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quackaddict 01-29-2013 07:52 PM

HVAC Question
Hey Everyone,

Was sheathing the backside of a partition wall in my basement and noticed the old owner had run a duct off the plenum vent to the "finished" (use this term loosely) side of the basement. It extends 5 feet or so and was very poorly done. No tape, no seal, two screws holding it on, etc, etc...

The finished side is divided into a larger living space (11x20) and a bedroom/office that is 11x14.

The gent has a 6 inch round duct plumbed in on a trunk that he used to get warm air to the big space, the one im questioning fed the small space. Both come into the rooms about 6ft up on the wall.

Is this proper placement for a register feed on the HVAC system? If not what would be a more ideal location? Do I spend the time to move the registers to the bottom of the wall?

There are also no returns in the space, all of the returns in the upper floor are ran in the joists, Can I pull air into one or some of those returns to get air circulation?

beenthere 01-29-2013 09:14 PM

Run new returns for those spaces. You can leave the supplies high, just get a register that will throw the air downward.

quackaddict 01-29-2013 10:11 PM

Can I have the returns for the basement in the ceiling? Or would they be better low in the wall?

The house has two large returns in the upper floor, one in the hallway for the three bedrooms and the bathroom and another (on the other side of the wall ) in the living room which returns air from the living room, dining room, and kitchen. These returns are connected by a very short, fat trunk that delivers the air back to the Furnace.

Would it be better if I plumb in new returns as dedicated branches or can I pull air from one of the short trunks that pulls air from the upstairs? Either is relatively easy.

how 01-29-2013 10:26 PM

The lower the return openings can be, the better.

quackaddict 01-29-2013 10:32 PM

Ok, I can handle that, I can easily add a trunk in the mechanical space to provide a return to the finished side of the basement. I will dig around and look at sizing. I know a friend that does HVAC install, I will be giving him a call as well.

beenthere 01-30-2013 04:25 AM

Better to run them the returns back with their own trunk. Your current return line is probably a bit smaller then it should be already.

quackaddict 01-30-2013 08:59 AM


Found an online duct sizing utility and here is what im coming up with...

space is 11x20 and 11x14, that translates into 220Sq ft and 154 sq ft. According to what I am reading, you figure one CFM per sq ft unless it is particularly hot or cold, the space is neither.

So into the calculator it goes, 154 sq ft requires a 7 inch round trunk at .1 friction (still trying to figure out how to measure this) in solid duct work and the 22sq ft space requires an 8 inch round. The return (It would be easiest to run one for both spaces would be a 10 inch round and all of the velocities would be in the 620-650 FPM range.

Does this sound close to real life in numbers? No experience with this just trying to learn as I go.

All of the returns and feeds would be actual duct, not flexible.

quackaddict 01-30-2013 04:22 PM

Am working on figuring out what is going on and have some additional measurements!

According to luxaire, the blower on my furnace is rated for 1200 CFM. I checked the return size (the duct work going directly into the furnace) and that dimension is 18x12. IF Im understanding this all correctly, this part is undersized, It is supposed to allow at least 1200 CFM back into the furnace at a rate around than 600 FPM (per Manual D velocities). The lowest I can get it to operate is 18x14 at almost 700 fpm, so im likely ramming air through it at a substantial rate with 2 inches smaller.

This brings me to the actual returns in the house, one is fed by a 7x5 duct made out of 2x8 joists and a metal bottom, one is 7x14 that dumps into the return plenum directly and the last is 31x7 and is fed by the main return in the upstairs hallway. Of the three the hallway is obviously pulling the most air.

When I add all of the square inches together for return ducting I get 288. According to the calculator I need a minimum of 210 inches to get the 1000CFM back to the return plenum.

So then the big question, do I have space in the system for another return duct from the basement? I am reading things correctly?

beenthere 01-30-2013 07:11 PM

If the plenum has room to tap the basement returns into it. Then tap them into it directly.

Your FPM calcs are correct.

quackaddict 01-30-2013 09:10 PM

Is there a minimum distance you can add a return from the filter and intake of the furnace?

Appreciate your comments been there, just wanna get this right the first go around!

Looking at a 7 inch round for the small room, an 8 inch round for the large, and a 10 or 12 inch round for the return air.

What is your opinion of the speedi boots and grilles?

Last thing, looking at sizing of grilles for the returns and feeds, is there a specific measurement I should be looking for? What will under-sizing cause? For instance, it would seem simple to me that a 12 inch round duct could be handled by a 12x12 square opening, would that be enough or would I need a larger grill surface area? On the 7 and 8 inch feeds, you can get 4x12, 6x12, 8x12 and so on. Im not stuck into any size cuz I havent fitted anything yet, is bigger better or is there a specific sizing you should follow?

beenthere 01-30-2013 09:28 PM

Not a min per say. But if the return was straight out from the furnace, it would transfer the mechanical noise. A 12X12 on a 12" round would give you a lot of air noise. Grille should be sized to 300 FPM, 400 is ok though.

quackaddict 01-30-2013 09:34 PM

Is this where the 2cfm per inch comes from? So 12 x12 would net me 144x2 =288, vs a 14x14*2 = 392 would be more appropriate?

beenthere 01-31-2013 04:26 AM

Yes. It gets you close to 300 FPM through the grille.

quackaddict 01-31-2013 08:10 AM

Not understanding the relationship between Calculated CFM and the 300 FPM value you reference.

After doing some looking it seems like the free area of a grill is not 100% of the area, I am also not finding values for free area.

So in simple terms, 392CFM x (free area % of grill) should get me the velocity in FPM? Is there an arbitrary value you can assign a grill to get your calculations close? Is it different for feeds and returns?

beenthere 01-31-2013 04:29 PM

Return grille, use 80%.

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