As part of an ongoing kitchen project, I'd like to relocate the existing a/c register in the ceiling (which is located next to an interior wall) to the center of the room and upsize the duct (and register) to get a better cooling in that area. The kitchen faces east and has two (2) large glass french doors and one very large picture window, so there is a lot of heat transmittance in the mornings during the summer which tends to make it a bit uncomfortable unless the a/c is on full blast. I think if I could get some more air in there and also "wash" the doors/windows better, it would be an improvement especially if cooking and/or entertaining guests.

So the existing duct that supplies the area (11' x 12' kitchen, BTW) is an 8" round galv. rigid duct, that is connected to an 6" x 10" register. The 8" round duct is being fed by a larger branch duct (off the main trunk) that is also supplying another 8" round rigid duct routed to a room on the other side of the wall (dining room). I haven't been able to measure the branch duct yet, but just running some quick calculations I'm assuming it's at least a 12" round duct or maybe larger (???). I'll try to confirm tonight, but maybe some of the experts on here can make a guess...

In any regard, let's assume it's a 12" round branch duct for arguments sake. My question is: Would it be too much volume/flow if I detached the 2nd 8" round duct from the branch duct, and routed the 12" branch duct directly to a 12" x 12" register in the center of the kitchen? Or would it have to be a larger register? If so, how big? I would obviously make a new connection off the main trunk for the 8" dining room duct, but my main concern is if the branch duct is too large to supply a register or does it just depend on the register and/or register box size? Will it whistle or make lots of noise? Or should I just upsize the 8" duct to a 10" duct and leave everything else the same (i.e. keep the connection to the other 8" duct off the branch duct)?

Unfortunately I don't have the time or budget to hire an HVAC engineer to do an appropriate design, but it doesn't have to come out perfect so just some general advice to get me in the right direction would be greatly appreciated. I'm pretty sure the ducting system in the house is already undersized anyway, cause all the ducts have a little whistling/noise as is; But I just don't want something overly obnoxious in the kitchen area if I can avoid it...

  • At minimum we would need to know size of house, type of AC unit, specs on AC unit, and how many total vents you already have. – DMoore Apr 6 '15 at 16:12
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    Ok - sure! The house is 3-2 rambler style 2,250 SF (1,850 SF living space), concrete & stucco construction, and has 10 windows and 7 french doors (387 SF of glass). There are nine total vents in the house, most of which are fed by 8" rigid ducts off the main trunk (16" x 16"). The A/C is a 5-ton Trane 16.3 SEER, and was installed about yrs ago. – Joe in Miami Apr 7 '15 at 13:17

Given that this question is 3 years old you probably moved on... But I can make a couple of non-expert suggestions...

The first thing I would look at is shade. Can you add some foil into the windows to cut down the heat in the morning? Or some shade cloth on the outside? I have large sliding doors facing west and in the summer I span some 50% black shade cloth from the eave to the ground about 6' out. One can see through quite fine and it cuts down the heat load tremendously (even with the low-e windows I have). This saves a lot of money too...

WRT your specific question, here is what I would do. I you want to go to a ceiling register, check out https://www.hvacquick.com/products/residential/Grilles-Registers/Curved-Blade-Grilles/TRUaire-A303-Series-Triple-Deflection-Curved-Blade-Grilles Click on the links tab and then on the performance data link. You get a table that shows grille size on the left and then performance data across the row keyed by CFM. Take a 10x10 grille, assume you have 200CFM, that means the air "jet" will reach 12ft horizontally ("throw"), and the noise is "NC 30", which is low (there's no lower entry in that table, but other types of grilles/manufacturers go down to NC 20).

Now the other two interesting measurements are at the top of the table, for that register @200CFM you have a "back pressure" of 0.022 "WC and 600fpm face velocity. You probably have something like https://www.hvacquick.com/products/residential/Grilles-Registers/Grilles/TRUaire-210-Series-Steel-Single-Deflection-Adjustable-Bar-Grilles now, look at the performance info for the 10x6 you have. At 200CFM you have 15' throw (interpolating a bit), 0.026 "WC pressure loss, and 650fpm face velocity. So you would be reducing the pressure by a rather small amount, so you wouldn't get much more air.

Now, if you have an an anemometer to measure wind you could work your way backwards by measuring face velocity. I got a https://www.amazon.com/dp/B01L9SC36A but cheaper will work too. So measure the fpm you have now by holding the anemometer in front of the center of the grille, use the table for the grille you have now to get CFM, then use that to look up the hypothetical new grille to see throw distance and noise level. Note that for ceiling you want curved blades so the air mixes at ceiling level and down't blow straight down on you. You could also go with a round ceiling grille...

If the new grille has lower pressure loss, you will get more CFM, stealing from other outlets. WRT duct, 8" flex doing 200 CFM has a friction loss of 0.024 "WC (http://www.hartandcooley.com/tools/friction-loss-calculator-for-flexible-ducts). Of course I don't know how long your duct is, but it doesn't look like it's going to be a limiting factor, but it may well be in the same range as the grille.

OK, I'm not an expert and an expert would tell you you have to recalculate and rebalance the whole system from scratch if you make a change. And the expert would be right. But you asked how to "wing it, and it doesn't have to be perfect" and I hope that the above gives you a way to make some back-of-the-envelope calculations so you end up in the right ballpark.

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