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Old 07-18-2013, 04:30 PM   #1
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DIY duct work?

I'd like to add air conditioning to my house, and trying to figure out the way to achieve the best value for my money.

The most straight forward way would seem to be adding mini-splits. The way my house is configured it would be best served by (2) outdoor units and (5) indoor units. It would not be ideal for keeping an even temperature in the large common kitchen/dining/living room area, but would be great for letting the (4) bedrooms get individual control. I had a couple of quotes and they agreed my (5) indoor unit system would be best, the two local professionals came in pretty close at about $20,000 for Fujitsu or Mitsubishi systems.

With a price tag that high I started to wonder if it might make sense to think about a traditional forced air system which includes a furnace at the same time.

Right now I have hydronic heat. First floor is in-wall convectors (1950s) fed by monoflow using iron pipes. Second floor is hot water baseboard loop with copper. The boiler is oil fired, about 10 years old. I just spent $2000 within the past couple of years for a SS chimney liner which I was told I needed urgently. The oil tank is original to the house and would be about $2500 to replace. Based on my oil usage, the price per gallon, the price for NG and the difference in efficiency ratings, I believe I could save a minimum of $1000/yr switching to NG as long as the price difference remains about the same between the two fuels.

I've had the entire house insulated with blown-in high density cellulose, new windows, and new doors. It really does a good job of holding heat and cold very well at this point--although I'm sure that the in-wall convectors must leak their heat like sieves in the winter since they are metal heat-conducting devices nearly butted directly against the sheathing in my 2x4 walls, I believe at best there is 60 year old R-5 FG insulation batts behind it, which is what had originally been used around the entire house.

So, my thought is, maybe it could make sense to replace the current heating system with forced air and use a heat pump with a natural gas / propane back-up? With the heat-pump I would use even less NG/Propane then a conventional heating system as well...

The big issue seems to be running ducts... but I'm somewhat fortunate in that I have an unfinished basement and I have access to knee-wall attics on either side of the second floor, which give access not only to the back of the lower halves of the second floor walls, but also to the first ~4' of the first floor ceilings at the exterior walls on the two long sides of the house. To get up there I could build a couple of chases into a two first floor closets giving me at potential for least an 18"x24" chase from the basement up to the knee walls attics on both sides, and since I'd be taking out the in-wall convectors on the first floor I would have the option of putting the registers for each room under each window, so those chases would mainly need to house ducts for a few ceiling vents, some returns and whatever the ~600sq-ft second floor needed.

So would it be insane to consider working out a plan for register placements and duct sizing for my house and then trying to run them all myself?

I figure in that case my professional labor would come down to someone installing the indoor and outdoor pieces of the heat pump, connecting the propane/NG, and then tying into the now-existing duct work.

I'm sure the materials alone for that piece of the job will add up of course, but if I could come in at around $20,000 or so for all the materials and professional labor for the job, I would have not just have a/c, but also a new heating system to replace my current aging setup. I know that hydronic is quiet and more comfortable heat then forced air, but with the improvements I've made to the shell of the house I feel like the performance could be similar to newer construction...


My advice is based on anecdotal knowledge or personal experience. I'm not a professional no matter how matter of factly I may say something
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Old 07-18-2013, 09:27 PM   #2
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don't like your chances..... you need a contractor.....


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Old 07-18-2013, 11:01 PM   #3
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Do the right thing and get three bids from contractors. You should ask them what THEY would recommend doing.
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Old 07-19-2013, 07:30 AM   #4
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Originally Posted by strategery View Post
Do the right thing and get three bids from contractors. You should ask them what THEY would recommend doing.
Just don't use them to design your DIY system for you.

Or, if the quotes are coming in beyond your ability to pay, offer to pay them for the design and DIY from a professional design.
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Old 07-19-2013, 08:17 AM   #5
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the design is very important ... you have to live with it forever... hammering ductwork together by yourself can be done but sizing critcal
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Old 07-19-2013, 09:14 AM   #6
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Anymore get 6 bids. They all are looking for jobs.
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Old 07-21-2013, 10:41 PM   #7
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I done this 6 years ago. It is 2 ton AC only system in the attic; with 3 returns; and all flex duct with triangle ductboard distributors. I ran all of the supply and return ducts to the air handler, and paid 1500 to a professional to install the condensor and air handler. It pass arch/electic/plumbing inspection in the second time. The total cost is about 4.5K. My house has another 5 ton system in the basement, and it not design well to push the dense cool air through the second floor. I under estimated the need of one room in overall design. Since I installed the original duct, so just modifying one branch was not to difficult. Everyone is happy with comfortable temp around the second floor.

Just to clarify a bit. I studied Mechanical Engineering in the first 3 years in college, so I am quite familiar with AC theory, cooling cycle. Adiditionally, I am professional who work computer room design, layout, and computer system setup.

It is very challenging job in multiple areas. The key to a successful new AC, first is the well engineered design, and then the second is the competent installation.

good luck.


Last edited by uid60001; 07-21-2013 at 10:44 PM.
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