Restrictions in return duct?

[mfb]

Now that I have fininshed sealing my ducts and returns my static pressure has rissen to .88 @ 1600 CFM. So my next project was to increase my return grills. But then I got to thinking. While I was sealing the return ducts, the builder only punched small openings through the floor. I didn't measure it at the time but I guess they were around 2"x4" or 3"x6" something in that general size. The returns are using the house beams and not seperate pipe.

So if I where to increase my return register size, I'd still be resticted in the walls due to these openings, woundn't I?

What size is normally put into the floors/cellings for returns?


Thanks

[-80guru]

Add up all the openings. What is the sum of the openings?

[metal magician]

When roughing in a house, cutting out the wall plates, I like to get an opening of at least 30 square inches per return. When piping single returns, usually 7" round or equivelent.

[dash]

Post the sizes of equipment,return grilles ,wall space,and cut out.

If your return static is the major portion of the .88,that may be the problem.

[pm4589]

Good question, was wondering the same thing... if grill size is correct but using the wall and floor cut outs reduce the return area then after the floor cutouts you size it correctly to the A/H? Hummm, just looking to monitor the responces

[dan sw fl]

Originally posted by mfb
Now that I have fininshed sealing my ducts and returns my static pressure has rissen to .88 @ 1600 CFM. So my next project was to increase my return grills.

What size is normally put into the floors/cellings for returns?

You should have >> 3 square feet TOTAL.

[mfb]

I have a total of 6 return grills
5-16x8
1-32x8
That makes for a total of lessthan 896 sq inches (since not all the grill area is open).

Since my returns use the wallstuds, the largest possible opening through the floor would be about 14.5x3.5 or 355 sq inches (one hole for each grill execpt for the 32x8 which has 2). I'm certian that my floor holes were smaller than this (I'll have break into my duct work in the basement to confirm).

So based on this it would seem that enlarging the return gills would provide no beifit, since enlarging the floor holes to there maximum size they would still be limiting the return air.

So it lookes like I need to:
a) enlarge the floor holes for the existing returns.
b) add additional returns.

Am I missing something im my analysis?
What size is normally used for air return holes in floors between studs?
How large can the floor holes be made without weekening the floor?

thanks

[dash]

Enlarging all the floor openings to the maximum,may solve the problem.


Per 100ft of duct which the space from the joist to the grille is not 100ft,the large one is around 225 to 300,cfm and the small ones are 120 to 140 cfm each.

The fact that the distance is so short,and depending on the sizing of the return thru the "beams",may allow them to be sufficent,if not,then adding more will be the way to go.

Do you know the static for the return and supply separately??

[mfb]

Unfortunatly the Carrier Infinity system doesn't seem to split return and supply static pressure. Unless its on a screen I haven't found.

[dash]

That's correct,but the good thing is you can test it anytime,so after enlarging the opening,you see what difference it made.

[trane]

Even if you use 150 cfm per stud space you are way short of having enough. The way I read it you are using 7 stud spaces and at 150 cfm each you would only be at 1050 cfm. If you need to move 1600 cfm, cutting out the existing returns to the full 3.5 x 14.5 opening will still not be enough. Looks to me like you need to find another way to get more return maybe from a hallway or other central area.

[dash]

The stud spaces run only a short distance,if they are low wall,just a foot or two.

So we need 229 cfm per space for 1600 cfm,which would be a Friction Rate of .15 per hundred,but adjust for the short distance ,say 3 feet,would a FR of less than.01FR.If the balance of the return is sized correctly for the system,elbows ,filter,etc. ,it may be okay.

[mfb]

The house is a 20 year old 2 story colonial that we recently purchased. 3 of the returns are at ceiling level on the second floor. The remainder of the returns are at the ceiling level of the first floor. The Air handler is in the basement.

When I needed to replace my heatpump I talked to the contractors about the lack of airflow to the upstaires. One suggested (the one I hired) sealing all the ductwork I could get to. Thats when I found that one of my return bays in the basement ceiling rafters didn't have a cover over it. Instead it just had a piece of fiberglass insulation filling the space. So I guess for 20 years the system had been sucking basement air and fiberglass into the heating system. That sure keeped the static pressure low all those years...

I should add that the airhandler installed was a variable speed one. It doesn't run at full speed much for heating but I suspect it will when its cooling season.

[pm4589]

Originally posted by dash

So we need 229 cfm per space for 1600 cfm,which would be a Friction Rate of .15 per hundred

I agree with what your saying here but was just wondering why you use .15 per 100FT? Is that due to the wall material?

[dash]

Originally posted by pm4589

Originally posted by dash

So we need 229 cfm per space for 1600 cfm,which would be a Friction Rate of .15 per hundred

I agree with what your saying here but was just wondering why you use .15 per 100FT? Is that due to the wall material?


Using the ductolator to determine FR,14"X3.5",wall space,looking at it as a 14X3.5 duct,if we had 229 cfm,it would be .15 FR per 100' of duct.

Since the original poster has stated they are high wall returns,then 10 ft of wall space/duct,would be .015 Friction Rate.

[pm4589]

Right after I posted I figured it out.....
Couldn't delete in time.

So were looking at about 620 CFM @ 700FPM

[Edited by pm4589 on 03-04-2005 at 02:46 PM]

[dash]

Originally posted by pm4589
Right after I posted I figured it out.....
Couldn't delete in time.

No problem,since you understand ,please post on this site more often,we can all use the expertise.