 Hi, I'm Nate Adams, CEO of HVAC 2.0, and often known as Nate the House Whisperer. And today I wanted to tackle a strange but surprisingly important topic, which is duck design. And to some degree, it defines itself as the design of the duck system for a house. Which you can see, this is actually my house that I'm sitting in right now, a duck design that I had done for this. It's a very important thing, so we're going to dig into why to get one in particular and then also what is it. So let's start with why. And here's the key piece. So it's a technical thing. If you have an uncomfortable room or you have moisture problems or there's dust or things like that, that is fundamentally caused by one of two things. So the first one is, there's too much heating or cooling escaping the house. So it's going from inside out and vice versa. The second one is, there's not enough heating or cooling being delivered to the room that is uncomfortable. And it can be either or, or it can be both. And we'll get into how you diagnose that in a moment. But here's what it looks like. You have to deal with heat, air and moisture flowing in and out, and air is by far the most important of those because heat and moisture travel on it. And then once you have a house reasonably tight, doesn't have to be perfect. What we find is a one-to-one ratio of blower-door leakage to square footage is about where things start getting decent. Sometimes you've got to be in the .7 range, but somewhere in that range. That's good enough to control problems with HVAC. And then you need to make sure that the HVAC is adequate. There's two sides of the HVAC, there's the equipment itself, and then there's the delivery system, which is the ductwork. Now to figure out what do you need when it comes to figuring out first is the house super leaky. Because if it's super leaky, it's like a boat that has a decent size leak in it. You don't need to buy a new bilge pump, you need to fix the leak. So for starters, you want to do what's called a blower-door test, which tests how much your house leaks. And then with that blower-door number and your energy use and your thermostat settings, you can do a load calculation, which figures out how much it takes to heat your house on the very cold days and to cool your house on the very hot days. And it needs to be accurate and to do that, you need a couple of extra pieces of information. Now once you have that figured out, that helps you decide what piece of equipment you need, what size equipment you need, and then you can decide about the ducts. Now this is most important for new construction or if a new duct system is in the offing, because fairly often in areas that have HVAC and the attic, it's all flex duct. So more often than not, duct systems get replaced when the system gets replaced. And if that's the case, having duct design for that is a really good idea. But a duct design helps you figure out what size ducts need to go to each room, how much flow they need to get, things like that. And so to fix leaky houses, it looks like air sealing and insulation. And the fix for getting air heat and moisture to the right places in your house, it starts with right-sized variable speed equipment. You're going to hear me say that again and again and again. It is the key to just about everything. And that is not the least expensive stuff. That's mid-range to up-range. There is no cheap variable speed stuff, doesn't exist. Okay, now switching over to what it achieves, there are two different things. So the most important one is that it increases the odds of success. So if you have lived in a home that has rooms that run much different in temperature, say 5 or 10 degrees different from other rooms in the house, if you make sure that there's enough flow going to those rooms and there's enough duct size going to it and there's enough trunk feeding that duct, you're much more likely to succeed at creating a really comfortable house. And the second part is it creates contractor accountability. So if you just ask somebody to put a system in and they put it in and it doesn't work, well, you don't really have a leg to stand on. But if you give them a design that shows exactly what size duct you want going to which rooms and how much flow it's supposed to get, you now have a leg to stand on if things don't work. So it's an accountability step as well. So is this critical on every job? No. In retrofits, probably 10 or 20%, something like that. If that. In new construction, I would recommend doing this every time. That's worth the exercise, because you'll also figure out where you have to put soffits and if there's a weird duct that has to go someplace, you're going to think about that before you build the house and the HVAC contractor comes in and is like, crap, we're going to put this thing. So it's important to think that sort of thing through. So let's go over what is a duct design. This is actually a duct design for my house that I'm sitting in right now because this is about to get a heat pump. And I haven't changed the duct system yet, but I wanted to get a design for it so that I can do that this winter. And let's talk a little bit about the industry standard side of this. So a duct design is a piece of the industry standard HVAC design process. So this is from ACA, which is the American, sorry, it's the Air Conditioning Contractors of America. The American Baseball Association of America. No, that's not it, ACA, Air Conditioning People. So there's a four step process. First you do what's called a manual J. I'm not quite sure why it's called a J, but it is. And that is the load calculation. That is how you calculate how much it takes to heat or cool your house on that very hot or that very cold day. The problem with MANJ is if you don't have a few extra pieces of information, namely energy use, blower door and thermostat sub points, we have seen it be off by as much as plus or minus 70% for cold climates and heating. So in heating, it can be crazy high. There's some houses we found we can drop the manual J 60% and still cover worst case load on that. So it's just like, it's not helpful sometimes. So you really need a few extra pieces of information to make that calculation useful. But what that ends up doing is it helps you decide what size HVAC is needed. Because then you can decide, all right, so I need 26,000 BTUs, am I just going to suck it up and run a two ton, which is 24,000? And like particularly for heating, run a little resistance heat to back up, you can do that or maybe you jump to a three. And so you're going to use the manual S calculation to figure out what is next. Now not everybody does this, but this is what the normal protocol is. So manual S takes your loads and then what piece of equipment can suit those needs. Once you get manual S done and you select equipment, you do manual D, which is the duck design and that determines what size trunks are, what size ducks need to come off to the different rooms, how much airflow the room should get based on how many BTUs, how much heat or cool they need. And you do that. And then manual T is not done very often at all, but that is outlet selection. So that is what kind of events are you actually putting in the ceiling or the walls of the floor. So that's it, manual J, S, D and T. What I'm going to show you is not that specifically, but I just wanted to give you an overview of what that looks like so that if you hear these terms, you aren't mystified. So manual D is the brand name. Like I said, a duck design may or may not use those protocols. One of my friends likes doing this more rule thumb than anything. It may sound funny to hear from me, but there you go. Now when it comes to load calculations, there are two primary kinds. So there's block load and then there is room load. A duck design deals with the room load. So a block load is for the entire house or sometimes it's one floor. So if you have a two-story house, it'd be the first floor load and the second floor load, maybe the basement. Room loads are just what they sound like. It's how much do you need for each room. So a duck design requires room loads, which requires a more detailed model than a block load. So a block load I can run quite quickly with a very basic piece of software and I can get a number together in minutes. Doing a duck design typically takes hours. It's a whole nother ball game, a whole nother level of modeling. So it's going to cost extra. It just is. Now the room by room, what it does is it helps you figure out how much does it take to heat and cool each room. This is going to take into account which way it faces because if you have a west facing bedroom with a large window, that's going to get beat on in the summertime because you can't shade western-faced windows because the sun is going to come in way too low of an angle where in the front, like say the front is south facing, the sun is going to come in. So you just need a little bit of shade and it will stop the sun from coming to the window. But western windows in the afternoon, you're going to cook. And so a duck design and a room by room load calc is going to take that sort of thing into account. What it also helps with is what air flow is needed. And if you know how many cubic feet per minute you want for heating and cooling to a room, that helps you figure out what size duct you need to run. And then what size ducts goes back to what size trunk. And then the trunk is related to what size equipment you buy because every ton or every 12,000 BTUs, rule of thumb is it's 400 cubic feet per minute. So you can go as low as 275 and almost freeze the coil. Out west I have friends that will run 600 or 700 cubic feet per ton because they don't want any dehumidification in the summertime. So there's different ways to do it but 400 is kind of the middle of the road assumption. So a two ton needs 800 CFM, three ton needs 1200 and so forth. Now there are two different main documents that you will see in the duck design. One is super numbers heavy, I'm going to show you that in a minute. And then the other which doesn't happen all the time. So the numbers document that's what you're going to see every time in a duck design. But 3D drawings can be really helpful for figuring out little details of again like where you're going to run the duct here, what size do you need and if it needs to be this size, what are we going to do with headspace, fit between the joys, things like that. So again like I said numbers lay out the loads, flows, duct sizes and so forth and the 3D really helps you visualize it. So let's look at what the numbers look like. This is on the house that I'm sitting in right now. So my friend John Askevel ran this load and as you can see there's a lot of numbers. So let's talk about what they are. So for starters we'll look at square footage. This is a little house, it's 1150 square feet. It was a good deal, it came with 5 acres, it's nice enough, it's fine. It is funny, I grew up in a house about 5 times this size. But this is fine. And so you can see how many square feet each room is as you go through it. And that's important for helping figure out how many BTUs it takes for each one. So then next up we'll highlight three different columns. These are the loads, the heating and cooling loads. So down here is your total heat load, which is 30,926. I suspect that this number is high on this house, probably not super high but a little high. And then there's the cooling load which comes in at a little over a ton. So a ton is 12,000 BTUs. So this is three window units and I know that I'll do it because we're actually running the house with two right now. Heat pumps getting installed this weekend as I'm recording this. But the heat load, you can see what it is for each room going through it. And the larger rooms are generally going to have higher loads. And then see how the master bedroom is 171 square feet but the living room is a little bit bigger at 220. Look, their loads are almost the same. This room has two walls exposed outside and two windows. So that sort of thing is going to get calculated in here. Next up you get cooling load and cooling is split into two. So you'll see something like that's sensible. Sensible is heat load, the heat that you can sense. Latent is humidity load. Latent actually means it's a phase change. So you have to take it from vapor and convert it down to water, which means you have to remove energy to be able to do that. And so it's going to depend where you are in the leakier your house is. The much higher latent loads are going to go up. And there's a problem there because air conditioners, they have a split. So they do 100% overall cooling, but they might only do 10% dehumidification, particularly if you have a high efficiency single stage piece of equipment. Avoid those like to plague. Those are a recipe for mold. You want to run right size variable speed equipment so it can ramp down and you get good dehumidification. But if you can run the coil in the air conditioner cold, so the thing that's in there, it looks like a radiator, picture a radiator car and you fold it into like a letter A, it's called an A coil. If you can keep that cold, you will do much more dehumidification and you can get to where it's like a 50-50 split between humidity reduction and actual cooling. So this won't matter out west where you don't have nearly as much humidity, but in the east this is really critical and makes HVAC design here a lot harder. I sometimes wish that I didn't have humidity to deal with. But in any case, room by room you can see what the loads are and like the kitchen is high because it's planning on the stove running part time basically. So you can kind of pull these apart and see what all things are happening. So that is the loads. Next up we'll look at air flow. So this is how many cubic feet per minute each space needs. And so the highest here is 64 for the master bedroom. There's a look at load is just enough higher to go from 63 to 64 CFM. And then in cooling, it can be quite a bit higher. So like look at the kitchen, it's asking for a whole lot of cooling for the kitchen because again it's trying to deal with that stove or the oven. And so then you come down to what your total air flow needs are so 642. Then you get to see what the duct size is. So 46 CFM can be served by a five inch. So it's one five inch duct and this is just one program that is used for doing this sort of thing. There's a number of different programs but they're going to be similar in their outputs. The bathroom and hallway needs a four inch duct. The big one is the kitchen it's suggesting two seven inch ducts. So that's what you're designing to and that's assuming 642 CFM of air flow. And then this is the air flow to design to which is usually the cooling particularly if you have a furnace and an air conditioner. The furnace doesn't need nearly as many cubic feet per minute to move a lot of BTUs because it's putting out 120 degree heat which is a huge difference is a 50 degree difference from indoor temperature one from 120 down to 70 where an air conditioner is typically putting out like 55 degree air. So if we're running that same 70 that's a 15 degree difference. So you have to move a lot more air the 15 degree difference to cool. So cooling is typically the dominant one although if you run a heat pump that will probably take it back to where heating is dominant. But this is what you have to design the system to that makes sense. It's a bunch of little steps. Now a good duct design I mentioned how manual J can be really wonky as to what size it suggests versus what reality is. So a good duct design you want to use a model that can trued energy use because that's a huge huge swing in what the load of a house is. You may pull up to a house and think that its load is double what it actually is when you look at the bills. You want it to be trued to blow it or leakage the overall leakage of the house. You want it to be trued to room leakage if at all possible. So you may run the blower door and you find out that one room is 40% connected outdoors where everything else is 10 or 15. That 40% room is going to be a lot harder to heat and cool because it's so leaky to the outside. So you figure that out with zonal pressure diagnostics which is one of the pieces of the HVAC 2.0 comfort console. It's one of the reasons it's in there. And then lastly it needs to be trued to homeowner experience. So if you know that a different room is too hot or too cold you want to be keeping that in mind as you do a duct design at least for a retrofit. New construction we're doing our best. So all of these like I said ideally if you're going to do a retrofit they should follow an HVAC 2.0 comfort console if at all possible. If it's new construction you aren't going to have these although you should A be building a reasonably tight house because new home should be tight. B you should be able to predict what the blower door number is on that house. So you want to set targets. We usually set a fairly easy target and then a tough target and we will typically give bonuses for the tough target so we suggest the client's pay bonuses. So that is the numbers page and then you can get a 3D drawing as well. Now I did something weird on my system. So remember how it said it wanted 642 CFM? The unit that I'm putting in here is either a 2 or a 3 ton. It's kind of a strange thing. Bosch makes it. And so I wanted to leave room so that absolute worst case scenario maybe I could run it in 3 ton mode to experiments. So I suggested that John design this to 900 cubic feet per minute rather than the 642. So he upsized the flow proportionally to each room and that resized the ductwork. So there were no eight inch ducts if you remember. So these are the two kitchen ones right here. It's suggesting two eight inches where the design showed two sevens. So this is where you get into if you have what it is by the books and then you put in a little extra input and you see what happens. So anyway, this is the unit here. It's a down flow, which for my markets, pretty unusual. So it sucks in the return from up top and this is an eight inch fresh air vents, but it sucks in down through the system and then it blows out underneath the floor and it goes two different ways there. So this is the duct system that we'll build soon. We're going to run with what it has for the moment. This is that same duct system put into a 3D version of the house where you can kind of see it floating. But I kind of like this one because this helps me visualize what the duct system looks like. Few things to note. There are no ducts at the very end of the trunks. Same here. You should always have a trunk extend out 18 inches past the last duct. And the reason for that is if you don't, the air will bias to flow through there. Everything when it comes to air flow in a duct system, you want to think about water because at the pressures that air works at in an HVAC system, it flows like water. So if you go to the end and you give it a place to go, it's just going to go like right through that end one where if you make it stop and it has to hit and then come back and bounce out, you're going to get much more even flow everywhere than if you let a duct come off the end. So that's one common thing to look for that we see it's an error in at least half of houses that I go look at because it's easy. There wasn't a duct design. They didn't know the rules. Again, this is an accountability step. And then this is another way to look at that same duct system. So this is overlaid over the layout of the house. So and again, 3D, this sort of thing. This is optional. But he's going to do it because this had to be done in a separate program. The reason is this program here doesn't true to energy use. And that was really important to figure out. So he basically just made this work and everyone that I've seen uses one program for doing the number side of things and then another program doing 3D. So depending on who you work with, they'll use different software. There's plenty of them out of that work. All right. So in conclusion, duct design, what and why? So the what's are, sorry, the why's are to increase the odds of comfort happening and then to create accountability for contractors. The what is detailed numbers on heating and cooling loads, how much flow needs to go to each room and then what duct size needs to go there. Then a 3D drawing, which is optional for visualization so you can help get your head around it and then particularly for new construction, you can think your way through where are we going to physically put these ducts because oftentimes you end up with one where you're like, I don't know where we're going to put this thing. So that's the basic idea. So hopefully this is helpful. You're going to see different charges or different prices for this sort of thing. But it's a reasonable amount of work to do these. And then there's typically revisions that come after this as well, particularly new construction. You'll put out a design. You'll notice this, this and this that need to move around and then you'll redo it. So these are things to keep an eye out for. But hopefully this is helpful. Again, you want to make sure that you're using right size variable speed equipment in a reasonably tight house and then you run the duct design. It's kind of the last step in the process. So I'm Nate Adams. Have a great day. And I hope you learned something today. Bye-bye.