Ventilating Dehumidifier Setup

[notanexpert]

Hey Folks,

Have been lurking around hvac-talk for a while, trying to absorb everything I can about residential hvac. I've read through tons of awesome threads, so thanks in advance for opening up the forum to homeowners.

I recently moved into a new construction, 3k square foot production-quality home in Houston. The short story is that I'm looking to add a ventilating dehum, and would like some second opinions on installation details and price based on a quote I received from a local company (that I've had good experience with in the past).

Here are some details:

The house is 1.5 stories, with one (probably oversized) forced air unit per floor (pretty sure I have 4 + 2 tons total, builder-grade Lennox single speed system). Ducts run in an unconditioned attic; each bedroom has its own return and supply ducts, and each floor has one additional larger return in a common area.

I don't have an ACH50 number for the home, but do know that CFM50 is 2218, and duct leakage is 49 CFM @ 25 pascals. Based on the CFM50 number and my rough estimate of conditioned volume, I'm thinking the house probably comes in around 4.7 ACH50.

I'm having some high humidity problems in low / no sensible load conditions (haven't been in the house long enough to go through a full summer), especially on the top floor. I'm not sure if this is due to the house being *too tight* or *too leaky*, but I somewhat suspect the latter since I don't have any humidity issues when outdoor humidity is low. To confirm my suspicion, I'm thinking of having another blower door test along with an energy audit to see if there are any big holes that need plugging in the air barrier.

In either case, I'd like to add a dedicated dehumidifier to my setup. A local hvac co quoted me $2,700 for an aprilaire 1850 on the main floor and $2100 for another smaller aprilaire up top. This is not including any sort of ventilation setup, which I'd like to integrate, since my house doesn't have any mechanical ventilation (other than point fans).

I'm not thrilled with the price or the equipment in this quote. First of all, I think I could get away with just one dehumidifier tied into the main floor ductwork (this is based on me experimenting with positioning of a standalone 70 pint dehumidifier). Second, the Aprilaire unit (reports of reliability problems aside) recommends a return to return installation. I'd like to avoid this install type because I'm not thrilled about running the inefficient blower motor every time the dehumidifier runs. Third of all, I have a MERV16 carbon clean filter box on the return, so going to a MERV 8 on the aprilaire (assuming it wouldn't be installed to dump into the return) would be kind of a bummer.

Instad, I've been looking at an ultra-aire 98h, which has a recommended install of new return + dump to supply. What do you all think about:

1. Going with one dehumidifier, on the main floor, instead of 2
2. Installation - where to pull air from (if one humidifier for two stories) and where to dump air to
3. Is it really OK to just use the dehum fan to distribute air through the duct system?
4. What's a reasonable range for installation cost of this type of setup?
4. Any preference for the aprilaire vs the ultra-aire?

Lastly, and most surprising to me, I've had a really hard time finding hvac companies that are good / familiar with best practices of dehumidifier installations. Most don't touch them, or haven't touched them in a long time. Being in Houston, I don't really understand why this is so. If anyone has a recommendation on a good dehumidifier installer (also - setting up the ventilation) in the Houston area, I'm all ears.

I realize this is a wall of text; hopefully it helps answer most of your questions up front. Thanks for reading and thanks for your time in advance.

[thermofridge]

I too am surprised you don't have luck finding companies with familiarity with dehumidifier installations in Houston. Here in Wisconsin we take this pretty seriously. Perhaps because we have very dry winters and our very humid summers are so far the other way that they feel extreme.
I would say from reading your post that you pretty much have it down.

[teddy bear]

I too am surprised you don't have luck finding companies with familiarity with dehumidifier installations in Houston. Here in Wisconsin we take this pretty seriously. Perhaps because we have very dry winters and our very humid summers are so far the other way that they feel extreme.
I would say from reading your post that you pretty much have it down.

I am with you on this. By the way, nice Therma-Stor reclaim installation. 90% of the hot water from the walk=in freezer in 80% of all the modern Grocery store in the US.
Now for other big problem. Basements do make easier to understand the absolute choice between a mold farm and basement that turns out to be the nicest space in the home.
Soon green grass climates will come to understand that a simple a/c setup properly with a small whole house can have 50%RH regardless the outdoor dew and low sensible cooling loads.
For a list of Houstin a/c contractor providing Ultra-Aire whole house dehumidifiers.
Keep us postedl
Regards Teddy Bear

[HVAC_Marc]

6 tons for that sized new house sounds excessive. That's probably why you feel uncomfortable in low load situations. It short cycles and never does what you need it to do.