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high velocity a/c returns

A neighbor of mine did something similar. One Unico serves 2nd and 3rd floor; another serves ground floor and basement.

One staircase to the 3rd floor and it's closed off by a door. Staircases between ground and 2nd floor are open.

Very good comfort on the ground and 2nd floors. Basement sometimes clammy, but nothing particularly unusual about that. 3rd floor overheats.

While hi-velocity systems <I>usually</I> only need one return, I suspect that serving two floors that are not open to one another is an exception. Another exception is a far-flung wing.

I've yet to see a heat gain calculator that compensates for the natural tendency or hot air to rise--particularly in an old home with high ceilings and multiple floors. With an old three-floor it seems best to calculate each floor, knock off 1/4 to 1/3 from the ground floor and add to the 3rd. I'm thinking here about a 3rd floor under a hip/gable roof, not a flat roof as in a townhouse.

"Dumping" a lot of A/C into the ceiling of an open stairhall works quite well with conventional systems to cool the ground floor provided that the ground floor rooms are quite open to one another--the kitchen is the typical trouble spot in such a plan and usually gets a "window shaker" in my area. I'm not sure if this would work well with hi-velocity systems however as they move MUCH less air.


  • Pete_18
    Pete_18 Member Posts: 197
    various questions

    I am currently spec'ing out a high velocity unico system for my home to cover the 2nd and 3rd floor. The 2nd floor is completely finished and the third floor is only partially finished, but we intend to finish the entire floor in the future and would like the unit sized for the whole floor. Neither floor is presently insulated, but we are hoping to be able to blow in insulation. The windows are original bubble glass and leak air but we are not presently planning to replace them.

    2nd floor - ~1000 sq ft (external measurements)
    3rd floor - ~200 sq ft (finished) / eventually 1000 sq ft.

    We have had very different opinions from the folks we have had out, so I have some questions:

    1. My experience with a typical a/c system is that the more returns, the better. I was thinking of a return on the 2nd and 3rd floor since we don't keep the door open between the floor and the 3rd floor stays warmer than the 2nd floor. Is this a good or a bad idea for a high velocity system?

    2. Is there a ballpark for how many drops belong in a room based on sq ft / sun direction? Everyone believes a different # of drops should go in each room ranging from (1 - tiny bathroom) to 3-4.

    3. If the 1st floor remains without A/C, even though it stays cooler than the rest of the house (today), should we be bumping up the 2nd/3rd floor AC as a result? I have people spec'ing 3, 3.5 and 4 ton units for the same job. The 4 ton spec is supposedly to give us another half ton to let air find its way downstairs. I don't mind adding a little excess air as long as the unit is not so oversized that is fails to remove the humidity as a result of not running long enough.

    4. Are there certain condensor brands that are more reliable or better than others?
  • Empire_2
    Empire_2 Member Posts: 2,343
    Your Unico system.....

    Although you situation is more complex than you think, you can rest assured that more return isn't necessarily better. Each area needs a predetermined amount in (CFM) to come back to the air handler and all returns should equal or slightly exceed the total cfm needed in each application. 4 tons of a/c out, 4+ back. Generally unico likes to see a minimum of 5 outlets per ton (delivering 40cfm)or 200 cfm per/ton. This is different from conventional systems. each run not to exceed 10' per run or it will decrease your delivery, thus more outlets will be required.

    There is no simple answer to your Q?, but find a contractor that you trust and can guarantee the work. Also like you stated, total heat gain is one of the most important factors of any structure, but not the only one.....I hope this helped you out a little...;-)

    Mike T.
  • terry_5
    terry_5 Member Posts: 92

    installing unico or space pak,its best to install 6-7 supplys per ton.Any less you'll have problems!
  • Bob Sweet
    Bob Sweet Member Posts: 540

    Unless your living at higher elevations, here at 5000 ft above sea level 8-10 outlets per ton is recommended, which also requires 250 to 300 cfm per ton.
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