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DIY Central A/C install, anyone done it? #7846283 05/21/20 02:11 AM
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Im tired of the window units in my 1940's farmhouse. Ive got a 3 ton package unit Im installing. So far Ive just gone through the unit making sure everything was good with it, changed out all the insulation inside the unit, poured a concrete pad for it to sit on, and starting gathering material. Man HVAC stuff is expensive. Even with my 10% military discount at HD Im going to end up spending right close to $2K on materials. I had a local company give me a quote on doing everything and it was $5400, SO, Im doing it myself. My house is 1150 sqft and Ill have 8 drops to rooms.

Im going to be running all flexible ducting with R8 insulation. 18" for starting out for return and 16" starting out for supply. Ill be going up the wall outside and into the attic, building an insulated box around it up the outside wall.

For the return, Im starting with an 18" R8 flex duct and then Y'ing into two 14" flex ducts going to two 20x25 return boxes and grilles. Im confident on the return side.

The supply side, without being able to do all the tests the pro's do to check resistance and flow and all that other crazy stuff, Im really just guessing. Ive planned it where the cfm's are correct going through the system with the different sizes but that's about the most I can do.

Has anyone ever done something like this themselves? A couple of things im not sure of are

Which size register vents to use for the drops
The 10" drop Ive got running to my LR would it be better to have a single 10" to a vent or Y that off to two 8" vents or would that make a difference?




The blue number is the size of the flex duct, the black is the cfm that duct carries. I think I have it balanced ok.


SUPPLY- 16" flex duct from unit, up wall into attic, then branching down to 14-12-etc.


[Linked Image]


RETURN - 18" flex duct from unit, up wall into attic, then Y's into two 14" flex ducts running to two 20x25 return boxes.

[Linked Image]

Last edited by Nathan at Fork; 05/21/20 02:12 AM.

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Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #7847203 05/21/20 11:51 PM
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Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #7847207 05/21/20 11:56 PM
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Better hurry, that Attic is going to get hot soon!

Re: DIY Central A/C install, anyone done it? [Re: Superduty] #7847334 05/22/20 02:28 AM
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Originally Posted by Superduty
Better hurry, that Attic is going to get hot soon!


Too late!

Re: DIY Central A/C install, anyone done it? [Re: Superduty] #7847356 05/22/20 03:02 AM
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Originally Posted by Superduty
Better hurry, that Attic is going to get hot soon!


It wont be too bad. IVe got a huge attic with 9' of headspace, keeps it from getting too hot.


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Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #7847471 05/22/20 07:11 AM
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Exhaust vents by windows, suck out that hot air before it has a chance to circulate with the cooled air.

A lot of good HVAC guys will tell you to install an exhaust vent in every room to help balance the entire house. I would at least put one in the far bedroom in addition to the living room and front bedroom.

Also, I would Y the living room vents into 2 vents


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Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #7847605 05/22/20 01:33 PM
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I would y the 10" and use 10x8 or 14x6 boots on everything but the 4 and 5 " use 10x6 or 8x4 on them you may wanna put dampers in the wyes on the main trunk

Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #7848762 05/23/20 07:26 PM
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Had someone tell me a setup using plenum boxes in the attic instead of all the wyes would be better. Any thoughts?

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Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #7848851 05/23/20 08:56 PM
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Originally Posted by Nathan at Fork
Originally Posted by Superduty
Better hurry, that Attic is going to get hot soon!


It wont be too bad. IVe got a huge attic with 9' of headspace, keeps it from getting too hot.



The first one I had replaced is stil in my attic. I don't go up there anymore.


Quail hunting is like walking into, and out of a beautiful painting all day long. Gene Hill


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Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #7849049 05/24/20 12:41 AM
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Originally Posted by Nathan at Fork
Had someone tell me a setup using plenum boxes in the attic instead of all the wyes would be better. Any thoughts?

[Linked Image]

easier to install, don't have to wrap if they are ductboard. you can resize the holes in them if ya need too. cheaper in the long run

Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #7999746 10/06/20 02:14 AM
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It took much longer than I originally planned but quite a bit of that was waiting on the fittings and ducting I had ordered. I finally got everything finished up last week. I have to say that having zero experience doing this and doing ALL work myself, it was quite the chore. I came out of the attic every days covered in mastic. That stuff gets everywhere.


Here is the layout I ended up with. It may not be perfect but it works with good pressure from all ducts. Im very happy with it. The First two big tees are attached together with no flex between them. My return air screen is 20x25 and above it is a big return air plenum box in the attic that the 18" flex runs to. I used all curved vein registers.

[Linked Image]


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Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #8000439 10/06/20 07:24 PM
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Nice. We have a cabin I would like to do that too but I don't have the patience you do.

Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #8000461 10/06/20 07:43 PM
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Originally Posted by Nathan at Fork
It took much longer than I originally planned but quite a bit of that was waiting on the fittings and ducting I had ordered. I finally got everything finished up last week. I have to say that having zero experience doing this and doing ALL work myself, it was quite the chore. I came out of the attic every days covered in mastic. That stuff gets everywhere.


Here is the layout I ended up with. It may not be perfect but it works with good pressure from all ducts. Im very happy with it. The First two big tees are attached together with no flex between them. My return air screen is 20x25 and above it is a big return air plenum box in the attic that the 18" flex runs to. I used all curved vein registers.

[Linked Image]

that will work but i would have put all my supplies on the interior walls blowing to the exterior. mixes the air better and makes shorter duct runs. you can install the grilles to blow on the exterior as you installed it or maybe you did, just used more duct than needed and had to lay out in the insulation to run the duct, cant even crawl that close to the eave.

Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #8000489 10/06/20 07:59 PM
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I have a tall high pitch roof so had plenty of room to maneuver. The areas I put the ducts around the outside wall had probably 4.5' of headspace space. There is around 10' headspace in the attic in the center of the house.

I had read and been told lots about the vent placement and it was about 50/50 on placing them on the inside wall or outside wall. The bottom of the diagram is the south side wall and lots of heat comes in those windows. I have the curved vane registers blowing on those windows. It is working well, no room colder or hotter than it should be. Of course getting finished so late in the year, next summer will be the real test. I think it turned out well though.


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Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #8001963 10/07/20 11:38 PM
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good job. ive ducted a bunch of houses in my time. "it cant be that hard, why does it cost so much" nice to hear the other side!

Re: DIY Central A/C install, anyone done it? [Re: Nathan at Fork] #8003806 10/09/20 02:25 PM
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Originally Posted by Nathan at Fork
I have a tall high pitch roof so had plenty of room to maneuver. The areas I put the ducts around the outside wall had probably 4.5' of headspace space. There is around 10' headspace in the attic in the center of the house.

I had read and been told lots about the vent placement and it was about 50/50 on placing them on the inside wall or outside wall. The bottom of the diagram is the south side wall and lots of heat comes in those windows. I have the curved vane registers blowing on those windows. It is working well, no room colder or hotter than it should be. Of course getting finished so late in the year, next summer will be the real test. I think it turned out well though.


Good job, but why a package unit?
They make some pretty compact air-handlers these days that would be easy to fit anywhere.

I 'diy' upgrade on my last house, 2 16seer 2T split heat-pump systems - saved about $14,000 -vs- what 'the man' wanted. Sil-phos brazing copper is easy - just flow nitrogen for a clean braze inside.
Units were just $2200ea delivered to my driveway.
Current house I switched out a 2yo straight-cool condenser to a 16-seer heat-pump condenser, making my system 'dual-fuel' electric/propane auto-switchable for heat - 1 tank of propane lasts me like 2 years!

Re: DIY Central A/C install, anyone done it? [Re: oldoak2000] #8005547 10/11/20 02:31 AM
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Originally Posted by oldoak2000
Originally Posted by Nathan at Fork
I have a tall high pitch roof so had plenty of room to maneuver. The areas I put the ducts around the outside wall had probably 4.5' of headspace space. There is around 10' headspace in the attic in the center of the house.

I had read and been told lots about the vent placement and it was about 50/50 on placing them on the inside wall or outside wall. The bottom of the diagram is the south side wall and lots of heat comes in those windows. I have the curved vane registers blowing on those windows. It is working well, no room colder or hotter than it should be. Of course getting finished so late in the year, next summer will be the real test. I think it turned out well though.


Good job, but why a package unit?
They make some pretty compact air-handlers these days that would be easy to fit anywhere.

I 'diy' upgrade on my last house, 2 16seer 2T split heat-pump systems - saved about $14,000 -vs- what 'the man' wanted. Sil-phos brazing copper is easy - just flow nitrogen for a clean braze inside.
Units were just $2200ea delivered to my driveway.
Current house I switched out a 2yo straight-cool condenser to a 16-seer heat-pump condenser, making my system 'dual-fuel' electric/propane auto-switchable for heat - 1 tank of propane lasts me like 2 years!


Because it was free! My dad bought it at an estate auction about 10 years ago for $100. It was 3 years old when removed. Its been sitting in my pasture covered with a tarp for the last 10 years. I had to replace all the internal insulation, the fan motor and patch a few wires but thats all it needed. So put less $100 into it getting it ready to use.


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