Duct work runs and knowledge | Inside 63

off with a zone. Um, you got flex coming in and tying to the box, and you got runs all over this side and all over that side, and sometimes they'll put them off of here and go that way. Which one's going to get the most air? The one straight from the box. One straight from the box. It's going to blast. I'm talking about be blasting because it's trying to force everything in there. Just like a duct system where you got a trunk line coming through here and it's reduced as it goes and reduced as it goes, and it comes to an end, you come back from that end cap a little bit before you put your first runs, because that air hitting that end cap is what pressurizes that duct system. It gives it back pressure into that duct system, which allows you to have a more balanced airflow. If I've got runs all over this side and I'm deadheading into here up against this wall, you don't know which one's going to be the path of least resistance, and you're going to have something that might not get any air because it might be down in the corner somewhere or something like that.. And then some that are getting way too much air. And to be honest with you, there ain't a correct way to balance one on a box. It's just, you take the air down off the one that's blasting and it still ain't fixed this one in the corner. And then you say, well, I dampered all of them down and I still ain't got it. It's because that's a void in that duct and that duct, that air when it's rolling in there is not ever hitting. It's not getting to. You'll create more static back on the blower and burn a blower up before you get airflow into that one run that might be in that corner. Pressure. Why are we on duct?

Like, I'm just curious so I know what I'm looking at, what I'm looking for. Uh, so you know, like, with a 6-inch run, it's 100 CFM, and an 8 is 150. So when somebody builds like a trunk line, right, so let's say they do 14, 12, you know, 10 when they reduce it down, like, by the size of 12 or 14, 12, how do you know how many runs come off each one of them?

Because I—

well, one, it looked like a piece of 10-inch pipe I swear it's like they had 4 6s coming off of it.

And that's fine.

Say, I want to know if that's going to work. That's fine.

That's my question. That's fine. A 10-inch run is good for 1 ton. That's 400 CFM. So if I got a 10-inch pipe and got 4 6s on it, that's 400 CFM. Actually, an 8-inch run, I heard you say a minute ago, is 200. 6-inch run is 100. 12-inch run is 800. Well, it's like 790-something, but we consider it 800. You can do right at 2 tons in a 12-inch run. So as you run your duct system, you reduce it to keep balanced pressure based on the size of the duct. Typically what I always told my guys is try to reduce somewhere around after you've taken a ton of air off of each one of them. So if I come in here and I take 2/8, that's a ton of air. I'll reduce it down to my next size, but I want the size to make sure, using your ductulator, to make sure that that next size will still maintain or be— had a capacity to hold, handle the amount of air that's there. So, um, and then you just, as you take the tons off, you size it down. You could go from a 12 down to an 8, or you know what I mean, from a 14 down to a 10, depending on how much room you got, how much air you're taking off. If this duct here, I handle, say, 1,200, I know it'll handle everything I got on the system. If I need to take 6 off of this instead of 4, then I might go from a 14 down to a 10 because that all will be left, you know what I mean? I won't mind. I had to go from 14 to 12, 12, 10. I just want to make sure that what's left when it comes out of here, this part here can handle it and I can maintain the same static pressure and pressure to get balanced airflow down through the system. Um, so, um, well, that's a good question, but it— rule of thumb is if you want to write this down, 6 is 100, 8 is 200, 10 is 400, 12 is, I would say, 800. That's going to be the basics that you see on a supply. So if you're looking at one and we ask you how many runs are on it and I get 14, there's 14 6s. Or I got 14, are they 8s or are they 6s? That matters too, because if I got an 8-inch run, I'm moving 200. Instead of 100. Also, if you're dealing with like bathrooms and closets, a lot of these will damper down by over 50% because we might have a 6-inch run in it, but you don't need 100 CFMs in a little bathroom. You don't need it in a closet. You just want— in a closet, all you're trying to do is keep the air moving to prevent mildew and stuff like that from growing and still, you know, in the closet on clothes and stuff. You're trying to keep humidity down and keep air circulating. Bathroom, you don't need it. You don't need but a certain amount in there, and we might damper it down to 50 CFMs. So even though it's a 6, might not be but 50. And you can kind of basically go through a calculation in your head when you're having problems. Maybe the duct ain't sized right. What size duct do I have? How many runs do I have? Let's count them and see what sizes they are. Looks like I got Even with the worst case scenario, say I had 2 bathrooms and I still added it, I got 1,400 on a 2-ton system. Now I got too much room because 2 tons is how much? How many CFMs is on a 2-ton system? Anybody? 2,000? 800. 400 per ton. 400 CFMs per ton. 2 ton and a half, 18. Say I had 12. Okay, now you can look at it and say that's way oversized. Then I might say how many's in bathrooms and closets? I got 4 that's in a bathroom and closet. Alright, so that's going to be instead of 400 CFMs that I counted, now I might have 200. So I can drop 200 off of here and now I'm down to maybe 1,000. Now I'm getting closer, you know what I'm saying? Now I'm getting closer to where I'm at.

Um, so that's how many CFMs hard pipe puts off, right?

Not flex?

Either one. Either one. So 12-inch flex is 800 CFMs?

Yep, if it's ran properly. Sometimes you can get noisy returns. Scott had one in his house when he first moved in, his return was too noisy, and, um, we went in there and did some changes. And you'd be surprised at what taking a return and going straight to your duct system, and that was— I just did that as a square duct, versus taking a return and coming up and looping it to your duct system does. This right here creates a— it cuts down on some of the turbulence and the noise, and it can quieten it down just by putting a little turn in your duct, an elbow or something. It can cut down on some of that noise. Here I've got, not only do I have air noise, I've got direct noise straight to, if this is a return box on the back of the unit, straight to the fan. Because I'm coming straight into the back of that unit, my blower's here and I've got radiant fan noise going down my duct system. But if I did this, then my air noise is bouncing around in here where it hits that thing, by the time it gets here you won't get that air noise. Motor noise down in here. It's the— from either I can't push enough air out of the unit or I can't pull enough back. I got to have just as much in that I got to push out. I can't push out 14 and pull back at 800. I'm going to die, you know what I mean? That's going to— your pressures are going to act stupid. So the best thing that can happen to especially newer people coming into this field is to go into the install department and learn ductwork. See how it's installed properly. See how airflow by your dampers affect in the house. Once you can understand that and you can visualize what's going on, it doesn't take a lot, but understanding how and what, you can actually on a startup play with it some when you got some of those startups that you got time on. Some of your new construction houses that you might be starting up, you can actually play with it a little bit when you have time. See what it does. Go in there and close the damper off, see how it does with your pressures outside. You'll see changes in it, and it'll help you when you run into those situations.

So I want to like a— if you're doing a, say, just a package in or whatever, do you want your return to be bigger than your supply? So say like if my supply is— I'm coming off here with 18 or 16, let's just say 16.

So when I come off my return with 18, or if it's sized properly, you can do the same size. Now what you can do, what you can, what that does do is it can slow the velocity. It will slow the velocity of the air down in the return, which can cut down on some of your air noise. You know how you can walk down a hallway, you can hear the air going in the return?

Like pulling real hard.

Yeah. If you make, if you're installing it or whatever and you do make that one size bigger, it's going to slow the velocity down. In that duct system, which is— which will cut down on some of the air noise. Now, you can slow it down too much to where you don't have enough velocity in your duct system. You've heard people say you can't ever have too much return. You can, because now you can slow it down to the point to where it won't pick anything up and bring it through the filter. That's right. Yeah. It won't face load the filter because you don't have enough velocity to pick it up and carry it. So you can get too big of a return. Yeah, because you're limited in two things. Then you'll have dust everywhere because it ain't picking anything up. You just, it won't, you change the filter in 6 months and it still looks as clean as it did before because I'm not strong enough to pick anything up and attach it to the face. So you can't get too much return. But yeah, if you got one that's a little noisy, you can upsize that return and it'll slow the face velocity down on that grill and cut down on air noise. And it won't hurt the velocity on the other side. 'cause my fan's still pushing like it can and I'm getting plenty of air in it. It just cuts down on face velocity on my return.