Converting Crawlspace based system to ducts?


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Old 06-24-06, 09:19 PM
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Converting Crawlspace based system to ducts?

My wife and I just bought a 1750 sq ft trilevel in the midwest that has a wooden foundation and a central heat/air conditioning system. The furnace is a ten year old Bryant gas unit with AC. On this house, the furnace outflow empties into a 16"x8" duct that is open to the crawlspace below the ground floor. This whole crawlspace acts as a single duct for the 4 livingroom, and kitchen registers. Another main duct feeds the upstairs via normal flexible ducts. Our question is, how feasible would it be to run a new duct attached to the 16"x8" duct and run sub-ducts to the registers directly? There is almost no airflow at the registers upstairs when the system is on and it currently gets quite warm during the day even with the AC set to 65'. Would adding ducts to the crawlspace require adding vents and insulation to the area, currently there are no vents or insulation under the floor?

Thanks in advance!

Christopher

Edit: A little poking around the net indicates that the system being used on my house
is usually reffered to as a "Plen-Wood" system. This is the system that I'm thinking of
converting to conventional ducts.
 

Last edited by ChristopherT; 06-24-06 at 10:40 PM.
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Old 06-25-06, 08:19 AM
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Let's make sure we understand your problem. Are you saying that the supply duct opens directly into the crawl space and there are no separate ducts that connect to the four registers in your living room and kitchen. If yes, that would explain why you get low airflow to downstairs registers and yes, you could install ducts to run directly from the 16x8 to those four registers. Yes, you would also have to insulate between the floor joists or insulate the crawlspace exterior walls as that area will no longer be considered "conditioned" space. I'll leave it to others to recommend what size duct you should install to each register.

There is one more consideration here worth mentioning and that is if you do this new duct thing, you may very likely create a problem in your crawlspace as it relates to moisture buildup, condensation, and mold growth. The crawlspace WAS conditioned space before you added ducts and that is what prevented these problems in the crawlspace.
 
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Old 06-25-06, 12:28 PM
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Originally Posted by N74TG
There is one more consideration here worth mentioning and that is if you do this new duct thing, you may very likely create a problem in your crawlspace as it relates to moisture buildup, condensation, and mold growth. The crawlspace WAS conditioned space before you added ducts and that is what prevented these problems in the crawlspace.
That's one of the things that concerned me, the possibility of adding a condensation issue into the solution. I could easily insulate the walls of the crawlspace but there are no existing vents down there. One setup I was considering would put a duct to the crawlspace with a manual damper on it so I could control how much air continued into the crawlspace for humidity control while the rest of the air was diverted to the new ducts for heating and AC. If I did insulate the walls of the crawlspace and vent the area, would it stay dry enough to keep the wood foundation happy?

Thanks, N74TG

Christopher
 
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Old 06-26-06, 06:03 AM
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I am building a new house that is similar to your problem except in reverse. Wheras your crawlspace is being used as the supply duct to each room, mine will be used as the return from each room. I will have ducts that run to each room (similar to what you want to do). There is also a 2nd register cut into each room (the return register). All returns go directly into the crawlspace, which returns to the system.

Why do I do it this way -- 1. I have a 12x22 basement room that I will let the return air cool and heat, and 2. it will help get rid of/prevent moisture problems so common in crawlspaces, and 3. since the supply ducts will be in conditioned space they won't have to be insulated much/if any.

If you did this you could maintain the conditioned nature of your crawlspace, while at the same time theoretically increase the airflow to your rooms.
 
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Old 06-26-06, 06:41 AM
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Originally Posted by N74TG
3. since the supply ducts will be in conditioned space they won't have to be insulated much/if any.
Oh, I think you will find you still need to insulate them well..
 
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Old 06-28-06, 02:17 PM
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Originally Posted by N74TG
If you did this you could maintain the conditioned nature of your crawlspace, while at the same time theoretically increase the airflow to your rooms.
I called two local A/C companies and the first referred me to the second. The second is coming by on Friday morning to take a look at this "Plen-Wood" system, apparently he has converted several houses in this addition to conventional ducted systems and several in another quite economically. He said if it's done right there is no need to vent the crawlspace/plenum or worry about condensation. He's also going to give me an estimate on replacing the 10 year old Bryant Gas/AC system with a heat pump.

Thanks Guys!

Christopher
 
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Old 07-01-06, 10:25 AM
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Wink

I think I read all the post here.
Now what you have can and does work well.
IF put in right. All walls 2 " insulation .then a R19 insulation put in each joist space just up on the belt board all around the home. A 6 mil poly down on the ground over lap 2' and tape all joints. Now do this and keep what you have .OR do this and then put the duct work in down there. BUT be sure and put 2 small registers and 1 return in it. No dont use insulated duct. You want the whole crawl space to work as a heat sink for you. Dont put insulation up in the joist all over put it on the walls . No outside vents down there at all. Just the registers.
Go read http://aboutsavingheat.com/crawlspace.html

ED just my .02 cents been doing it like that for over 30 years
 
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Old 07-02-06, 02:37 PM
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Originally Posted by Ed Imeduc
I think I read all the post here.
Now what you have can and does work well.
ED just my .02 cents been doing it like that for over 30 years
Thanks, Ed. I did have the guy come out and look it over. For around $750.00, he wants to run a second metal trunk line into the crawlspace and wrap it with insulation. He wants to run ductwork to the new register boots and insulate those too. Last he wants to put a register into the crawlspace so that stays "conditioned space" and we avoid the condensation issues. The crawlspace floor and walls are already covered in a heavy,black water/vapor barrier that actually is wrapped over the top of the foundation and stops before it gets to the grade on the outside of the house. He's also putting in two registers in the finished basement, something the previous owner neglected to add.

Christopher
 
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Old 07-02-06, 06:33 PM
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Wink

Poly on the floor and sealed good. You still need insulation on the walls and in the joist space just up on the sill plate all around the home. Why oh why insulation on the duct ??????
when you will have registers in the duct for heat and cool down there. Makes it a "conditioned space" just metal pipe to the registers off the trunk line
Registers in duct in basement good . But put a small register in cold air return drop also so you can play with it to open or close it a bit as you need heat or cool down there.
Did you go read that www. I gave you--do

ED
 
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Old 07-03-06, 04:59 AM
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Originally Posted by Ed Imeduc
Registers in duct in basement good . But put a small register in cold air return drop also so you can play with it to open or close it a bit as you need heat or cool down there.
Did you go read that www. I gave you--do

ED
Yes, I did and I put installing insulation on our TDBW (To Do
Before Winter) list.

Thanks, Ed!

Christopher
 
 

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