and what I learned from this forum :-)
7) Cycle time downstairs is not too long -- ~20 to 30 minutes each cooling run.
8) Insulation in home: average to above-average; crawlspace insulation is like a finished basement (foam on the inside walls).
9) Note: My walls have no OSB sheathing; they have 1" foil-faced foamboard on the outside of the walls covered by brick veneer
10) Yes, am interested in fresh air supply during calm weather but don't know how to solve that.
11) Leakage at the unit is always a possibility. But both my supply and return ducts are tight with mastic and good takeoffs.

Thanks!!!

Backpacker

[udarrell]

5) Upstairs has no return duct, just a box - unit is in a closet right beside the grille.

You have to be sure the closet is airtight sealed from the hot high grains of moisture attic air or the heavy-loaded coil may not be operating cold enough for optimal dehumidification.

1) Outdoor condenser’s discharge-air-temperature ______-F
Subtract Outdoor air temperature: _______
Outdoor Condenser Air-Temp-Split _______

2) Need the Indoor percent of relative humidity at time of temp-tests - in the middle of the rooms or, at Return-Air inlet grilles ___
3) Indoor Return-Air Temperature ______
Subtract Indoor Supply-Air Temperature ______ -F
Indoor temperature-split _______-F

It will be a TXV with 410A.
If U can find a turn in the small liquid line insulate the temperature-probe; get its temperature for the subcooling analysis; tubing sensors are best:
Small liquid line temperature ____F

To measure return air the wet bulb temp, wrap a small/thin, wet (not dripping) piece of cloth around the probe and put it into the Return Air grille & then close to the closest Supply Air diffuser near the air handler (need air movement). Wait until the temperature stabilizes - may take a little while.

Return Air wet bulb ____F
Supply Air Wet Bulb ____F

Need the above information for troubleshooting & performance analysis.

Example below:
A Goodman 2-Ton 13-SEER condenser, @800-cfm indoor airflow; 80-F indoor dry bulb & 50% relative humidity; Indoor temp-split 18 to 19-F.
@ 85-F outdoors; 103.9-F - 85-F outdoors or around an 18.9-F temp-split;
@Indoor 75-F, 63-wet bulb around 50% RH - condenser temp-split is only 14.9F

A 20-SEER system will have a much lower outdoor condenser split than the above 13-SEER example, we can look it up.

[jtrammel]

Is your crawled pace damp from groundwater

[skippedover]

Did anyone ever do a load analysis before sizing the equipment? How about proper sizing using Manual 'S'?

[skippedover]

Awesome posts, everyone! Answers to several questions on the replies are:

1) Location of home - North Alabama. RH issue is by far worse at night.
2) Upstairs is 1400 sq ft
3) Upstairs unit is a 3 ton split system Am Std, 20 SEER Communicating. It almost never needs to run at "full blast"
4) Upstairs has a bonus room over a garage with heavy heating/cooling load. A single 1-ton commercial-style supply and register are dedicated to that room.
5) Upstairs has no return duct, just a box - unit is in a closet right beside the grille.
6) A load calc was done on the 1800 sq ft downstairs at the time of unit installation. Result was 3.5 tons. I talked the tech down to a 3.0 ton based my known RH problems and what I learned from this forum :-)
7) Cycle time downstairs is not too long -- ~20 to 30 minutes each cooling run.
8) Insulation in home: average to above-average; crawlspace insulation is like a finished basement (foam on the inside walls).
9) Note: My walls have no OSB sheathing; they have 1" foil-faced foamboard on the outside of the walls covered by brick veneer
10) Yes, am interested in fresh air supply during calm weather but don't know how to solve that.
11) Leakage at the unit is always a possibility. But both my supply and return ducts are tight with mastic and good takeoffs.

Thanks!!!

Backpacker

This entire post screams over sized. You state that the 2-stage 2nd floor unit rarely runs "full blast", meaning that it's operating day and night on 1st stage. Just that alone is a huge statement. If it's a 1-compressor unit, then it's running on approximately 2.0-tons on 1st stage, even during the day. What does that tell you? It tells you that 2.0-tons is plenty and maybe even then, too much for the space and tightness of your home. During the day the unit may run 20-30 minutes per cycle but I suspect those are optimistic times. I'd suspect the unit is running 10-15 minutes during the day and even shorter cycles at night. For each cycle, the first 8-10 minutes of run time is just allowing the compressor to 'organize' the refrigerant in the system. The indoor coil begins to cool but does not get down to the air temperature dew point until about 10-minutes have passed. Clearly a 10-minute cycle will then remove little or no moisture from the air.

The simplest and least expensive remedy for your situation is to have a whole house dehumidifier installed. This will operate continuously under most summer conditions and keep the RH in the house much lower. It will also negate the need for the AC system to operate longer cycles to maintain RH, so the AC can just be used to maintain temperature. Someday, when you're ready for a new system, have an accurate Manual 'D' done by a well qualified and experienced company/person and size the equipment according to that load and Manual 'S'. You'll be surprised what the results will show.

[hcong]

Awesome posts, everyone! Answers to several questions on the replies are:

1) Location of home - North Alabama. RH issue is by far worse at night.
2) Upstairs is 1400 sq ft
3) Upstairs unit is a 3 ton split system Am Std, 20 SEER Communicating. It almost never needs to run at "full blast"
4) Upstairs has a bonus room over a garage with heavy heating/cooling load. A single 1-ton commercial-style supply and register are dedicated to that room.
5) Upstairs has no return duct, just a box - unit is in a closet right beside the grille.
6) A load calc was done on the 1800 sq ft downstairs at the time of unit installation. Result was 3.5 tons. I talked the tech down to a 3.0 ton based my known RH problems and what I learned from this forum :-)
7) Cycle time downstairs is not too long -- ~20 to 30 minutes each cooling run.
8) Insulation in home: average to above-average; crawlspace insulation is like a finished basement (foam on the inside walls).
9) Note: My walls have no OSB sheathing; they have 1" foil-faced foamboard on the outside of the walls covered by brick veneer
10) Yes, am interested in fresh air supply during calm weather but don't know how to solve that.
11) Leakage at the unit is always a possibility. But both my supply and return ducts are tight with mastic and good takeoffs.

Thanks!!!

Backpacker

At night the fan won't need to be on "full blast" and actually the lower the fan speed/CFM the more moisture is removed from the conditioned air. The Santa-Fe Dehumidifier is for your crawlspace and doesn't directly dehumidify your house correct?

I am wondering if it gets cool enough at night that you reach your thermostat setpoint, so the AC turns off but it is still too humid to be comfortable. What temperature are you setting your thermostat at night? Is your fan on "auto" or "on" position? I would say set your temperature between 65-70 F (adjust until you feel comfortable), make sure your dehumidifier is on, and make sure your fan is "on". Usually 20 SEER units will come with thermostats with humidity controls. This will drive up power usage, but you should feel more comfortable.

[teddy bear]

Awesome posts, everyone! Answers to several questions on the replies are:

10) Yes, am interested in fresh air supply during calm weather but don't know how to solve that.
11) Leakage at the unit is always a possibility. But both my supply and return ducts are tight with mastic and good takeoffs.

Thanks!!!

Backpacker

The Santa Fe Advance is a powerful dehu and able to handle 3,500 sqft. of space Get a duct kit for the SF A. The intake comes with a 6" and 10" return. Have your a/c tech bring a 6" fresh air duct in through the rim joist to the 6" return. Connect the 10" return to a duct from the main floor.
On the supply side, attach a 10" Tee with a damper on the side of the tee open to the crawlspace. Connect a 10" supply duct to the mainfloor. Use insulated flex duct for all of the ducts.
Partially open the supply to the crawlspace enough to keep the crawlspace 50%RH. Open the 6" fresh air enough to introduce 70-80 cfm of fresh air to the dehumidifier. The fan on the dehu can operate 24/7 to provide circulation and fresh air throughout the space.
Keep us post on your results.
Regards TB

[beenthere]

6 tons for 3200 sq ft. 533 sq ft per ton. Probably a bit over sized.

[backpacker]

My RH problem is downstairs; upstairs is always much better. Agreed that my upstairs unit is oversized, but it almost always runs on 1st stage cooling only. I assume that is because I set the humidity on the tstat to 45%. The RH upstairs is almost always below 60%; 55% on avg.

Back to the downstairs unit. Today the unit ran for 18 minutes to drop the temp by 1 ^F. That's probably about what it runs per cycle to maintain as well. The delta temp was 14^F.

My tech comes tomorrow. Sounds like the order of business is:

1) Have the tech check refrigerant charge/line per udarrell's list.
2) Add duct kit and ducts to the SF A dehu per TeddyBear's list.

Please confirm nobody is recommending to:

1) Increase the size of the return duct/grille on the package unit. I'm at 20x25 grille, 25 ft of 16" round flex for a 3 ton.
2) Change the DIP switches on the package unit to reduce airflow down to 1000 or 800 CFM.

I'll definitely keep everyone posted!

[54regcab]

I think your manual J may have been "faked". When they do a load calculation inputting exaggerated values for air leakage and heat gain to get to that magic "500sqft per ton" result they want. 3.5 tons times 500sqft is 1750. Scary close to the 1800sqft that your downstairs is...

[beenthere]

Still should have more then a single 20X25 return for a 3 ton.

Your upstairs unit is doing some of the cooling work for the first floor. Even with slowing the blower on the first floor unit, it will only be able to do so much toward moisture removal.

The dehumidifier will have to do most of your humidity control.

[teddy bear]

The important point is that the a/c cooling coil must be <45-50^F when cooling. This maxes the amount the moisture the a/c will remove while doing routine cooling. T-stat setup when the home is unoccupied also provides an extended a/c run on cool down which will remove extra moisture. This minimizes the dehumidification needed by the dehumidifier.
Keep us posted.
Regards TB

[backpacker]

The important point is that the a/c cooling coil must be <45-50^F when cooling.

I put a wireless thermometer on the bottom right next to my coil drain pan in the unit. The lowest temperature over a 24 hr period is about 57^F. I assume that's probably a few ^F over the actual coil temp??? The hygrometer on the sensor says 99% :-)

Tech came yesterday and says there are no refrigeration issues whatsoever. He made a small "tweak" to slow down the blower, but it must have been a very small adjustment. He's sending the ductwork fellow to give me a bid on a mixing box and a second return, and hopefully an increase in return duct diameter to 18". He's out of ideas other than that.

When placing my thermometer in the package unit, I noticed that the floor insulation is wet. No visible water is present, but it's "mushy". This must have been this way ever since my drain pan cracked and was replaced early in the spring. Is this part of my issue? How can I get that fiberglass dried out?