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Old 10-19-2010, 10:08 PM   #1
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New duct work design help!!!


Hello and thanks for the help.

My current set up is a 1970's America Standard 80k btu forced hot air Natural Gas furnace.

The supply lines are currently on the interior walls and the returns are on the exterior walls which is opposite of what its supposed to be now. There is a supply and a return in every room except the bathroom. The living room has a larger return than rooms.
My house is a 1950's cape and the duct work is only on the 1st floor. I want to extend the duct work up to the second floor. I am also planning on finishing the basement.
I already know that I need a 3 to 3.5 ton unit. I am thinking about a York LX series Furnace and matching evap coil and condenser. I also know what size ducts need to go to each room. I want to run them in the joists so I can finish the basement.

What I need help with is figuring out my main supply and return trunk lines.
The unit is capable of 1600cfm and I am not sure how large the trunk ducts have to be. Also is it easier to run one main trunk up stairs then break off that or just run long runs of say 6" flex ducts to each room through the walls. Long run is 15 feet max....

I have attached some pictures I drew up quickly. Anything will help.

Thanks

Joe
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Old 10-20-2010, 07:12 PM   #2
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Any help would be great I want to finish my basement so I don't want the main trunk line to take up a lot of head room....

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Old 10-20-2010, 07:40 PM   #3
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Get your check book out.
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Old 10-20-2010, 08:48 PM   #4
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$10k-$12k are my quotes for 80% and 13seer
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Old 10-20-2010, 09:17 PM   #5
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If you are dropping that kind of money it makes better dollars and sense to go with a 90% and a higher SEER ac...

Do you know about the rebates and tax credits available? A 95% furnace would pay for the cost diff between it and an 80% furnace. Gas is only going to go up in price.
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Old 10-20-2010, 10:00 PM   #6
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Thats whats I was getting for quotes for those units I want 90% + 16 seer + I can get the units for cheap my uncle has a contractors discount at F.W. Webbs which I will be using to buy some sort of York LX Series furnace and condenser...

Yes I know all about rebates and the tax credit...I know pretty much everything the only thing I don't know is if the main trunk has to be able to support all 1600cfm for example if the main trunk is split equally say 8 feet to the left of the air handler and 8 feet to the right do they both have to be able to support 800cfm.

I might just see if I can hire someone to put in my main trunk and plenum and then do the rest....

I just have a very hard time spending $12,000 pretty much on someone running the duct work for me....which is easy when I can get better parts for $2500 total..
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Old 04-17-2011, 07:56 PM   #7
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So I put this project off but now that its getting warmer out I am going to tackle it. What is a recommended plenum size? and main trunk line those are pretty much the only 2 things I can't figure out....80K btu York Affinity LX 95% unit I'm going with a a 1 seer 2.5 ton compressor...

Any thoughts

Thanks
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Old 04-18-2011, 05:19 PM   #8
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Never heard of a 1 SEER A/C. sorry, couldn't resist.

The plenum should be the size of the opening of the furnace/A/C case coil.

Trunk line size? Probably 28X8, maybe a bit larger, depending how long it is.
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Old 04-20-2011, 07:13 AM   #9
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haha 16 Seer I mean't...... yeah my house is about 30 ft long and the current unit is in the middle of my house. I think it will be about 15ft - 20' I'll have to measure from one end of the current to the next because I don't want to have to start cutting all new vent holes in every room.
Most rooms are either 8" x 3" metal duct or 12" x 3" metal duct which is fine I just want to flatten the main trunk line but make it wider to keep the same air flow obviously. So that it gives for more head room in certain area's for when I finish my basement.

My house is about 30' x 24' (720 sq ft) and an upstairs that is 30' x 18' (540 sq ft) so I am thinking a 80k york LX with a 3 or 3.5 ton 16 seer unit.

Let me know what you think I am having a friend who owns a HVAC company come over to help me tomorrow with the correct size of the ducts and what not....I just can't justify giving someone $11k to put together $1,000 work of duct work. and to hook up electrical and connect a few pipes....plus the units obviously but those I can get shipped to my door for less than $3,000... so If I can get everything for $4,000 why are they charging me $11,000...thats the cheapest offer in my area I got. I live in a very expensive area. I'll save my $7,000...and put it toward my new Audi S4....
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Old 04-20-2011, 06:16 PM   #10
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Well, in their price is an allowance for warranty labor, design cost, staying business to serve you down the road, paying their employee's, employee's vacation pay, 401K, etc. All the things you want and probably get from your employer.

80,000? For that small of a place? 3 tons? maybe 60,000 and 2.5 tons. At 1650 sq ft, my place is oversized with a 2.5 ton A/C.
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Old 04-22-2011, 02:10 PM   #11
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I think it will be a little bit over sized. How many heat runs do you have?
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Old 04-22-2011, 02:28 PM   #12
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I will have a main trunk line with a 12" round going upstairs to the second floor that has 2 bedrooms and a bath room. (3) Then 2 bed rooms a bathroom a living room that I'll have 2 runs going to and a kitchen (6) and then 1 in the basement. So 10 runs total....
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Old 04-22-2011, 02:51 PM   #13
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Are all the runs 6" except for the second floor
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Old 04-22-2011, 03:57 PM   #14
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Quote:
Originally Posted by Chris4mx View Post
I think it will be a little bit over sized. How many heat runs do you have?
I have 8. But the number of supplies means absolutely nothing. Its the total C FM of the supplies that counts.
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Old 04-22-2011, 04:25 PM   #15
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That's y I was asking what size they were. A 2.5 tone needs about 1000 cfm. To work correctly. Each 6" run is about 100 cfm. 7" about 150 cfm.

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