Central AC install in existing house

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Old 04-07-11, 08:20 AM
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Central AC install in existing house

Here is the scenario: a 2500 square foot colonial house that does not have central ac installed and has hot water baseboard so there is no ductwork installed. I know I would have to do a J load calcuation to get the right sized unit but I just want to get a feel for the project first before I commit.

I believe using two zones, would be the best and easiest way to install since running ducts from the attic down would be a nightmare. I know that you can run them through closets but running up through the basement to first level floor you can position the ducts anywhere as is the same down from the attic for the second zone.

If you do run them up from the basement can you run ac ducts up through the floor? By this I mean registers in the floor? Given that cold air sinks would this work, or should they come from the ceiling?

If I were to go closet runs and install one unit and run them down from the attic to the first floor what size do the ducts have to be? I would be using rigid ducts so they do not get damaged in the closets. Once they are in the attic I could use insulated flex to run each to the plenum.

Would it be better to remove drywall and put the ducts in the wall then re-drywall? I assume that doing this you would have to cut the top plate also to get the duct into the wall.

Thanks for the help
 
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Old 04-07-11, 09:59 AM
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It would be best to have a licenses HVAC contractor come out and look at your house and discuss your options. Only they will be able to give you usable answers.

It is common to run ducts for the first floor in the basement or crawl space so the registers are in the floor. This is good for winter when heating but not optimal for summer when running AC. Upstairs it's common to come down through the ceiling with registers and then you have the opposite issue where they are in good position for cooling but not heating. If you are only doing AC it would be best to have all the registers in the ceiling but that's probably not economically viable (more work = more money) when doing a refit. My house has the registers in the floor and while not perfect for cooling it works very well. I believe the result of a properly sized system providing proper airflow.

In most cases running ductwork in standard thickness walls is not an option. The ducts are just too big to fit. Cutting out top and/or bottom plates is not recommended since that may be part of the load bearing structure.

The size of ductwork, registers, quantity and location will all depend on the house and size of system you install. Also do not forget that in addition to ducts for the registers you will also need return air ducting and intake grills.
 
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Old 04-07-11, 11:35 AM
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Thanks for the reply and I do plan on having a contractor take a look. I am just trying to get an idea of what needs to be done and the $$$. If I could do some of the work myself I will. Wiring and setting up the units wouldnt be a problem and I would have it inspected for safety and charged by a contractor and inspector upon finish.

One question I have is on the return if I go with the one unit in the attic. The house I currently live in has one unit in the attic which has a large return on the second floor ceiling. Its a large square grill in the ceiling with a filter. There is a ceiling register in each room that the cool air comes out and the house cools eavenly and quickly. Wouldnt using one return like this be the easiest to install? I understand that a return in each room (minus bathrooms and kitchen) would provide best circulation but if I have to run ducts for the air wouldnt one large return on the second floor be fine? The house does have a large two story foyer so the air that is being pumped out on the first floor can easily recirculate up to the second floor.
 
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Old 04-07-11, 02:18 PM
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Most do not install a return in each room. Quite often cutting an inch or 1 1/2" off the bottom of the doors will allow enough airflow even when the door is shut.

Depending on your location code may not allow just one cold air return on the second floor. They may require at least one return one each floor. I'm not licensed in HVAC so I don't know but if you can go with just one in the ceiling on the second floor it would be easiest and therefore the cheapest.
 
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Old 04-07-11, 07:44 PM
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I would look at two units and not a zone system. Companys can't get zone systems wright on new installs much less on finished homes. I'd estimate around 4 grand a ton depending on the system u choose.
 
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Old 04-08-11, 03:40 PM
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The closets on the second floor and the first floor line up perfectly. So if one unit was used:

Is it ok to install the unit in the basment, install the runs to the first floor through the floors and then run a duct up through these two closets into the attic and then down from the attic into the rooms?

Also can this be reversed... install unit in attic with runs into the rooms ceilings, and then one duct down through the closets into the basement and then up into first floor?

Would this be too far of a distance?

Again I am just getting an idea of what could be done so I appreciate the comments.
 
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