Technical > Home Improvement > A/C Ductwork Question

A/C Ductwork Question

The airflow is always going to take the path of least resistance, so closer takeoffs will usually get a better blow, unless they have a high resistance built in due to design.
The system can deliver X amount CFM, and it all has to go someplace.
Manual dampers are the best way to balance branches off the main trunk. Moving the take off point further down makes no sense.
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jack b is correct.
Air likes the path of least resistance.
All the balancing dampers do is increase resistance so that the air would rather go somewhere else.

reply to emptywig

Coil, Transition and Plenum with First Duct

Looking Further on Down the Plenum

The 10" supply ducts are too small for the ducts they are feeding. In the first case you have a 10" feeding two 7" and one 5". Calculating cross sectional area using the formula pi2r you find the following.

10" ~ 33 square inches
7" ~ 21
5" ~ 15

So you have 33 square inches feeding 57 square inches.

In the second case you have a 10" duct feeding an 8" and a 6" - so

10" ~ 33 square inches
8" ~ 24
6" ~ 18

In this case you have 33 square inches feeding 42 square inches.

In both cases the 10" duct is too small for what it is feeding.

reply to emptywig
Maybe they didn't do very much math in the early sixties

Looks like I have a little rearranging to do.

wig
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"There is nothing- absolutely nothing- half so much worth doing as simply messing about in boats."

There were no shortcuts (it took a full week in the middle of summer) except that we couldn't afford to have them strip/seal/reinsulate the old ductwork. I have been doing that bit-by-bit in the couple of years since then.

The system was installed by Central City Air, and has been spectacular. They did all the calculations, and initial inspections, and they recommended we keep the old ductwork. They were impressed that it had dampers installed. We even added two more ducts and a return air on the far end of the house, and a fresh-air return.

You're correct, the original ductwork was probably for heating only. Some of it was not even connected with screws - just pressed together.

But the airflow problem predated this system. It has always been a problem on that side of the house, and that side has always been fed by those two 10" pipes. All of the other rooms are fed by ducts that connect directly to the main plenum.

I have no complaint about the way the system performs. My electricity bills are lower by a third. There is no problem getting the house either cool or heated. The humidity is rock-solid at 50% during the most humid days , and there is plenty of extra overhead for really hot days. And the imbalance is not really even that great - 1 to 2 degrees. Still, if its something I could sort out pretty easily, I'd like to.

Thanks,

wig

PS I met all of the goombah A/C guys while I was getting my estimates.

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"There is nothing- absolutely nothing- half so much worth doing as simply messing about in boats."

Oh, sorry - yes, the ductwork is 40+ years old.

wig

reply to emptywig
I'm very familiar with Central City Air. I should have known when I saw that two-staged evaporator coil. Oh, well, maybe you should call them and have that air balance problem taken care of. Good luck!
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reply to emptywig
if you already have dampers installed, couldn't they be set incorrectly?

other than adjusting those, you are probably stuck adjusting the registers to try to generate some back pressure (and i think this is generally frowned upon).

the problem could be, if your house is sealed well enough, that you have pressures inside the house working against your distribution scheme. probably hard to fix even with dampers, and maybe exacerbated by the return duct added recently.

reply to SandShark
I'm still smarting from their first bill. Followed the very next year by a new roof. Do you think I play with hardcast and duct insulation for fun? LOL.

Like I said, this is not one of those things that keeps me up nights. But I've been putting radiant barrier up in the attic while its been cool, and you know, you just start looking at things and wondering...

Thanks again.

wig
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"There is nothing- absolutely nothing- half so much worth doing as simply messing about in boats."

reply to jig
Its not exactly what I'd call a tight house. Its a ranch from the 50's, original aluminum windows. Can you explain a little what you mean? I was under the impression that in a tighter house, return air and a fresh-air return were even more important.

It could be the dampers. I've just been hesitant to adjust any of them too much for fear of messing up the excellent results in the rest of the house without actually fixing the slight problem in that one area. I don't generally like to fiddle with things that 1) are working more or less fine and 2) I don't really know much about. I know just enough to be dangerous.

wig
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"There is nothing- absolutely nothing- half so much worth doing as simply messing about in boats."

on the return duct issue, the rooms closest to the return will have the lowest air pressure, especially if the rooms are pretty much sealed from other areas of the house, and those other areas aren't leaky themselves. lower pressure means lower resistance through the supply ducts to the "return" rooms, and so you end up with a kind-of a cycle that somewhat bypasses the rooms that aren't line of sight to the return.

in other words, you can have sections of your distribution system that are blowing air into closed spaces like trying to blow into a full balloon, and other open sections that are like blowing air into a toilet paper tube. the tube sections get more wind.

the fresh air return is somewhere near the suction side of the system, or it'd probably be more like a stale air exhaust. i think fresh air returns are built with a high enough constraint to air flow that they don't make a large difference to the (already small) pressure differentials throughout the house (they rely on the suction of the return). i don't know what the code specifically says, but i seem to remember it's something like 10% fresh air.

side question: when they installed the fresh air return, did they put any kind of separate filter on the return, maybe on the outside, or do you just rely on the main intake filter for everything? my neighbor burns something in his fireplace that sticks to the ground and would ruin our house air if we had much of a fresh air inlet, let alone 10%, filtered or not.

Thanks for the explanation.

All of the return air (fresh and inside) runs through 5" media filters located just before the furnace. I have to change the filters about once every 5 months or so.

wig
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"There is nothing- absolutely nothing- half so much worth doing as simply messing about in boats."

reply to jig
OK, I think I understand what you're saying here. Unfortunately, the rooms that are having problems are on a hallway with one of the air returns, so those rooms _should_ be getting the good, easy airflow if that was the problem. But they're not.

Thanks,

wig
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"There is nothing- absolutely nothing- half so much worth doing as simply messing about in boats."

reply to SandShark
Sandshark, what were you seeing in my photos that let you know I had a two-stage coil? What would look different on, um...a not-two-stage coil?

wig
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"There is nothing- absolutely nothing- half so much worth doing as simply messing about in boats."