Whole House Dehumidifier with two air handlers

[bronx]

I have a two story house, about 2700 square feet and I'd like to reduce the humidity, which is mostly a problem on the 2nd floor. My home has two air handlers, one for each floor, and there is a large opening between the 1st and 2nd floor at the foyer. I'm curious what's the best practice with dehumidifier installation, assuming I went with an ultra-aire 98h or 120h and also took advantage of the fresh air supply as well. Connect the dehumidifier to the 2nd floor unit where I have the highest humidity, or connect the dehumidifier to both air handlers?

[teddy bear]

Is there a basement or crawlspace?

With more of humidity problem on the second floor, make sense to connect the dehumidifier supply to the a/c supply for the second floor. Fresh air from outside to the dehumidifier return and a return from the a/c supply of the first floor a/c. This will circulate the first floor air mixed with fresh air, filter both, and supply throughout the second floor. The excess air from the dehumidifer to the second floor will move slowly throughout the lower level of the home. Locate the control for the dehumidifier in the most humid open area of the home probably on the open area of the home. If there is a basement or crawlspace, route a 4-6" a dampered supply from the dehumidifier to the that space. I suggest the Ultra-Aire 120 because it has a more powerful fan.

A damper in the fresh air air inlet to adjust the amount of fresh air. You will need a %RH controller. Also, fresh air on/off is needed unless the home is occupied mostly 24/7. A on/off switch or CO2 controller for fresh filtered air is also suggested.

I also suggest that make sure your a/cs are setup to max the amount of moisture they remove while doing sensible cooling. During any long sensible cooling runs, the a/c should remove enough moisture to get below 50%RH. Adjusting the a/c air controls how much moisture the a/c removes while maintaining your desire indoor temperature. What is the indoor %RH during hot days and with normal outdoor dew points? Where is the home located?

Keep us posted.

Regards Teddy Bear

[bronx]

Thanks for the reply Teddy Bear,

My house is on a slab foundation, no basement, both furnaces are up in the attic. I'm located in Austin TX, so a lot of humid days. My downstairs AC will pull the humidity down to ~50% on hot days, the upstairs which is new, and was sized, stays closer to 60%. I was curious if splitting the dehumidifier output to both furnaces would improve fresh air / dry air distribution, or of it was not worth the complexity and additional installation cost.

[teddy bear]

Thanks for the reply Teddy Bear,

My house is on a slab foundation, no basement, both furnaces are up in the attic. I'm located in Austin TX, so a lot of humid days. My downstairs AC will pull the humidity down to ~50% on hot days, the upstairs which is new, and was sized, stays closer to 60%. I was curious if splitting the dehumidifier output to both furnaces would improve fresh air / dry air distribution, or of it was not worth the complexity and additional installation cost.

I am concerned about your upstairs a/c setup. A whole house dehumidifier will not make up for a less than ideal setup a/c. During long sensible cooling runs, the upstairs a/c must do it's share of moisture removal. To confirm the setup, I suggest you measure the return and supply temp/%RH from both a/cs.

Dehumidifiers are intended to supplement humidity removal during evenings and rainy days. Well setup a/cs remove about 3 lbs. of moisture per hour per ton of cooling.

This is important. Usually, the problem is that too much air flow flowing through the cooling coil does not allow the cooling coil to get cold enough to remove the moisture needed. A 3 ton a/c needs to remove 9 lbs. of moisture per hour of sensible cooling. We must have the upstairs unit removing 6-9 lbs of moisture per hour plus the mainfloor unit and plus the dehumidifier 4-5 lbs. per hour. We may need the dehumidification on all the units during some of the humid times of the year.

Lazy a/cs will not any moisture as the dehumidifier lowers the %RH in the upstairs area. I have learned this lesson several times the very humid green grass climates.

So measure the temp/%RH in the space and at the a/s supply grills without the dehumidifier running to confirm that the a/c is setup ideally to get your entire home <5%RH during extreme high out door dew points.

Looking forward to the info.

Regards Teddy Bear

[bronx]

Thank you for the guidance, do you have any recommendations on devices to measure the temp/humidity?

[bronx]

I got a temperature delta of 16 degrees Fahrenheit at the closest register to the plenum

[teddy bear]

I got a temperature delta of 16 degrees Fahrenheit at the closest register to the plenum

To help determine how your a/c is doing at removing moisture, I need the temp/%RH in home and at the closest a/c supply.

Looking forward to the info.

Regards Teddy Bear