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Central Air Sizing


We have a two-storey house, and we have conventional air on the second floor and Unico on the first floor. I much prefer the conventional system. Contrary to what all of the Unico fans say, our conventional air conditioner takes out more humidity.
I have also been told that it is more efficient for each floor to have its own system, but if I were to do it again I would get all conventional.


  • steve_173
    steve_173 Member Posts: 140

    I am considering putting central air in a 1920s house. In the past I sized window units using an ASHRAE worksheet that provided fairly good results. When considering central air, is the system size generally the sum of the individual loads. I guess it drops a little as neighboring rooms are cool, but the main external load is still there.

    Any general thoughts on high velocity versus traditional AC? What about other tips such as brands or how to find a good installer. I'm getting prices and sizes that are all over the place. Some want a return in each room, some one return. Some want to come through the basement to the first floor, others drop down through closets.

    Is there a concern that the plaster keys holding the plaster up may break when they slide the flex ducts through the ceiling? Is the flex harder to upkeep and clean than metal ductwork?
  • Gregg Jackson_7
    Gregg Jackson_7 Member Posts: 2
    high velocity air conditioning


    I have done many homes similar to yours with high velocity air conditioning. I can say that i think (especially for your application) that it is the best option. A knowledgable installer can make use of closets and chaseways to hide the small ductwork and blend the system into the homes decor (usually). I personally use the Unico System for all of our installations and have had great success in cooling some very odd and hard to reach areas. I will say that this system is 90% design and layout. Many people do it by "rule of thumb" and this is how high velocity received some negative press about being noisy. A properly designed and installed high velocity system provides a better leel of comfort in my opinion. It will remove more humidity than a conventional system which i feel gives you greater comfort without having to plunge the thermostat. This system also operates with one central return (typically) which reduces penetrations throughout your home. I also have to say that the Unico airhandler is one of the best i have ever dealt with. High quality and very low failure rate. The most important aspect of your project, however, is a proper load calculation. I would not guess or add up individual window units to get your cooling load. A good contractor will have modern heat gain software that will calculate your load based on actual construction conditions (windows,R-values,etc). I have installed high velocity both in ceilings, floors and in some cases both in the same structure. I dont find much of a difference in performance but i prefer ceiling installation when accessible. Hope this helps.
  • tim smith
    tim smith Member Posts: 2,544
    Flex only for last 5 feet of each run on std velocity system

    Returns in each room is the ideal way. Zone it if you can swing the extra $$. Use variable speed air handler and 2 stage condensor. All metal duct except for few feet of flex at outlet terminals. Best way to do job. Hi V as other said may be more practical depending on construction of house and lack of room for regular velocity trunks. I would prefer std velocity if I had my way but sometimes not feasible. Tim
  • steve_173
    steve_173 Member Posts: 140

    Why do most guys want all flex in the attic? Why do you suggest minimizing flex? Is it best to not use it at all?

    When you say zone, doesn't each zone need a separate condenser and air handler?

    Do you still have a central return with a return in each bedroom? Or is the idea to ideally have a return in every room including hallways, but not bathrooms?

    If one adds better insulation later, will the system be oversized?
  • tim smith
    tim smith Member Posts: 2,544
    Flex is cheaper and easier to install. The zoning I am talking

    about would be with zone dampers and integration w/ air handler for air volume and maybe staging on the condensor side. Carrier/Bryant have a good zoning system for their higher end systems. Good luck, Tim. PS, the reason for a little flex right before register boot is to help quiet down and velocity noise coming to register.
  • steve_173
    steve_173 Member Posts: 140

    Using zone dampers how would you suggest zoning a two story house? I assume you would use two systems (house needs more than 5 tons), but how would you split those systems up?

    Most installers tell me not worth the bother for zone dampers.
  • tim smith
    tim smith Member Posts: 2,544
    Depends on the direction exposure and the type of use in rooms.

    Sometimes zoning can be very helpful and as they have said, sometimes not. Just depends on the overall evaluation of the house.
This discussion has been closed.