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Old 11-12-2008, 03:12 PM   #1
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Number, Size, and Location of Celing Vents


I have a 1750 sqft home built in 1978. All of the HVAC vents are ceiling mounted and located close to the exterior wall of each room and blow in the direction of the window/wall. My AC repairman said that they used to do this on some homes with the idea that cooling/heating the exterior walls worked well in adjusting the temperature of the home to a comfortable level...he disagrees. Anyway, I will soon be replacing my 3.5 ton unit with a 4 ton and I want to relocate all of my ceiling vents to the proper location. What size and location should I use for the the rooms in my house:

Laundry Room, 13' X 6' (1 vent)
Kitchen/Nook, 30' X 15' (2 vents)
Living Room, 19' X 18' (2 vents)
Hall Bath, 8' X 7' (no vent, should I consider adding one?)
Bedroom #1, 12' X 11' (1 vent)
Bedroom #2, 17' X 11' (1 vent)
Master Bedroom, 18' X 13' (1 vent, should I consider adding another?)
Master Bathroom, 9' X 6' (1 vent)

Also, Ceiling height in all rooms is 8 feet.
Is there a website that gives out these specs?

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Old 11-12-2008, 05:39 PM   #2
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Number, Size, and Location of Celing Vents


Which room is the room that you added, that you need a 1/2 ton larger A/C.

Depends how close the grilles are to the outside wall. It does make a difference.

If you didn't add a room. Might want to see about improving your homes insulation, and infiltration. And save money by being able to use a smaller unit.

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Old 11-12-2008, 05:40 PM   #3
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Number, Size, and Location of Celing Vents


you neglected to mention what your impression of how the system operates currently are you generally happy with its performance? if you have rooms that are problematic than you can consider adding registers if your just doing it for the hell of it than id say if it aint broke dont fix it
your hvac guy should do a manual j (heatload calc.) on your house it will determine the tonnage you need and btu requirement
if he says you dont need it (he probably doesnt know how) and you should find another company
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Old 11-12-2008, 07:20 PM   #4
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Number, Size, and Location of Celing Vents


I didn't add any rooms. The AC man recommended that I go with a 4 ton based on the living area and that the existing unit was not sized correctly. Vents are about 4 feet from the exterior walls.

Currently the system does not operate well. Then again I should just be happy that it operates at all being that its the original exterior unit from 78'. Air Handler was replaced in 92'. I was told the SEER rating on this unit is roughly a 6. But the master bedroom and bedroom #2 get very hot in the summer.

The attic could definitely use some more insulation.
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Old 11-12-2008, 07:41 PM   #5
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Number, Size, and Location of Celing Vents


regardless of weather you added rooms or not manual j is the industry standard and i belive a code requirement when pulling a permit even for a replacement i do realise that la. is probably a little more lax when it comes to permit requirements but the man j takes into account the direction your house faces types and number of windows,insulation type and amount,sq footage of the home etc
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Old 11-12-2008, 10:43 PM   #6
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Number, Size, and Location of Celing Vents


Four tons is a bit big of your size home even with the heat and humidity of La.

Seems you got a larger latent load to worry about from the design data I have before me.

I would definitely add a third diffuser to the nook and kitchen area.

I would suggest quite strongly that you have a Man J heat calc done on the home to see exactly what your latent vs sensible heat loads are. Some equip combinations address this more efficiently than others. i would also suggest you ac man does a survey on your duct work to make sure none of the flex is in need of change.
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Old 11-13-2008, 04:58 AM   #7
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Number, Size, and Location of Celing Vents


As others, a load calc should be done.

I have 1650 sq ft.circa 1956.

I have a 2.5 ton A/C, and can maintain 72, when its 98 outside.

True, I have good shading. But if sized by sq ft, my A/C would be 1 to 1 tons too small.
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Old 11-13-2008, 11:40 AM   #8
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Number, Size, and Location of Celing Vents


Another consideration. You AC guy is just expressing his opinion.

Understanding heat gain/loss is important.

Windows are the major cause of heating/cooling. Even with efficient low E and double glazed windows they are the major heat loss/gain for any home.

Conditioning of air (heating or cooling) should involve mixing the hot with the cold to get a nice temperature.

Having air blow towards the opposite wall is one of the ways to go. It is not designed to heat or cool the wall, it is designed to mix the existing air with the conditioned air. Since there is a lot of heat/cool around windows this is a reasonable approach.

Any kind of forced or convective heating/cooling is designed to do just that with the exception of hydronic heating which actually heats the structure (floor).

Position of the vents to mix the existing air is the key. Having the vents high on the wall or in the ceiling so that it can't be blocked by furniture or people is desirable. Also having it come out at sufficient velocity to mix the air is also good (beenthere will disagree but hvaclover and I are on the same pager here ).

This is not so much opinion as scientific studies have shown that this does indeed work better to keep you home more constant as far as temperature.

Bottom line is that if you have good ducts that are free of obstructions and are blowing with sufficient velocity to mix the air in the room then it doesn't matter where they are aimed. In fact my whole HVAC system has vents that blow towards windows and outside walls (this is where the heat/cool is that I want to condition).

I really see no reason to make any changes or location in your situation. If you don't like them there just put in a deflector to push the air towards the center of the room.

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Last edited by Marvin Gardens; 11-13-2008 at 11:43 AM.
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