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syclnjr 02-05-2011 10:47 AM

Return Air Duct size question
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Hey there all, new here and I am looking for some advice.

I am getting ready to install a Goodman GMVC 95% 2 stage, variable furnace and I am revising my ducting in the house, it was a good design but I want to make sure that the furnace is going to run to it's best potential.

Some specifics on the house 1200 SQ FT. Cape cod style house, built in 1938, located in SW Michigan. We had new windows installed in the house, which is a drastic upgrade over the single pain units:)

I purchased all the software and tools and what not and did a ton of research on equipment sizing and all of that. That being said with everything calculated out I bought a 70K, well actually 69K input furnace and 2 ton 16 SEER AC.

Currently the 30 year old furnace heat exchanger has a small hole in it and the gas meter is humming when it is running, it is currently 105K input 80% eff. with no AC. So I need to get all this done in a rather quick fashion. :)

Now for the supply ducting what I have calculated out is that the current supply from the furnace is 8X16, to 8X14 to 8X12, isn't adequate and I need to up size the supply ducting to 8X20, to 8X16 to 8X14 Forgot to mention that the furnace is on one side of the basement. This is one portion that I am modifying, but I am also going to convert the supply registers in the bedrooms to return air ducts and move the supply's to the outside wall as they are all on the inside wall except for the living room. I am also going to cap off the big return in the kitchen, add a supply register in it's place, run another supply to the upstairs as there is only one feed currently and it needs at least two more as well as return air up there.

This brings me to the question on the return air duct, currently the duct right above the plenum down to the furnace is 8X24 then as it runs down it drop down to 8X12 about 4 feet after the furnace. Question is being that I am converting the supply vents in the lower bedrooms to returns, then I should upsize this return duct as well, correct. It should note that the 8X12 duct also has the return air from the kitchen, which I really think is grossly undersized. as there is only two returns in the house currently, one directly above the furnace near the front door and one in the kitchen which is on the other end of the house.

Any advice would be great, thanks for the help. Also attached is the PDF of the drawing that I made of the way the system is currently, yes I am an engineer ;)

Anyway, thanks again in advance.

Jackofall1 02-05-2011 11:00 AM

You have mentioned all but the one thing that is all important......what volume is the fan moving at what rated static pressure in inches of H2O?

That will determine both supply and return air duct sizing.


syclnjr 02-05-2011 11:36 AM


Originally Posted by Jackofall1 (Post 585095)
You have mentioned all but the one thing that is all important......what volume is the fan moving at what rated static pressure in inches of H2O?

That will determine both supply and return air duct sizing.


Are you talking what the fan in the furnace is rated at or what it can supply, or are you talking the calculated number that it has to supply?

Jackofall1 02-05-2011 11:41 AM

The calculated volume, and the fans ability to deliver that volume are two very different things.

The size of ducting is based on velocity, velocity determines resistance, summary of resistance is system total pressure.

So that being said, the size of the ducting is determined by the capacity of the fan, not the calculated needs.


syclnjr 02-05-2011 12:20 PM

Got ya sorry.

Maximum rated CFM per the manufacturer for the size unit that I have is 1760 CFM @ .1 - .5 ESP

Jackofall1 02-05-2011 12:33 PM

I believe the target velocity for return air ducting is 200 FPM.


syclnjr 02-05-2011 01:08 PM

Thanks but isn't 200 FPM rather low. I thought that 700 FPM is optimum.

The rough calculations that I can come up with is this.

Relative CFM that I have calculated for the house is about 1150 CFM,

Duct sizing for this I would think or at least the way the calculations come out to be the supply needs to be 8X22, which I kindof don't have room for, where the furnace is located. and the return would need to be 10X22

So if the main portion or section rather that is at the furnace is 8X24, I would think that I could change out the drop down to the furnace to 10X25 and branch this to the 16X25 filter opening. Also I guess working on some more of the calculations I think that the 8X12 after the 8X24 duct should be fine as it is only going to be servicing two rooms. and they are small.

That being said another question came up in my head, would it be better to have the returns closer to the ceiling, I rear somewhere that this is the preferred method? Any ideas on that one? Thanks again for the insight.

Jackofall1 02-05-2011 01:22 PM

The best return air considerations are for both high and low wall return air openning, High in the summer and low in the winter, makes for the best heat and AC distribution.

You can run the RA that high, but you may want to think of turning vanes in the duct if your filter is perpendicular to air flow at the base of the furnace.

syclnjr 02-05-2011 02:14 PM

Ah good call thanks for that, I planned on running a radiused transition into the furnace return air as well as supply would this be enough of would I still need the turning vanes?

Jackofall1 02-05-2011 02:16 PM

I personally would use and have used turning vanes, just helps better load the filter.

syclnjr 02-05-2011 04:06 PM

Ok thanks I will look into that.

beenthere 02-06-2011 04:29 AM

The current return in the kitchen spanning 2 joist bays is more then the 8X12 can carry.

The size return duct you'll need after you add your return to the second floor. Depends on how much return your getting from the one near the front door, and how much you need from the one your adding to the second floor.

700 to 800 FPM minute is fine in a return duct. Its your register grilles that should have a face velocity of 300 FPM. But often 500 FPM is ok, and not to noticeable, as far as noise.

syclnjr 02-06-2011 08:48 AM

That is what I had thought, thanks for the clarification, what is weird about the two joist span is that one end is open to the basement and the grill in the Kitchen is kind of a loop through. Just weird and I am not sure why that was done, oh well getting that fixed now.
For the return upstairs, now if you think about my house where it is pretty much open in the upstairs portion and just split into two rooms down the middle. The supporting wall that runs through the center of the house from th front door to the back door, is where the return is currently, which is right above the furnace, by the front door. I used this wall space to carry the return upstairs, I pulled some of the floor boards up, and opened up the wall space and used the joist space for the floor upstairs to a 6X30 grill for the return. I made sure the the studs for the wall are directly above the floor joists for support before I cut :) I think that it is coming out ok, just trying to find the time to accomplish this task, but I am revising all the ducting first then I am going to set the furnace and get it all going. Figured I better have everything on point before I set the furnace and run it so that I can limit any issues.

Thanks for the insight on the ducting and the grilles. From what I have seen they don't list the face velocity of the grilles from grille manufacturer, sure it is pretty easy to figure out.

beenthere 02-06-2011 04:25 PM

Most register/grille manufacturers list the face velocity at different CFMs. Haryt&Cooley does along with Lima and Titus.

syclnjr 02-06-2011 04:48 PM

Cool thanks been, I will check them out.

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