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honola 08-06-2009 01:00 PM

return air
 
I've had a few hvac contractors come through and they don't seem to believe more return is better. Seriously, this "technology" has been around up north for a long time - add a return in each room, minus the bath/kitchen. Down south the rule of thumb is a central return.

After spending months searching, it appears that 160 -200 sq in of return is optimal rule of thumb per ton. right now I'm at about 336 sq in (actual opening) and 500 sq in (filter opening)

So 480 - 600 "should" be optimal on a 3 ton unit. The return and t-stat are at 2 different ends of the house and the way the house is designed I get very little return air flow on 1 side of the house due to a stairwell with cathedral ceiling in the middle.

What I plan to do is add 1 8" return in the stairwell and 1 8" return in between my master bed/bath. I can't fit a bigger takeoff due to the plenum being in a closet. This would effectively give me an additional 100 sq in of return correct? Should I split the 8" into additional 4" returns or leave it as an 8" with a 5x10 or 6x10 grate?

Thanks for the help.

Plumber101 08-06-2009 05:29 PM

Quote:

Originally Posted by honola (Post 311269)
I've had a few hvac contractors come through and they don't seem to believe more return is better. Seriously, this "technology" has been around up north for a long time - add a return in each room, minus the bath/kitchen. Down south the rule of thumb is a central return.

After spending months searching, it appears that 160 -200 sq in of return is optimal rule of thumb per ton. right now I'm at about 336 sq in (actual opening) and 500 sq in (filter opening)

So 480 - 600 "should" be optimal on a 3 ton unit. The return and t-stat are at 2 different ends of the house and the way the house is designed I get very little return air flow on 1 side of the house due to a stairwell with cathedral ceiling in the middle.

What I plan to do is add 1 8" return in the stairwell and 1 8" return in between my master bed/bath. I can't fit a bigger takeoff due to the plenum being in a closet. This would effectively give me an additional 100 sq in of return correct? Should I split the 8" into additional 4" returns or leave it as an 8" with a 5x10 or 6x10 grate?

Thanks for the help.


Not sure where you researched but you need a total approx of 210 sqin for a 3 tn. Just adding more is not the best approach because you will change the static pressure of the system. Most residential system are designed for .5 in wc or .2 in the return and .3 in the supply.

Research static pressure.

Why are you trying to change the design are you having problems or try to bust the chops of a trained tech who says changing it is not good?

newtech 08-06-2009 05:49 PM

Quote:

Originally Posted by Plumber101 (Post 311340)
Not sure where you researched but you need a total approx of 210 sqin for a 3 tn. Just adding more is not the best approach because you will change the static pressure of the system. Most residential system are designed for .5 in wc or .2 in the return and .3 in the supply.

Research static pressure.

Why are you trying to change the design are you having problems or try to bust the chops of a trained tech who says changing it is not good?

I think what he is saying............ is that one central return is :furious:. Which I agree with 100%:yes:

Plumber101 08-06-2009 05:58 PM

Quote:

Originally Posted by newtech (Post 311345)
I think what he is saying............ is that one central return is :furious:. Which I agree with 100%:yes:


Still with a central return it has to be sized for the system so that static pressure is correct.

I like central returns because it moves air in each room yet today it is cheaper to just have one or two returns to the AH

newtech 08-06-2009 06:39 PM

Quote:

Originally Posted by Plumber101 (Post 311348)
Still with a central return it has to be sized for the system so that static pressure is correct.

yet today it is cheaper to just have one or two returns to the AH

Like you said the cheap way :furious: Do it right....."Life is tough and it is even tougher if you are stupid"

honola 08-07-2009 01:16 PM

Wasn't trying to bust chops on any competent techs. However, the 4 techs I had in my house said it's a big open house, the ac is going run longer and depending on the outside temp it will dictate the inside temp. I understand there is only a certain amount an ac can drop temp. But if you have a return on the left side of the house and t-stat on the right side of the house with a large stairwell in between that is wide open to the downstairs. It seems a smart solution would add additional return in the center and right side of the house. Considering the bedroom is 20x15, walkin closet is 15x15, bath is roughly 15x15 and the only way the air can get back to the return on the opposite side of the house is through a 32" wide door and across an open stairwell and through another 32" wide opening.

"if" 210 is the magic number of return I'm even shorter on sq in than I thought. 336 current and 210 * 3 = 630 or 294 sq in short.

upstairs configuration is 2 bedrooms and full bath on left side of house with the only return, hallway and 32" opening, center is a stairwell front to back of house with cathedral ceiling and 6" supply, right side of house has master bath/walk in closet and master bed with the thermostat and 32" opening to access the stairwell.

I wasn't planning on "maxing" out the return air, just adding adding additional return. The biggest I can add is 8" take offs and can possibly get 3, but would be safe with 2. The way I figure it an 8" flex would really be a 7" and measure out to about 40 sq in of return x 2 would be an additional 80 sq in or return or 336+80=416 total. Which is still shy of "rule of thumb" standards. Does this seem like a good idea or is there a better recommendation. Should I run the 8" into the attic with a box in the attic and put several 6" returns?

160*3=480
200*3=600
210*3=630

Thanks for the help

Plumber101 08-07-2009 06:27 PM

Quote:

Originally Posted by honola (Post 311589)
Wasn't trying to bust chops on any competent techs. However, the 4 techs I had in my house said it's a big open house, the ac is going run longer and depending on the outside temp it will dictate the inside temp. I understand there is only a certain amount an ac can drop temp. But if you have a return on the left side of the house and t-stat on the right side of the house with a large stairwell in between that is wide open to the downstairs. It seems a smart solution would add additional return in the center and right side of the house. Considering the bedroom is 20x15, walkin closet is 15x15, bath is roughly 15x15 and the only way the air can get back to the return on the opposite side of the house is through a 32" wide door and across an open stairwell and through another 32" wide opening.

"if" 210 is the magic number of return I'm even shorter on sq in than I thought. 336 current and 210 * 3 = 630 or 294 sq in short.

upstairs configuration is 2 bedrooms and full bath on left side of house with the only return, hallway and 32" opening, center is a stairwell front to back of house with cathedral ceiling and 6" supply, right side of house has master bath/walk in closet and master bed with the thermostat and 32" opening to access the stairwell.

I wasn't planning on "maxing" out the return air, just adding adding additional return. The biggest I can add is 8" take offs and can possibly get 3, but would be safe with 2. The way I figure it an 8" flex would really be a 7" and measure out to about 40 sq in of return x 2 would be an additional 80 sq in or return or 336+80=416 total. Which is still shy of "rule of thumb" standards. Does this seem like a good idea or is there a better recommendation. Should I run the 8" into the attic with a box in the attic and put several 6" returns?

160*3=480
200*3=600
210*3=630

Thanks for the help


If you have 336 sqin then you are over you only need approx 210 sqin. But 336 is OK.

I understand what you are trying to do but if you run an 8" you are only going to be able to move approx 225 cfm so if you connect four 6" to a VAV and four 6" total approx 440 cfm you are only going to move about 225 because you are limited to the 8" pipe.

And when you start changing static pressures and changing ducting this will also change delta "T" and return/primary air difference.

THe problem I don't think you understand is it should be OK to add return but you may need to restrict existing returns.

By the way what is the static pressure of the return and supply?

And where can I get a rule of thumb book. I think yours needs some updating.

The only rule of thumb I have is that you need 350-450 CFM per ton of cooling but then I use a Ductulator for proper sizing

newtech 08-07-2009 07:03 PM

Quote:

Originally Posted by Plumber101 (Post 311677)
.

I understand what you are trying to do but if you run an 8" you are only going to be able to move approx 225 cfm so if you connect four 6" to a VAV and four 6" total approx 440 cfm you are only going to move about 225 because you are limited to the 8" pipe.

I think this a constant volume unit not a vav system. I didn't see him mention anything about 6" runs???? VAV system http://en.wikipedia.org/wiki/Variable_air_volume

Plumber101 08-07-2009 07:32 PM

Quote:

Originally Posted by newtech (Post 311689)
I think this a constant volume unit not a vav system. I didn't see him mention anything about 6" runs???? VAV system http://en.wikipedia.org/wiki/Variable_air_volume


Thanks for the correction I meant to type CVU and not VAV. Yes I know the diff just brain to finger didn't make the connection

And I guess you didn't read when he said "Should I run the 8" into the attic with a box in the attic and put several 6" returns?"

It was an example of how onle an 8" pipe to a box can only pull some much air reguardless of how many 6" you connect.

honola 08-10-2009 08:28 AM

Quote:

Originally Posted by Plumber101 (Post 311697)
Thanks for the correction I meant to type CVU and not VAV. Yes I know the diff just brain to finger didn't make the connection

And I guess you didn't read when he said "Should I run the 8" into the attic with a box in the attic and put several 6" returns?"

It was an example of how onle an 8" pipe to a box can only pull some much air reguardless of how many 6" you connect.

Yes, I typed that wrong. What I meant to say was considering 6" flex is roughly 28 sq in and an 8" flex is 50.2 sq in. Would it be better to add 2 6" flex on that 8" or leave it 8" and put a 5x10 grate.

Thanks

tubapix 08-11-2009 09:25 PM

Find a Ductulator online and figure .1 static for std. motor and .08 for a variable speed motor. 400 CFM per tonne and go from there.

hvaclover 08-13-2009 07:41 AM

Quote:

Originally Posted by tubapix (Post 313261)
Find a Ductulator online and figure .1 static for std. motor and .08 for a variable speed motor. 400 CFM per tonne and go from there.

Thoseain't for static press, but for 100 equivalent length of trunk,


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