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Dutch2 · 10-19-2009, 09:25 PM

I'm not sure what information is needed to answer my question, but here's a start...

I have 3 bedrooms and a bonus room upstairs. My house has 2 air conditioners. The one for upstairs has 3 return vents, each with a 14x20 filter and a 12" flexible duct going to it.

One return is in the bonus room by the thermostat. The other 2 are in the hallway at the top of the stairs.

The problem is that my kids all sleep with their doors closed. At night the bonus room is nice and warm but the kids are freezing

I'm wondering if I could add small (12"x12") returns in each of the 3 bed rooms. I could take one of the 12" return ducts at the top of the stairs and split it into two 8" ones and run them to two of the bedrooms.
I could then split the one in the bonus room with the last bedroom the same way.

The two 8" ducts have about the same area as the one 12" duct so the flow would be about the same?

My kids are looking forward to your response

D2.

**beenthere** · 10-20-2009, 06:13 AM

An 8" round has 50 sq in cross section
A 12" round has 113 sq in cross section.
Although 2-8" are close in cross section. The small difference makes a big difference. Along with the fact that they also have more surface area for less cross section.
At a fixed .1" friction rate, an 8" round sheet metal duct has a rating of 240 CFM.
A 12" at the same friction rate has a rating of 700 CFM.

Your idea would starve your system, and cause lots of trouble.

Good chance, you are already short on return.

Best thing. Is ADD new returns to the bedrooms, and leave the existing ones alone.
When the bedroom doors are closed. The new returns will draw in air.
And you will find that the supplies blow in more air.

Dutch2 · 10-20-2009, 08:11 AM

beenthere, thanks for your reply.

The bedrooms are about 12'x14'. Can I just add a 12"x12" ceiling return in each room?

Should I use 8" duct from there to the existing flexible 12" duct? What is the best way to connect the new duct to the existing one?

Thanks again,

D2.

**beenthere** · 10-20-2009, 10:01 AM

The 12X12 return grille is fine.
An 8" duct for it will probably be fine.

DO NOT connect it to the 12" duct from the hallway or bonus room returns. Run it all the way back to the return plenum at the furnace/air handler.

If you connected the 8" to the existing 12". You would just decrease the amount of return the furnace/air handler can get from the new returns. Sort of defeating adding returns.

Dutch2 · 10-20-2009, 11:27 AM

I just checked the attic and it turns out that one of the air returns at the top of the stairs goes to the downstairs unit.

In other words, I have only two 14x20 returns upstairs. Each one has a 12" flexible duct going back to the upstairs unit.

The two 12" return ducts are connected to the left and right of the unit. For the three new 8" return ducts can I disconnect one of them, add a duct board box, and connect the 12" and three 8" ducts to it?

Thanks for all your help,

D2.

**beenthere** · 10-20-2009, 12:08 PM

If you make it big enough, you can.

You said left and right. Usually, that leaves a center position open for you.

Dutch2 · 10-20-2009, 01:30 PM

Both units are in an open area in the attic, no obstructions around them so no reason why they wouldn't use the back of the unit.

There is a large Centrifugal Blower in the bottom of the unit where the returns connect. The back of the blower is pretty close to the back of the unit so there would not be a lot of room for the air to flow. I'm guessing that's why there are no ducts on the back.

A funny thing I noticed is that on the downstairs unit there is a large flexible duct (14"?) going from the top of the unit right into the air return box at the bottom. Doesn't that connect the hot/cold air directly to the return???

The downstairs unit has two zones if that matters.

Confused now

D2.

**beenthere** · 10-20-2009, 04:42 PM

Post pics of both units.

Dutch2 · 10-20-2009, 05:32 PM

This is the upstairs unit with the two returns. The duct on the top right is not connected to this unit.

As I was typing in yellow I couldn't see it and I typed "fron" instead of "from"

This is the front of the downstairs unit. the duct on the right comes from a return. The box on the left has two returns going in it.
But there is another duct connected to this box...

Same box, same ducts... with one more duct going from the return box all the way to the top where other ducts come out and go to registers... huh?

Maybe hard to see, but this is the inside of the downstairs unit where the return ducts come in from the left and right. There is not a lot of room behind the blower which is why I think there are no returns on the back.

I'm really curious what the duct from the top to the bottom of the downstairs unit is for

D2.

**beenthere** · 10-20-2009, 05:51 PM

Is the downstairs unit zoned. Could be a bypass for it.
If its not zoned. then no idea why they would have done such a thing.
And I would put a manual damper in it. Shut teh damper, and see if the furnace operates ok. They may have over sized it. And had noise in the duct system. And that was their half a$$ solution.

Second floor unit. What a crappy return install.
The return opening into a furnace. Should be basically full size of the cut out on the furnace. That gets rid of, or atleast minimizes turbulance at the blower.
Build a plenum box on each side of that furnace. And on the side opposite of the blower motor. make that the side that the new returns tap into.

Should also do the same as far as a return plenum on the right side of the first floor unit.

tinmanrob · 10-20-2009, 06:19 PM

Am I missing something?
Where are the returns filtered?
At the grilles?
I'm not familiar with attic installs.

**beenthere** · 10-20-2009, 07:00 PM

Thats where they should be in that install.

Dutch2 · 10-20-2009, 07:03 PM

The returns are filtered at each of the grilles.

The downstairs unit is split into two zones. The left and right duct coming out of the top each have a damper (if that's what you call it) with a motor that is controlled by the zone system.

I could add a damper in the bypass and experiment with it. What is a bypass for in the first place? Is there somewhere I could go read about that?

From what I understand about the returns is that they should always be connected to a box that is connected to the side of the unit, and that the whole side of the unit is open to the box, not just a 12" hole.

Thanks for all your help,

D2.

**beenthere** · 10-20-2009, 07:16 PM

A bypass is used to maintain proper air flow through the furnace when only one zone is calling for heat or cooling.
There should be a damper in it already. A barometric, that is set to open when only one zone is calling. And closed when both zones are calling.

Yes, a full opening on the side of the furnace eliminates turbulance. And helps keep static pressure down, so the blower can move air easier.

Dutch2 · 10-20-2009, 08:21 PM

I'll look for a damper in the bypass duct tomorrow. Is this going to be a damper that is electrically controlled? What do I look for?

I'll start my project by adding a box to the upstairs unit to create more space for the 8" returns from the bedrooms.

D2.