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Old 05-21-2010, 09:24 PM   #1
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adding duct work and outlet


I have had four AC contractors at my house and got 5 different answers. I want to add an outlet/vent? in what was a porch. ONe said. can't be done. #2 said can be done for $1000, he said I need new main run. #3 wants $400, 4 wants ah maybe $250?
I know nothing about AC. This is a Florida Concrete block house with 2X4 trusses for roof. The problem one they don't want to work in the attic and the clearence between the top of the concrete block wall and bottom of plywood sheeting on roof. The 2X4 truss leaves a 3 1/2 inch space to run duct through. The x porch has a flat roof married to the end of the pitched roof so no help there.
In FL all I see is flexible duct, to knock a few inches out of top of wall to run a 6 inch duct between roof sheeting and top of wall would be a bear of a job. As the top row of block is filled with concrete.
Why cant I run the 6 in flex duct in attic and adapt it to 3x 7,8,9,10?? metal duct where it needs to pass through 3 1/2 in clearance??? It seems the metal duct could be mostly insulated by gluing on foam.
To me this looks like the way to go, but again, I know nothing about AC or duct work. Do they make an adapter for flex to metal duct? What size 3X metal would I need to use.
Thanks in advance JIm

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Old 05-21-2010, 10:13 PM   #2
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adding duct work and outlet


Duct has to be insulated in unconditioned space so your 3 1/2" space has much, much less area for air to travel through. So figure a 1" liner in the duct makes an opening 1 1/2" tall. That little space will not provide enough air to do anything so you'll be wasting your money. Better to rip the flat roof off of the porch room and put a pitched roof on or cut out a block and build a soffet to cover the new duct. Second option would be cheaper but first option would look better.

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Old 05-21-2010, 10:46 PM   #3
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Probably why the one contractor said can't be done.
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Old 05-22-2010, 08:39 AM   #4
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Thanks for the responses. I was thinking of insulating the exterior of the duct with 1/2 in foam board. The duct would be passing through the space at the angle of the roof, so the 3 1/2 in would only be a few inches. 1/2 foam on top and sides full length. then as far as possible on bottom, should only leave about 4 inches on the bottom uninsulated.
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Old 05-22-2010, 09:31 AM   #5
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As long as you don't use it for cooling, it might work.

Use it for cooling, and that 4" of uninsulated metal, will sweat, and soak the area around it. And give you a nice mold problem.
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Old 05-22-2010, 09:33 AM   #6
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Any uninsulated area is going to sweat and cause mold and 1/2" foam doesn't have enough R value so will do the same. Still will have too small an area for air flow. Give any thought to a through the wall unit?

Beenthere beat me to it.
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Old 05-22-2010, 10:53 AM   #7
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Condensation, never thunk of it
I've framed in for a through the wall unit. I want a small AC to use on a small generator, Honda EU2000i when the power is out.

I'm hoping to find a way to run a duct off the central air, for a reasonable amount of effort and money. It is not looking promising.

Another option I see (remember I do not know anything about AC) is run flex duct through the garage. I could put the vent in the wall a couple of rows of block down where the block is hollow and more accessible it would be eaiser to cut through wall. The drawback I see to this is the length of the duct. How long can a flex duct be? This would be getting in the 15 to 20 ft long area.

I have had four AC contractors here and have not had a good feeling with any of them.

I know it is hard to comment on something without looking at it, but I really appreciate your time and effort. You guys want to come over to the house I'll buy dinner!
JIm
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Old 05-22-2010, 12:03 PM   #8
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Depends what you tap the take off into.
Your current trunk line isn't sized for the porch. So you would need to tap into the plenum. Might need to use a 8, 9, or 10" flex, depending how big the porch is.
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Old 05-22-2010, 02:46 PM   #9
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Thanks for the response. The x porch is 12X15. It had 9 37X62 single pane awning windows. Three on each wall. I removed the 3 on the west wall (sun side) framed it in and insulated to r 26, 3 1/2 unfaced and 3 1/2 faced. I replaced the other 6 windows with vinyl, insulated, argon filled, low e glass. I furred the ceiling out and insulated that to R30. The rest of the house has 3 1/2 inch 40 year old fiberglass in the ceiling and nothing in the walls.

The main plenum is approximately 20x17x12" It feeds a 12x12" trunkline/duct? which runs about 14 foot the length of house which feeds the vents through various size flex duct. The 12X12 main duct is feed by a round flex duct that is about 14 inchs long and has an outside dimension of about 12 inch dia.

The area that could be tapped into in the main plenum is the 12 inch face. This would require a run of about 36 feet to where I would want to vent it. Seems like a long run??

While I was looking at this the AC came on and there are several small leaks in the existing duct work. All the duct work is 18 year old duct board.

Can't think of anything else to add, any questions.

JIm
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Old 05-22-2010, 10:06 PM   #10
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12" outside diameter? That would make it a 10 inch flex. Look on it, to see if its size is still listed on it.
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Old 05-24-2010, 11:28 AM   #11
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Over the weekend the relay/contact? points stuck, something else to add to the never ending list. While I was freeing the contacts I noticed a schematic, the date on it was 1983. This unit cannot possible be that old, can it??

The plenum is 20X17X12, the 20X17 being the outside dimensions of the unit. It connects to a 12X12 main duct by a flex duct. The flex duct is about 8 inch long and has an outside diameter of 12 in, it fills almost the entire end of the 12X12 duct. There is no size listed on it.

I will try to add a pic, it is in a very small attic, with almost no room to maneuver. I wish I had done this before the roof, I could have pulled part of the old roof and had tons of space to work.

The first picture should have been rotated, the 1 foot mark is actually at the top of the duct. What appears to be a wide duct is actually the end of the cold air return next to the supply duct, it is also 12X12.
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Old 05-24-2010, 01:45 PM   #12
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A 12" round only moves about 76% as much air as a 12x12 duct. At the same pressure loss.
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Old 05-24-2010, 03:12 PM   #13
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adding duct work and outlet


Thanks for the response. I just had contractor #5 here, this is ridiculous. I got 2 prices on my roof and hired the first guy. Got one price on my floor and hired the guy, I'd seen his work.

#5 seems knowledgeable, easy to talk to and trustworthy. He wants to bring 2 6" flex duct into the room compressed through the 3 1/2" opening. Claims he does it often.

Also wants to relocate the air handler to the opposite end of the house. When he explained why it made sense.
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Old 05-24-2010, 03:19 PM   #14
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So why does he want to relocate it to the opposite end of the house.
Ask him for references to these houses that he squashed the flex duct. And then call or visit them. And see how bad the room is.
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Old 05-30-2010, 11:58 AM   #15
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Thanks for the response beenthere. I responded to this post and it seems to have disappeared??

I see what I believe is a better way to run this duct and have mentioned it to 2 contractors who have brushed it off. They want to squash the duct through a 3 1/2 in space or build a "special duct". Actually they want to bring 2 ducts into room.
The new room is across the back of the house with 11 feet against living area and 4 feet against garage. By lengthing the supply duct by 2 feet I could bring it through the garage, and through the wall into new room. I checked the wall is hollow concrete block and easy to break through. The difference would be the vent would be in wall 4 inch down from ceiling instead of through ceiling.
The duct would come through the garage ceiling along wall between living area and garage and through garage wall to new room. This is above electric hot water tank and creates no space or clearance issues. It would have to be boxed in with fire rated board. I don't know anything about AC, but it sure looks like a no brainer. I could be wrong.

Relocating unit. Air handler is in tiny attic built with trusses. I believe its a 4/12 pitch. It is accessed by a hole in the end of house. The hole in end of house is directly over condeser unit complicating entry. Air handler is close to end of house making access difficult at best. You have to kind of wiggle, squirm around return plenum to gain access. The guy that put this in must have been 5' tall and weighted 98 lbs. It is one of those why did they do it this way jobs!!
If moved to other end there is a fold up stair leading to attic. It's still a small space but the air handler could be set to one side and make access to the control panel much easier. Wiring would need to be cut shorter, not made longer. Most of the system needs to be replaced so it will only create a little extra work.
I like this part! Present system could be left in operation while bulk of new system is being installed.
Condenser theft in Florida is rampant, this would put my new condenser inside the fence.
It really seems like the optimal way to do it. But I have been wrong before.
JIm

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