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Old 04-25-2020, 09:10 PM   #1
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Dehumidifier position within crawlspace


Good evening!

Anyone here have a Santa Fe Compact or similar in a ducted installation?

I have a 26 x 60 crawlspace with the dehumidifier located at the lowest corner, near the sump pit and access door. The far opposite corner tends to have the highest humidity, but the dehumidifier doesn't reach it as effectively; it dries the corner where it's installed but doesn't have quite enough "throw."

I have considered moving the unit to the center of the crawlspace, where headroom and access are less but overall crawlspace circulation would be significantly better. A gravity drain line would be at least 25 feet long and subject to damage, so I'd likely use a condensate pump to send water to the sump pit.

The other obvious option is to leave the unit where it is, with drainage and service conveniently located, and duct the discharge straight up and over 10-20 feet toward the middle of the crawlspace.

I am inclined to go with the second idea, basically directing the discharge air in the correct general direction. There should be less rebreathing with a duct, but obviously airflow will be reduced. Will there be a noticeable performance penalty with a discharge duct?

A.
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Old 04-25-2020, 11:41 PM   #2
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Re: Dehumidifier position within crawlspace


I would try it in the middle but maybe it is not large enough for the conditions. Vapor pressure makes the air try to reach an equilibrium so it should tend to dry the whole crawl space. I am curious, how is a gravity drain line in a crawl space subject to damage?
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Old 04-27-2020, 03:22 AM   #3
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Re: Dehumidifier position within crawlspace


If sized properly, the duct will have minimal effect on the air flow volume.
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Old 04-27-2020, 05:42 AM   #4
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Re: Dehumidifier position within crawlspace


First you need to get a reading on both ends of that crawlspace. RH is relative to the temperature as well so each reading needs its accompanying temperature then you can adjust each to a common base.

Is the crawlspace encapsulated, walls and floor, and insulated?

Bud
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Old 05-03-2020, 05:21 PM   #5
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Re: Dehumidifier position within crawlspace


Quote:
Originally Posted by Old Thomas View Post
I would try it in the middle but maybe it is not large enough for the conditions. Vapor pressure makes the air try to reach an equilibrium so it should tend to dry the whole crawl space. I am curious, how is a gravity drain line in a crawl space subject to damage?
This is a rather atypical crawlspace that probably could have been a basement if the home had been built slightly higher. The foundation steps down from 3 to 6 feet deep, and the dirt floor mostly slopes toward the sump pit.

The lowest area has good access, with 5 feet of headroom, exterior stair access, and has mechanical systems including the dehumidifier and sump pit. I'd like to make it long-term storage space eventually. Any other dehumidifier location will put the drain line across the foot traffic path. The floor is 6 mil poly on top of 6-8 inches of pea gravel; the gravel was added because the original owner wanted a basement and dug the floor out, cutting the original poly and creating a muddy mess that caused a lot of the moisture issue. He tried to make it into a "basement" complete with a work sink and paneling on the ceiling.

The shallow end of the crawlspace, 50 feet away, gets the brunt of ground water and runoff from uphill. I've replaced gutters, regraded, and added a driveway drain; these have helped immensely, but it's an ongoing effort. It would certainly help if my neighbor's downspout didn't shoot water right toward my house. There is no interior or exterior drain tile, but the slope generally carries any infiltrated water toward the sump pit. The block walls get damp in the upper area, as does the soil next to the footing. The vapor barrier covers about 95% of the floor area, but the edges are not sealed. I haven't bothered yet because I knew the poly would get damaged from all the construction traffic and have mostly assumed it would be temporary.

The area has an air circulation dead zone, so any localized humidity can cause mold even if the rest of the crawlspace seems fine.

I do plan to install an interior drain tile, but the foundation changes elevation five times from one end to the other, so the drain tile would have to be in sections and would really just be helping water to find a path downhill toward the pit.

A.
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Old 05-03-2020, 08:51 PM   #6
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Re: Dehumidifier position within crawlspace


Quote:
Originally Posted by Bud9051 View Post
First you need to get a reading on both ends of that crawlspace. RH is relative to the temperature as well so each reading needs its accompanying temperature then you can adjust each to a common base.

Is the crawlspace encapsulated, walls and floor, and insulated?

Bud
Good point. I don't have any instrumentation. There's definitely a humidity gradient, but I'm not sure how much.

The crawl was built vented and has not been encapsulated. As I find various quirks (such as the lack of brick ledge), it seems important that the perimeter framing needs to be able to dry to the interior, meaning no foam board on the band joists. Venting makes a huge difference in radon readings, so I would prefer to vent when the ambient dewpoint is low. Of course, as summer approaches, venting is no longer helpful and I have to switch to the dehumidifier, so I want to make sure it reaches all corners of the crawlspace. Some air bypasses the vents, and the vapor barrier isn't sealed, so it is far from perfect. I plan to eventually do a GOOD vapor barrier and maybe a dewpoint-controlled ventilation system such as ATMOX or a homebrew system.

What insulation was there was ripped out for mold remediation a few years ago and has not been replaced. Since I am not keen on insulating the perimeter, I would need to insulate the floor to pass code.

Part of me wants to set up a computer and DAQ system down there and control fans, dehumidifier, etc., with some data logging. It's been on the back burner until I fix the obvious things, though.

I see this got a bit off topic, but it should provide some background. Basically I want to maximize the effectiveness of the dehumidifier until the construction activity is done, which could take a couple more years.

Last edited by ablodneyget; 05-03-2020 at 08:53 PM.
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Old 05-03-2020, 08:57 PM   #7
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Re: Dehumidifier position within crawlspace


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Originally Posted by beenthere View Post
If sized properly, the duct will have minimal effect on the air flow volume.
Thank you for your input. I'm starting to think I could get away with moving the unit a few feet over and running the exhaust out the top and adding a single 90-degree bend to point the discharge air to the opposite end.
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Old 05-04-2020, 04:04 AM   #8
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Re: Dehumidifier position within crawlspace


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Originally Posted by ablodneyget View Post
Thank you for your input. I'm starting to think I could get away with moving the unit a few feet over and running the exhaust out the top and adding a single 90-degree bend to point the discharge air to the opposite end.
That may work fine.

I personally, prefer the running of the duct to the other area. As it creates a positive pressure in the other area forcing an air flow from the far end toward the dehumidifier. Ensuring an air mixture in the crawlspace.

There are of course more ways than one to skin a cat.
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Old 05-06-2020, 04:58 PM   #9
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Re: Dehumidifier position within crawlspace


I have a 700 sq ft basement next to a 1300 sq ft crawlspace. We keep a 70 pint dehumidifier at the basement end on auto. It drains into the HVAC sump pump.
The vents stay closed year round, and eventually will be sealed.

At the narrow far end of the crawl I placed a 20x20" box fan to keep some air moving 24/7. Just months ago I added another fan near the center of the crawl to help the wall around the sewer pipe exit stay dry. All parts of the basement and crawl space keep reasonable humidity this way.
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