Sizing and recycling of used duct parts

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Old 01-08-20, 05:47 PM
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Sizing and recycling of used duct parts

Hello everyone. We are a DIY couple that is trying to save as much money on a next to nothing budget and hope to get a bit of advice. One estimate was for $7000 and that's really not doable for us. We've already done our own foundation work, electrical, plumbing and insulation after lots of research and advice.

I've been researching HVAC duct installation and worked on my calculations but have some questions regarding some recycled ducts we were offered from a friend. Mainly if the size is too big for my estimates.
We also purchased a used 3 ton HVAC unit from a Habitat for Humanity store and just need it to get us through a couple of years so we can save up for a new unit. It is a 3 ton unit and calculated based on 1200 cfm.

The sections we were given are foam board: one main 8ft section14x20 and five sections 14x14 for at least two offshoots
Most info I found online shows that for 1200 cfm a round duct of 16" or rectangular 16x12 or 14x16 is adequate as a main trunk. Then use the other sections for reduction ducts.

Also thinking of hanging the unit from basement ceiling so that the drain can run outside without using a pump system.



 
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Old 01-08-20, 06:39 PM
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Are you measuring the used duct size inside or outside? Ducts are always measured inside.
 
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Old 01-08-20, 08:24 PM
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Thank you Grady for the quick reply.
I've been measuring the outside. I believe the material is usually 1.5' thick from what I've read, so measurements should be closer to 11x17... and 11x11. Doesn't seem so big now.
I've been comparing it to the size of ducts in the house we're in now, and they're maybe half the size of the one's we were going to use. That's why I wondered if they were too big and would need to be reduced..and this house heat/cool is fine, except for two rooms upstairs...
Now I feel a bit more at ease using those and makes me wonder if the house we're in now is undersized.

We will still have to build some additional duct at the end of the run and for the upstairs and of course the returns, but the donated boxes definitely gave us a jump on the work.

Thanks again.
 
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Old 01-09-20, 05:23 AM
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According to my chart, the 11 x17 would be right for 1200 CFM. The 11 x 11 would carry about 650-700 CFM.
 
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Old 01-09-20, 04:10 PM
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The chart I have shows that as well. I was basing those calculations on the reduction for lines coming off the main duct after a few supply runs totaling 238 cfm with an offshoot for other supply ducts for another 230 cfms according to my diagrams. I read that reducing the offshoot ducts by 2" on each side would help keep the air flow constant. That's why I was curious if these might be over sized.

Since we were thinking about hanging the indoor unit part I wasn't sure if I should build a plenum in a Y-shape to supply the main 11x17 and the other offshoot I mentioned above at 11x11. or have the offshoot attach from the main duct.
 
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Old 01-10-20, 07:37 PM
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A wye, generally being 45*, provides for smoother air flow. I'm not sure I understand exatly what you propose.
 
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Old 01-11-20, 09:38 AM
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We would be suspending the indoor unit from the basement ceiling joists to allow the drainage pipe to flow outside through a nearby opening with the water hose connection (all concrete basement floor with no drains), the unit would be forcing air forward into our ducts (instead of up and out of a plenum if the unit was sitting on the floor) This was a suggestion from one estimate we received so we wouldn't have to install a pump system to remove the water from the condensation line.

I was wondering if a wye plenum connection on the unit would be better to split the flow into the two trunk sections (11x17 and a smaller 11x11) or should the smaller trunk be connected as an offshoot branch from the 11x17 after it is attached to the unit? (Y or L shape configuration from a top down view of the layout)

I'm thinking that the L configuration is probably the way to go since more of the air needs to be forced through the main duct (11x17)for the further parts of the house and to ensure the offshoot for upstairs will have enough flow as well.

I could be overthinking the need for max air capacity/velocity thru the system. A blog I read suggesting going with the next size up so that the system has the ability to handle at least what you need. Its easier to dampen the air flow than try to increase it after installed.
 
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Old 01-11-20, 06:40 PM
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Resizing donated ducts

We are a DIY couple trying to build a new HVAC duct system after ripping out the oil furnace asbestos octopus that was there previously. A friend removed some ducting from a remodel he was doing and gave us an 8ft section 18x12 and five 4ft sections 12x12. We also purchased a 3ton HVAC (inside unit and outside fan unit) from a local Habitat for Humanity store. Hopefully this will get us through a few years to save up for a newer system as an install quote was $7000. House is approximately 1352 sqft

I have calculated that the one prebuilt box of 18x12 should be sufficient to move 1339cfm for the house. And the 12x12 section should be adequate for 524 cfm. I'll have to buy some additional duct board to build the return ducts. I didn't have an actual ductulator but did find several similar sizing charts online.

My question is if its advisable to cut down the remaining boxes for a couple of sections that would need 10x8 or 8x8 or would that make them to unstable since all the sides would need to be separated and cut?
Or should I just leave the 12x12 ducts as is and run into those areas? A few blogs said that delivering more air to rooms is better than not having enough, but wonder if that will slow it down too much.

Main trunk will run approximately 20 ft before attaching to another cross duct approximately 8 feet long.
Another section extends off from the beginning of the main truck for approximately 8ft with a 90* left turn for another 8ft. Upstairs never had ducting, so we're calculating 10ft vertical and 12ft long sections approximately 8x8 for just under 300 cfm for two supply outlets.

Again, return ducts will have to be built with same dimensions due to space limitation. Expecting at least two vents 20x25 and 12x12 with a possible 3rd replacing a downstairs supply duct if needed.

It was also suggested that we could suspend the indoor unit from the basement joists so the condensation line can be ran to an outside area with the water hose connection. Concrete basement floor with no other drainage available. This also seems like it would allow the full air velocity to travel down the main trunk.

I have attached my schematics and calculations to better understand the layout if needed.
Any suggestions are appreciated.
 
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Old 01-11-20, 06:49 PM
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Threads combined. Starting a new thread with usable information in another is not helpful.
Keeping them all together will allow all the available info to be known.

Also moved to the ducting forum.
 
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Old 01-12-20, 06:31 AM
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Not being able to either enlarge the images so I can see them well nor picture in my mind's eye what you're trying to do, I'm going to have to bow out.
 
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Old 01-12-20, 09:44 AM
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The second thread was after I went back and remeasured everything again to double check my numbers. The duct board is only 1" thick instead of the 1.5". And i had several sizing charts from different pro sites and they were all a bit different by a few inches, so that had me question how the different sizes could affect anything substantial.
I can send the images separately if that would be better.
Just looking for some advice on the things I should or shouldn't do.
I just keep second guessing everything because the house we're in now has ducts that are all the same size all thru the system and doesn't seem to hinder any air flow to the rooms, other than upstairs due to only one supply register in each room..
 
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Old 01-14-20, 04:06 PM
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I did find a local HVAC supplier that not only verified my calculations and suggested a few less ducts, but also gave me my own ductulator.
 
 

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