Whole House Dehumidifier - Duct Installation Options

[steger424]

So I live in South Carolina and have a 6 year old 2200 sq ft home on a slab. We have to keep their air on 69 degrees with the indoor humidity around 58%-62% during the summer. I was thinking of adding a whole house dehumidifier to get the humidity down and possible raise the AC temperature if it will be more comfortable. I am looking at the Aprilaire 1870 unit and they recommend installing RETURN DUCT to RETURN DUCT with at-least 6ft spacing and the system fan ON. I've been doing some reading and that seems like a common install but I found one install that doesn't use the system fan and might make more sense to install this way. They suggest installing a new return duct in the home and connect direct to the dehumidifier and then connect the dehumidifier to the supply duct (after the Ac unit in the attic) to feed the dry air into the home. They stated the air will not be "conditioned air" but once the inside temp rises to the AC set level, then AC will cool the space down. What are your thoughts on this method of install. I like the idea of not having to run the system fan to aide the dehumidifier and it seems this alternative method may use less energy and maybe more productive.

Thanks for any advice!!

[teddy bear]

I am impressed that you understood all the tech talk about the different ways to install a whole house dehumidifier.

Basically you got it right. With an independent return duct from the open part of the home, the a/c fan is not needed to circulate the dry throughout the home. There are more benefits to this type of install.

Injecting the warm dry air down stream of the a/c cooling coil improves the moisture removal during the joint operation of the a/c and dehumidifier. The way this works is that the warm dry air from the dehumidifier will slow the a/c ability to dry the house to remove moisture from the house air. Also a/c fans use as much electricity as a dehumidifier, so keep a home dry with the dehu fan only reduces the cost of keeping the home dry. One more be benefit is that the moisture left on the cooling coil at the end of the cooling cycle will rapidly evaporate back into the home if the dehu warm dry is blown in to the a/c return. This is important during evening short cycling to keep the home dry.

There are exceptions to these suggestions but are generally true.
Hope this all makes scents.

One more basic, consider getting a Ultra-Aire dehumdifier instead of the other brands. Its is the pioneer of the concept and has the most efficient units. We are hear to help.
Made in Madison WI for 25 years.
Ultra-Aire.com
I do not mean infer that other not ok.

One more suggestion, your a/c is not setup ideally. A ideal setup of the a/c should get you down the %RH you really expect after several hours of running in the heat of the day. Lets say you want 75^F, 50%RH, a 55^F dew point 24/7. Set the t-stat at 75^F and cool through the heat of the day. By late afternoon after several hours of cooling, a properly setup a/c should be able to provide <50%RH. If not low enough, have a tech slow the air flow through the cooling coil to get a 30^F lower than the return air temperature. This will lower the a/c supply air to <50^F dew point, which will remove enough moisture to get you to a 75^F, <50%RH, a 55^F dew point.
Adding a dehumidifier to a poorly setup a/c will fix the high %RH problem, because the a/c will get lazy while the dehumidifier tries to help the a/c. Trust me.
Keep us posted.

Regards Teddy Bear -one of the pioneers of this concept. +25 year!

[steger424]

Thanks teddy!! I will check out the Ultra Aire and also will mention the slowing down of the fan. I had a guy come service the unit and seal all the ducts in the attic again because I had mold on my fan inside the unit. Seems the return duct was leaky and sucking in attic air with the house air. I also taped all the joints of the handler in the attic as it was sucking in attic air with my smoke test. I plan to check the fan this season to see if the mold returned. We also added a bi-polar 2400 air ionizer to help with the mood issues. He mentioned everything looked fine with the install. I have a 3.5 ton unit for 2200 square feet home.