How to Design Duct Work for a 3 Ton Air Conditioning system

[Music] welcome i'm going to show you how to design the ductwork system for a three ton air conditioning unit inside of this house right here now most of the hard work has already been done we already have our cfm values and this is cubic feet per minute how much air needs introduced into this room how much cooled comforted air needs introduced into this room in order to combat the heat gain that is coming in through our windows our ceiling our floor etc and in order to get this value here we have to know how much heat in btus is is coming into this room during the summer so what we've done prior to me showing you this is we've done a load calculation on this house and we've determined that you know with the building materials the orientation of the home the color of the roof the type of insulation construction materials all of that that this room needs 300 cfm of air in order to maintain you know a 72 degree temperature on the hottest day of the year for the area that we're in so each of these rooms has a value associated with it to for us to design duct to carry that air into that room in order to maintain a stable temperature during the summer so we're going to use this i'm going to show you we're going to draw on the duct and show you how to kind of lay this out but to know what size of duct we need for the airflow we're going to have to use a duct calculator so this is the super cool slide rule it's a pretty pretty handy tool there's a lot of different types of formulas and data tables and things like that on it so i do recommend it it is a pretty good duck calculator and has a lot of useful information on it so um so let's look at this top room here so we have a hundred cfm that that room needs it's a laundry room i believe and 100 cfm in order to maintain the temperature and so what we do is we take our cfm value right here and we line it up with .08 inches of static pressure now sometimes with flex duct you can go up to 0.1 but just somewhere right in that neighborhood now if you're designing for smooth sheet metal you know non-non-insulation lined we would go down here to 0.08 just like where we are here so um if we do go up to 0.1 it doesn't really change our value all that much in this the small amount of cfm so let's start off here at 0.08 and we'll look down here at our round vinyl flex our round vinyl flex so it's gonna fall in between a seven and an eight and like i said if you designed a .0.1 you know we would be a little under seven so um seven inch is is approximately what we're uh what we want and this is the size of duct that we're gonna run to that room let me pull out my next sheet i've got some of this drawn up already so you'll see we've got a seven inch run right here coming up behind our our door so duct placement register placement is also a very important part of this you know in this if this is a laundry room and the washer and dryer is here you wouldn't want to put the the supply register under the washer or dryer and it's going to make it very difficult you know to do its job if it's being blocked so you know typically out of the way in some some form or fashion um you know just so that you know the homeowner doesn't place anything on top of it or close it off for any reason so so looking at this 100 cfm we've ran a seven inch duct to it now if we look at our seven inch duct you know we may or may not be at 100 cfm typically at 0.1 inches of static it's about 102 cfm and so that number is really close but not exact so you'll notice these little symbols here looks like an l kind of um that is a volume dampener to control our airflow so that would typically come off of the ductwork you know pretty close to the main section and then you would balance that by closing it just a little bit in order to reduce the airflow to that register you do that in conjunction with taking an airflow measurement here and then that way you know for sure that you're getting the correct amount of airflow to that room in order to maintain that temperature during the summer so you can also see we've got a seven inch here seven inch here a seven inch here so all of these are pushing out approximately 100 cfm of air and they all have volume dampeners on them you know this room is the only one so far that needs a hundred so we have 200 cfm in the kitchen that this room needs so this is only half of the airflow that this this room needs and then our two runs down here is only two thirds of the airflow that this room needs but i've only drawn this small section of duct um and there's a little bit of information here and let's talk about that so our duct our main duct section is going to be sized using also the super cool slide rule or the duct calculator but we have a three ton system in this building typically a a system air conditioning system requires 400 cfm of air per ton to support the refrigeration process and so what we've done is we've taken that 1200 cfm value and lined it up with 0.08 so 0.1 0.09 0.08 1200 cfm and we've come down here and looked to see what size of duct we have so you look at the smaller dimension most of the time if you're buying prefabricated sheet metal the smaller dimension is going to be an 8 a 10 or 12. typically 8 is your most common size that has the most options available to it as far as as various sizes of duct it's what i typically ran whenever whenever i was doing duct work and had my business so um so we will look at the eight and come up here so it's not an eight by thirty but it is an eight by twenty eight those are increments of two you know in between those so an 8 by 28 which is what we have here is capable of producing 1200 cfm so 1 1100 1200 at approximately 940 feet per minute so the velocity scale here is also important because if our velocity gets too low then the ability to move that air that that a volume of air and through the duct work system diminishes so the lower the velocity the slower the air moves and if the air is moving too slow it's just going to to dissipate in that duct work it's not going to make it to where it needs to go so typically the value we're looking for is is approximately 800 800 to 1000 is usually what we shoot for inside that main trunk line in order to carry the air through the entire system um you know without it stopping so if we look at again 1200 cfm so 1 1100 1200 you know we have between eight and a thousand feet per minute and actually 940 feet per minute which we've got written right there so um this first section of duct is able to support our our three tons of refrigeration by moving the correct amount of air 1200 cfm it's 400 cfm per 10 and moving it at um the required velocity of 940 feet per minute or in between 800 and a thousand so um that's something else this super cool slide rule has is our recommended duck velocities um you know our main duct hazard 800 commercial you know obviously everything increases and the reason for this is that you know residential is so much slower than commercial and industrial because you know you're sitting there in your in your living room trying to read a book if you have 1500 feet per minute of air going through your ductwork you're probably going to hear your system running a significant amount so you'll hear that wind noise that air noise and it's not always what you want so but in commercial and industrial environments it's not a big deal they don't particularly care about the air noise that much but residential they do we do bring it down so that you can't hear the air coming out of the vents as much so but between 800 and 1000 is typically okay i usually try to hover about that 900 mark on on the main duct so we don't ever want to drop below 600 so if it looks like we're getting to that point then we need to size down the the duct to the next size down in order to increase our velocity so it's kind of like pushing water through a smaller hose it's going to move faster coming out of that hose you know just think of a pressure washer versus a garden hose you know even though even if the pump's not running it still has a tendency to throw that water out further because of the smaller opening that it has so so what we've done is we've got our first four runs off of the system this reduces our total error volume by 400 cfm so we've got one two three four hundred cfm that we've removed from this main duct and now we are less 400 cfm less than what we are than we started with so this this duct this 8 by 28 now has 800 cfm of air flowing through it you know when we get to this point so if we look here and to just verify our velocity if we take and line up an 8 by 28 which is the duct we have right now slide that just a little bit and look up at 800 cfm so that's the 400 cfm that we've removed from the 1200 that leaves us with 800 we'll look and see we are getting really close to 600 feet per minute so that's where we want to stop with that 8 by 28 duct and we want to size down to the next size so if we come up here again to our our top scale with our friction loss and line up 800 cfm with our 0.08 inches of static and then come back down here and look we have an 8 by 20 that would meet the requirement of 800 feet per 800 cfm with a velocity between 800 and 1000 feet per minute so the next size of duct that we would use would be a 8 by 20. so this is called a reducing plenum system a reducing duct system um you know it is going to reduce the size of that duct as it as it travels the length of the building and as it does that it you know it deposits the air where it needs it but also reduces the size of the duct in order to increase our velocity so we have an 8 by 20 with three more runs that come off of it one seven inch so now we have 200 in our kitchen we have 50 in our bathroom this is a six inch duct which if we look at six on the scale lining up the 6 with our diameter and look up here at 0.1 to 0.08 we have between 60 and approximately 67 cfm that this six inch run will carry into that bathroom well 67 cfm is too much air for this bathroom you know if we get introduced too much cooled air into a certain room then that room will become cooler than the other rooms because the ductwork system is not balanced it's not depositing the air evenly throughout the structure so we also have a volume dampener installed here that after the installation of the duct you can turn this down to reduce the airflow that is being introduced into this room take a velocity measurement take a cfm measurement in that room and determine you know what it is that you need specifically for that in order to maintain the temperature that it requires so you can see we've also put the other seven inch to make up the 300 cfm so take note also of where our supply registers are placed we have our supply registers are typically around our highest heat gain areas that would be around doors it would be around windows and in places like that that is is allowing more heat to enter the structure than other areas so so we have 800 cfm in this 8 by 20 duct at 850 feet per minute we subtract 250 cfm from that that leaves us with 550 cfm in that 8 by 20 duct so if we look here real quick just to verify our velocity um 8 by 20 at 550 cfm we are we just barely dipped below our 600 feet per minute so we definitely want to to move to that next size of duct um and it would be at 550 cfm so once again come up to this top scale here line up 550 cfm with 0.08 and then we'll look down here at our duct so we fall a little in between now so we're we're an eight by fourteen eight by fifteen um you know unless you're making your own duct work uh the size you're probably gonna find uh is going to be an eight by fourteen so most of most of our duct is in even increments on the long side so if we slide that just over just a little bit to 8 by 14 so that we can have our correct measurement then we can see that 550 cfm is approximately 800 feet per minute so you could go down to the next size even if you want to go down to an 8 by 12 and then look at your velocity so now we we're up above 900 feet per minute but i'm gonna i'm going to keep using this 0.08 value for our static pressure so we're going to maintain that 8 by 14 that we started with just a second ago so 550 cfm is you know right at 800 feet per minute a little closer to 790 actually so now we've got our next section of duct which is an 8 by 14 like we just discussed 550 cfm at 790 feet per minute we just have two runs that come off of this you'll notice it's kind of an exponential decrease so the less volume of air we have the quicker we lose our velocity in this duct because the amount of air is is constantly being decreased as we release it into this house so 550 cfm we subtract 200 from that and give us 350 so now if we look at our 8 by 14 at 350 cfm we are once again below our threshold of 600 and we need to bump it back up so we will come up here again to our top scale look for our cfm f350 line that up with 0.08 and then we'll come down here and find that next size of duct so it looks like 8 by 10 would be a good logical choice 8 by 10 350 feet cfm will give us about 700 feet per minute so you'll notice that that even though you know we're using this this scale and the same numbers of friction loss or 0.08 you know our velocity is slowly decreasing and again that's because the volume of air inside of our duct is is becoming less and less as we make it towards the end of this system so so our next run would be an eight by ten or as i did in this one we went ahead and jumped down to an eight by eight so eight by 350 cfm at .08 inches of static would give us an eight by ten but you'll notice if we drop it down to an eight by eight we're looking at a little bit higher on this friction loss scale and then it also increases our velocity though so 350 cfm that brings us up a little above 800 about 825 or 850 850 feet per minute and so we have 350 cfm at well i put 825 but i think it's 850. so um but this would you know finish off our our ductwork system for this house for the supply it would deposit the rest of the air into the room the rooms that it needed so and you'll notice too we have 200 cfm in this room but we ended up putting 250 into it assuming that this door is going to be open most of the time that there is an exhaust fan in this bathroom that when it is closed it's going to be on and pulling a little negative pressure that way so it'll pull some of that conditioned air towards the bathroom um and concerning bathrooms whenever you do place a supply run in here uh be very careful not to put it underneath the toilet or next to the toilet um you know it's it's a it's not the most pleasant thing in the world to have the air conditioning blowing on you while you're using the restroom so if you want to avoid you know complaints or conflicts with customers you know try to try to be mindful of things like that as you're designing and installing this ductwork system so that's a overview on how to design a three-ton ductwork system for you know a pretty generic house that we have here but that's about it thanks

Channel: Teach Me Everything

Views: 178,688

Keywords: HVAC, duct design, load calculation, heat loss, ductwork, duct calculator, super cool slide rule, air conditioning, diy, construction, air flow, 3 ton, refrigeration, sheet metal, ductboard, flex duct, carrier, rheem, trane, bryant, goodman, york, luxaire

Id: v-dzVgTi2ag

Length: 20min 37sec (1237 seconds)

Published: Mon Sep 27 2021