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Conditioned air in crawl space question

Old 03-04-2012, 06:54 AM
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Default Conditioned air in crawl space question

We are buying a new home that has over 2200 sq ft of crawlspace. The first thing I am having done is encapsulating the entire space. My hvac guy has suggested running supplies and returns to the space as a means of drying out the air. In my current house I had a dehumidifier installed after we encapsulated the crawlspace. Any experts here that can give me objective advice on which method is preferable?
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Old 03-04-2012, 08:52 AM
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I'm not sure it's even allowed by code to duct off of your central system into a crawlspace. I suppose it will depend on how you encapsulate it.

Personally, I think the crawlspace should be treated as a separate zone. The dehumidifier acts as a separate zone and is much better at removing humidity. The only downside to the dehumidifier is that it gives off heat during the summer, I doubt it could give off enough to be a problem. I have seen dehumidifiers that were ducted to the outside during the summer and kept the heat in the crawlspace during the winter.
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Old 03-04-2012, 08:53 AM
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I don't know how much help I can offer. I have 20+ years in the HVAC industry, but being in florida I have never encountered this crawl speace qustion, or been posed with this problem/issue.

Here's what I would say - to attempt to condition 2200 s.f of crawl space seems like a lot of area. A dedicated dehumidifier seems more practical and efficient, By adding supply, and return dustwork you are now bringing that air into the entire house. This could bring in indoor air quality issues, and additional humidity into the home, not to mention killing efficiency. It likely would not meet energy code requirements. Residential systems are sized from 1.5 to 5 tons. This would also seem to bring the load in excess of 5 tons when you combine the home size, unless he is speaking about a dedicated unit for that area, and again a dehumidifer would seem to be a better option. IF and I still think it's a bad IF on the surface of the question - if a system was installed for that purpose, insist on a variable speed air handler, and look a a 2 speed or dual compressor system. With a non-variable speed (single speed air handler) you have a 100% full speed or zero (off) system. No humidity removal when off, and poor humidity removal when on in full speed as you have good velocity, but the air stream is pulled past the evap coil quickly resulted in little dehiumidification. A variable speed system pulls out about 3-4 times more humidity than a single speed. Not a huge cost difference with variable speed air handlers (about 400.00 - 500.00) as the labor is the same. A 2 speed compressor does add significantly to the equipment cost.

I would not do it if it were not a dedicated system. I would not want to bring that air into the home on a continious basis. A dedicated dehumifier seems a lot more logical. We have done serveral in Florida attic applications that suffer from "sweating attic syndrom" You may also want to look at a form of ventaltion or "fresh air make-up" system used in conjuction with a dedicated dehumidifier. Any questions feel free to pm me or I can give you my number if you or the a/c guy want to talk. - Jon.

Last edited by jon in florida; 03-04-2012 at 09:03 AM.
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Old 03-04-2012, 09:16 AM
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Thanks guys. Let's start off with what I mean by "encapsulate". The pics below are from our current house. The first is the before, the second and third are after encapsulation.









This system completely seals the crawlspace. The vents are closed and my furnace, which is a high efficiency model, draws air from outside.

I did a limited search on the method proposed by my hvac guy and it appears to be a viable alternative. None of the discussions I read worried about code issues. Supposedly with the crawlspace completely encapsulated, the air coming from that space will be the same as the rest of the inside of the house. I can tell this....if you go down into my current crawlspace, you will not smell a damn thing .... no earthy smell, no moisture, nada. I love the encapsulation system.

My hvac guy is pushing me towards the Trane 16i (dbl speed) .... he would love me to buy the mac-daddy xl20i. I am probably going to go with two Trane xl16i. I did read some guy argue the single speed xl15i does a better job on humidity than the 16i. I have a xl15i in my current house and it is a very good system. I may need to consider the cost differential between the two. It also seem to me I will have to buy and operate considerably more ac tonnage to condition the 2200 sq ft. I should find out what it cost to operate two separate dehumidifiers when compared to the larger compressors, if I go that route.

Anyhoo. I appreciate the input. I am trying to find someone that has done what is being proposed.
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Old 03-04-2012, 09:26 AM
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There is no question in my mind that I would go with dehumidifies that are made for encapsulated crawlspaces. I believe Fine Homebuilding (the "green" section) had an article on it.
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Old 03-04-2012, 09:41 AM
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Originally Posted by davedowneast View Post
There is no question in my mind that I would go with dehumidifies that are made for encapsulated crawlspaces. I believe Fine Homebuilding (the "green" section) had an article on it.
Thanks buddy, I will check it out.
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Old 03-04-2012, 10:26 AM
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I just got done ducting and installing a new funace and air in a home with that done to it. They do recommend (insul company) having grills run into the space. This was a really old farm house with 5 seperate crawls. There is no smell. It encapsulates the space from the outside preventing any moisture to wick in through the ground. It keeps the moisture out better than a poured basement. I was VERY skeptical of this, but it works. If the space isn't tempered than all the money was for nothing. It will condensate and cause problems. It doesn't need to be the temp of the living space, but it does need air flow. One option is to put returns into the floor and have a larger return grill cut into the duct work at the air handler. Put them around the perimeters so all corners will draw and get air flow. If it's a two story (or three, with your money) make sure the returns have proper draw. Things will need to be modified to get airflow to the crawl as well as having proper draw to the rest of structure. Don't rob Peter to pay Paul. A variable speed fan is recommended. I would insist on a load calc and NOT have it oversized at all. You will want it on the smaller side to be able to pull the humidity out. Bigger is NOT better with air.
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Old 03-04-2012, 11:26 AM
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Originally Posted by tinmarine View Post
I just got done ducting and installing a new funace and air in a home with that done to it. They do recommend (insul company) having grills run into the space. This was a really old farm house with 5 seperate crawls. There is no smell. It encapsulates the space from the outside preventing any moisture to wick in through the ground. It keeps the moisture out better than a poured basement. I was VERY skeptical of this, but it works. If the space isn't tempered than all the money was for nothing. It will condensate and cause problems. It doesn't need to be the temp of the living space, but it does need air flow. One option is to put returns into the floor and have a larger return grill cut into the duct work at the air handler. Put them around the perimeters so all corners will draw and get air flow. If it's a two story (or three, with your money) make sure the returns have proper draw. Things will need to be modified to get airflow to the crawl as well as having proper draw to the rest of structure. Don't rob Peter to pay Paul. A variable speed fan is recommended. I would insist on a load calc and NOT have it oversized at all. You will want it on the smaller side to be able to pull the humidity out. Bigger is NOT better with air.
I found a company that sells a simple fan system you use to push some conditioned air from the house into the crawl space. They recommended passive grills, like you recommended, to allow the air to return to the interior space. I need to talk to my hvac guy about this considerably less expensive alternative.

I totally understand the problem with oversized ac units. We had an old 5 ton unit cooling one zone in our house. It died last year so I had the hvac guys come estimate what I needed for that space. They figured I needed a 2 ton unit, not the 5. The smaller unit did an incredibly better job at removing humidity.
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Old 03-04-2012, 11:31 AM
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You have to watch when it comes to fans. They tend to be loud and will drive you insane. If installed, have it well insulated around it and any and all duct lined on the inside for sound attenuation
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Old 03-04-2012, 12:13 PM
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Originally Posted by tinmarine View Post
You have to watch when it comes to fans. They tend to be loud and will drive you insane. If installed, have it well insulated around it and any and all duct lined on the inside for sound attenuation
Roger that. My family can't sleep without white noise so a fan probably wouldn't bother us a minute.

I received a link to a professional hvac forum from a friend here. The more I read at that forum the more confused I became. Apparently the pro's don't agree on this issue. Many would avoid at all cost bringing air from the encapsulated crawl space into the household. Others cited building codes requiring this system. Right now I am leaning to simply dehumidifying the space. That approach works in my current house.......if it isn't broke, don't try to fix it. I'll let you know what my hvac says after we talk about his recommendation tomorrow.

Thanks again to all that posted comments.
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Old 03-04-2012, 12:46 PM
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"My hvac guy is pushing me towards the Trane 16i (dbl speed) .... he would love me to buy the mac-daddy xl20i. I am probably going to go with two Trane xl16i. I did read some guy argue the single speed xl15i does a better job on humidity than the 16i".


Just to give you a little backround on the Trane units. Both the 16i and 20i are great systems. Both will pull out more humidity than a single speed compressor 15i. Both would almost always be matched with a variable speed air handler like I first suggested, The 16i is a 2 speed single compressor, the 20i is a dual compressor (a 4 ton would have a 2 ton maintence compressor and a 4 ton primary compressor). both systems (and all 2 speed/dual compressor systems) are only available in even ton sizes 2/3/4/5 ton. The 20i can be a "communicating" system which I personally would stay away from. Trane begins their spring rabates on March 1st. The 16i should be a 500,.00 rebate and the 20i a 1000.00 rebate. Keep in mind the dealer co-ops these rebates @ 50% so the 1000.00 Trane rebate has a direct cost of 500.00 to the Trane dealer. Most utility companies also pay a high efficiency rebate that becomes larger as you go up in efficiency.

For example a 4 ton 20i cost exactly 641.00 more than a 16i (they would generally use the same variable speed air handler model) 500.00 more Trane rebate (really 250.00 net with the dealer co-op cost) IF there are any utility company rebates - often the 16i and 20i become about the same cost. Does the dealer mark-up the 20i more? I don't now but those are the real costs. Just did a 20i system on Friday that was literly 46.00 NET more than a 16i. Again, I don't know your utility co. of if it's new construction or an existing home.

Two speed or dual compressor systems are the best way to go, Ultra high efficiency, more consistant temps and highest dehumidification. Think of a system running on the seconday or low speed compressor like cruise control vs. stop and go city driving. They are awesome but be aware that duct sizing becomes much more critical, and 2 speed-d/c systems do not like high static presure. On the communicating systems, ductwork flaws make that system a nightmare. Installed properly - there is no way a single speed 15i has better humidity removal than a 16i oor 20i. The 16i/20i have longer more efficient run times. Longer run times = more dehumification, that simple! The idea is that very seldom do you need full capacity. It has the ability to use full capacity of course, but 95% of the time you can control comfort better with less capacity. Best of both worlds a 4 ton (5 ton is a 2.5 & 5 ton etc.) will run 2 ton almost all the time - but if needed it's a 4 ton when it has to be.

BTW 2 speed/dc with variable speed ahu's are the quietest systems made - which is another benefit.

Last edited by jon in florida; 03-04-2012 at 01:07 PM.
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Old 03-04-2012, 01:06 PM
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I think that if my HVAC guy was telling me to put a return in a sealed crawl space, I would be looking for somebody with more experience. All the ones I am familiar with have a supply which maintains a small positive pressure, and a pressure release vent to the outside.
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Old 03-04-2012, 01:42 PM
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There is a Complicated science to inclosed crawl space! If not done correctly you are dealing with serious mold, dry rot issues!! There is a specialist on a spray foam forum that goes through the science, reasoning and codes of inclosed crawl spaces, you mat want to check that out . Sorry can not recall his name of the top of my head? Never mind just googled it for you. Sprayfoam.com .... Mason Knowles. He will personally answer you questions or steer you in right direction . Some very technical info !,
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Old 03-04-2012, 02:14 PM
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I really appreciate the feedback jon. You convinced me to go with the 16i or the 20i.....I need my guy to break out his fee quote for each item before I can decide. Great tip on the rebates. We are with Ga Power, they often have rebate programs.

Glacier, his plan is to install both supplies and the proper amount of return from the encapsulated crawlspace. I think the EPA endorses this as well.....not that I have a lot of respect or trust in the EPA.

Thanks Jehovah, I will look him up.
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Old 03-04-2012, 02:24 PM
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Your before and after pictures are exactly what I had done to my crawl space about 3 years ago. The company that I hired to do the job recommended a large,heavy-duty crawlspace dehumidifier, which I did buy. I don't remember the name, but it is a horizontally-installed unit, with a condesate pump. It keeps the crawlspace 35-40% RH.

I WOULD NOT, introduce crawl-space return air into the living space of the home. De-hu is the way to go.
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Old 03-04-2012, 03:02 PM
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Craw spaces have been getting along for years and years without anything except vents. I agree with putting wrap around all the pipes that go through them, other than that leave them be with vents. If you don't believe it, I have a bridge I want to sell you, the Green thing is just going way to far.

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Old 03-04-2012, 05:08 PM
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have you considered putting a vapor barrier on the dirt, leaving the vents for outside air flow, and spraying closed cell foam on the ceiling of the crawl space?
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Old 03-04-2012, 05:26 PM
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I guess I'm missing something here. If the space is encapsulated, where is moisture coming from?? If anything, I could see it being too dry. I did it once and was very pleased with the outcome but there was no moisture to dry out. Maybe it's a different application?
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Old 03-04-2012, 05:35 PM
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Put a 12K BTU through wall A/C unit in and run it on low. Friedrich has one wired for 240VAC. That's what I did. Dry as a bone and cool. Doesn't draw much more current than a dehumidifier and you don't have to worry about draining the collected water.
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Old 03-04-2012, 05:37 PM
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Originally Posted by Pierless View Post
have you considered putting a vapor barrier on the dirt, leaving the vents for outside air flow, and spraying closed cell foam on the ceiling of the crawl space?
No, I haven't considered that option given my experience with the humid air in the SE and mold in crawl spaces from open vents.
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