If you tie a non sealed Crawlspace dehumidifier into the home system, you will introduce microbial issues to the home. I would recommend two dehumidifiers, or have the Crawlspace sealed. The vapor barrier needs to be contiguous… two sealed layers is best.
Crawlspacedepot has a lot of instructions, and many companies will do the work…

 

Thanks for the reply! The crawl space is sealed. The vents, wall penetrations, and crawl space door is sealed. The moisture barrier is in really good shape and overlaps. With that said the moisture barrier is not taped and doesn't go up the walls or piers. The crawl space walls are not insulated. I had a guy come look at encapsulating it and he said there wouldn't be much benefit due to it being in such good shape and sealed up pretty good.

With that said, you still think I'm better off with a separate dehu in the crawl space?

Again thanks for your time!

 

Congrats on recognizing the need for supplemental dehumidification in your home. With adequate fresh air change, you moisture levels should even be higher.
With the air handler in the crawlspace, treat the crawlspace and mainfloor as one space. The total space is +-4,000 sq.ft. space. Three level structures have strong natural stack effect which cause air to move vertical depending on the the inside/outside temperature difference. Keeping crawlspace dry make them easy to have air quality equal throughout the home. Your humidity levels indicate minimal fresh air ventilation. You need a fresh air change in 3-5 hour when occupied to purge indoor pollutants and renew oxygen. With adequate fresh air change your home would have more moisture problems during evenings and rainy days.

A 150 pint per day dehumidifier located in the crawlspace with a fresh air inlet and a main floor return connected to the dehumidifier return with an adjustable dry air supply to the crawlspace and the majority of the dehumidifier output connected to the mainfloor cold air supply. With addition of fresh air to the dehumidifier will force the dry air slowly up through the home. This may control the top floor humidity.

I would suggest a unit like the Santa Fe Ultra 155 which is very high efficiency dehumidifier with Merv 13 air filter.

Your a/cs should be re-setup to remove enough moisture during significant sensible cooling loads (+50% duty cycle) and high outdoor dew points. Well setup a/cs will maintain <50%RH during typical moist season high outdoor dew points.

Thanks for an excellent post about a common problem in most well built home. And also coming up with a fix. Well done.

Keep us posted on issues and results.

Regards Teddy Bear

 

Thanks for the info! So I assume my downstairs ecobee could control the dehu based on my preferred downstairs humidity setpoint on the ecobee. I also assume the dehu would turn on and off for the house only based on my ecobee. What would control the dehu for my crawl space? Would the dehu ever turn on and off for the crawl space only? ie. the house humidity is 50% but the crawl space humidity is 75%...what tells the dehu to turn on for the crawl space only? or when the crawl space is being dehumidified is the house also being forced to be dehumidified at the same time even though it's not needed.

Thanks for the help!

 

A second dehumidistat can be added to the dehumidifier to activate the dehumidifier or allow the dehu fan to operate 24/7 to slowly mix the entire home, making the mainfloor tstat reflect the mix %RH of the home. Humidity is very aggressively to mix throughout the home.

Is the home occupied on a schedule or mostly 24/7?

Regards Teddy Bear

 

The home is occupied mostly 24/7.

The installation would be like this...right?

What tells the dehu duct damper for the crawl space to open and close?

Can the dehu dehumidify the crawl space without dehumidifying the house? Or would that require a 2 dehu's...one for the house and one for the crawl space?

Thanks!

 

A second motorized damper can be added to the duct to the basement dry air supply. I would try the dehu without the motorized damper first. The basement will be similar to the rest of the home with the dehu fan circulating the air of the home. Basements provide some cooling to the entire also and are warmed by the circulating air.

Keep us posted.

Regards Teddy Bear

 

Can the dehu dehumidify the crawl space without dehumidifying the house?

 

This would require a second electric damper in a return from the crawlspace into a duct to the dehumidifier return. The crawlspace dehumidistat would open the crawlspace return to the dehumidifier and open the crawlspace dehumidifier supply. The majority of the air flow through the dehumidifier would be from the crawlspace through the dehumidifier and back to crawlspace because of the short duct.

I would start out simple only add more electric dampers if needed. If the home is routinely unoccupied on a schedule, damper to control fresh air should be considered.

Keep us posted.

Regards Teddy Bear