Engineering ToolBox - Resources, Tools and Basic Information for Engineering and Design of Technical Applications! Out of these, the cookies that are categorized as necessary are stored on your browser as they are essential for the working of basic functionalities of the website. To get the air moving slower, we need bigger ducts so theres more surface area, too. Including recorded tech training, interviews, diagnostics and general conversations about the trade. Take my house, built in theTake my house, built in the 1920s, frame made of Redwood, Im located in L.A./OC area of California. By clicking Accept All, you consent to the use of ALL the cookies. He has installed insulation on the outside so I cannot tell if the seams are properly sealed. But opting out of some of these cookies may affect your browsing experience. This cookie is set by GDPR Cookie Consent plugin. Maintaining sufficient velocities to prevent dust from dropping in the duct is important, but there are negative impacts resulting from excessive duct velocities. These cookies track visitors across websites and collect information to provide customized ads. These cookies will be stored in your browser only with your consent. This cookie is set by GDPR Cookie Consent plugin. In general, the heavier a particulate the higher the conveying velocity required to keep it moving through a duct. The cookie is used to store the user consent for the cookies in the category "Analytics". The house does not cool till after 12am & all the hot air in the attic goes nowhere but down the side of the walls & out of every open surface available. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); HVAC Design As for air residence time, that has no direct influence on duct heat gain or loss. NOTE: Comments are moderated. In fact, if branch runouts are short enough, a duct can be increased by one size with minimal impact on surface conduction. In those cases, redundancy is desirable to prevent overflowing. For example, some jurisdictions require mastic instead of UL-181A-P tape on metal seams. Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your Sketchup model with the Engineering ToolBox - SketchUp Extension - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro .Add the Engineering ToolBox extension to your SketchUp from the SketchUp Pro Sketchup Extension Warehouse! This website uses cookies to give you the best browsing experience. Learn more about how the velocity varies by dust types by reading Control Costs by Carrying Dust at the Right Speed. Lets start with Q=UxAxDT. You also have the option to opt-out of these cookies. If I surround, at least my main rectangular ductwork with R-45+ foil-faced polyiso rigid insulation, am I at least minimizing the effects of my hot attic on my ductwork? It implies that absolute zero [], 2021 - HVAC School | All rights reserved. This 900 fpm design translates to even more resistance going upstairs than my normal 600 fpm design and my normal is still a challenge to cool the hot upstairs. Unless you put it in writing. 1Industrial Ventilation: A Manual of Recommended Practice for Design (28th ed.). To me, this is the problem with most upstairs designs. Watt's law is just as easy: Watts = Volts x Amps With this newfound knowledge, the student walks confidently into the real world with two equations and some elementary algebra [], When someone says there is no such thing as cold, only the absence of heat, you can point out that while cold is the absence of heat absolute zero is the definition of COLD. If I seal off my attic, andIf I seal off my attic, and my gas furnaces are also in the attic, where does the combustion air come from? Air-to-water pool heat pumps are seeing more and more popularity in the climates that can support them over the more traditional gas and electric pool heaters we usually see. The takeaway here is that low velocity is great for air flow but sometimes bad for heat transfer. And heres what I wrote about flex duct a couple of years ago: An excellent article andAn excellent article and chart to teach the science of velocity vs duct loads vs fitting losses, etc. I see no air going upstairs. It does not store any personal data. The cookie is set by GDPR cookie consent to record the user consent for the cookies in the category "Functional". I know, but getting your grandparents to understand what needs to be done is like teaching that old dog new tricks, some dogs dont even listen to you! I am currently analyzing ducts in conditioned spaces more carefully, but have done detailed experiments on the interactions between ducts and variable capacity heat pumps. Heres the bottom part of the chart, covering air flow rates up to 300 cfm: If you need a duct to move 100 cfm, for example, youd go with a 7 duct if its in conditioned space, a 6 duct if its buried deeply in the attic insulation, and a 5 duct for exposed ducts in an unconditioned attic. So now you suggest a higher velocity in the attic is better? In fact, the title asked the question, Is Low Velocity Bad for Air Flow in Ducts? And the answer was that, in terms of air flow, you really cant get the air moving through the ducts too slowly. Control Costs by Carrying Dust at the Right Speed. Real training for HVAC ( Heating, Ventilation, Air Conditioning and Refrigeration) Technicians. My larger ducts help so many airflow resistance challenges. This device monitors the static pressure in the dust system just ahead of the collector and constantly sends feedback to a variable frequency drive on the fan to adjust fan speed. ThatsBlow cigarette smoke? I am reconsidering unvented attics. Analytical cookies are used to understand how visitors interact with the website. Thomas, yes, the first priority should be to get the ducts into the conditioned space. My point is that hot air in the attic goes nowhere but down & all around inside the house, even if theres a cool breeze & clouds that came out of nowhere on which was a 95 degree day at around 4pm late afternoon in which the day was humid. I have a subcontractor installing ductwork that he sealed with non UL listed tape and now I am examining everything very closely. Condensate drains have uphills and downhills, and they may have traps, vents, and cleanouts throughout. Cookies are only used in the browser to improve user experience. Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors. It is a win all the way around. If you have bends in the ductwork, the more air will be bouncing all over the place inside that duct. Secondary drain pans should be large enough to overlap with the primary pan, especially in horizontal air handlers. 1 story with a attic and 3 gable openings & crawlspace. And its not just time of contact thats responsible. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. I still subscribe to larger ducts in the attic (500 600 fpm) and simply adding a few seconds travel time and a little bit more surface area. The location of a vent can help equalize the siphoning effects of pressurization. Often the damper is left in the fully open position when new filters are installed and the system runs at an increased air volume. Industrial Ventilation Manual of Recommended Practice for Design. If you have an iPhone, subscribe to the podcast HERE, and if you have an Android phone, subscribe HERE. Eric Kaiser returns to the podcast to discuss high-voltage wiring, low-voltage wiring, and condensate assemblies as they relate to systems thinking. Great article and very usefulGreat article and very useful guidelines for considering supply and return air velocities! Other very important aspectOther very important aspect is to maintain a certain minimum velocity of airflow at discharge grilles and diffusers to provide good throw of the air jets and good mixing of air in the room (which is another great topic for article). Theres another relevant fact, however, and ignoring it can lead to trouble. I like your article, as itI like your article, as it gets across an important point about thermal versus fan energy issues. The factor that governs this is velocity: The slower the air moves in a duct, the more BTUs each cubic foot gains or loses. I mentioned that in the previous article, and there was a lot of discussion about it in the comments, too. The air duct velocity also known as air flow velocity is a measurement of the air that moves through your HVAC systems air supply and return grilles. Our industry expertise, engineering and design, innovation and client focus is recognized around the world, demonstrated by our ever-expanding international network of industry partners and clients. Even the best tapes will eventually lose adhesion (faster if located in attic or crawl). But that just tells you how many BTUs go into or out of the duct in an hour. Heating furnace with duct branches in a uninsulated attic. Ducts can and will carry the right amounts of air to the required space if the design is accurate and based on design conditions. In winter, the warm air in our ducts can lose heat to the surroundings. What about blowback from letsWhat about blowback from lets say a bathroom exhaust/vent. By selecting duct sizes that yield velocities appropriate for the conditions, you get the best of both worlds. We use cookies on our website to give you the most relevant experience by remembering your preferences and repeat visits. Your email address will not be published. The Industrial Ventilation Manual1from ACGIH is the industry-recognized resource for determining recommended design practices in industrial ventilation, and it includes guidance on carrying (or conveying) velocities. I hate to be the bearer ofI hate to be the bearer of bad news but all seams (including boots) should have been sealed with duct mastic, not tape! Supply grilles and diffusers should be sized for the appropriate CFM and throw based on the manufacturer's register specs like the ones from Hart & Cooley shown above. He knows the principles and has studied the research and he installs some of the best duct systems in the country. When that conditioned air is moving through the ducts, the second law of thermodynamics comes into play because we have a temperature difference between the inside and outside of the ducts. Advertisement cookies are used to provide visitors with relevant ads and marketing campaigns. That was the main point of my last article. Flow velocity in air ducts should be kept within certain limits to avoid noise and unacceptable friction loss and energy consumption. Do not run high voltage wiring in parallel with low-voltage or control wiring. Just stuck my head in a plenum and it seems very sloppy to me. If the duct does not remain free of debris, the duct or its supports may eventually fail due to the additional loads, or the accumulation of material may create other concerns such as combustibility or corrosion. Low velocity design is very important for the energy efficiency of the air distribution system. Code Compliance Now more resistance is better? Heres my article about that: The Science of Sag Flex Duct and Air Flow. Buying a fan with more static capacity will increase the cost of initial capital and will increase energy costs every day the fan is running. We don't save this data. Measuring grille/registerface velocity is much easier and can be done with any quality vane anemometer, with my favorite being the Testo 417 large vane anemometer. Cleanouts are also capped when in use, but vents are not. SUBSCRIBE TO THE ENERGY VANGUARD NEWSLETTER! These applications will - due to browser restrictions - send data between your browser and our server. So now that we all understand duct design a bit better, time for a bit of reality. It may also increase abrasion rates in the duct system, as well as dramatically reduce the effective life of the filters, requiring more frequent replacement and servicing. Without a written contract that calls out this detail, your only recourse may be the building code. Q = Airflow (sometimes referred to as volume), Fumes and metal smoke produced by welding, Light or fine dusts such as cotton or flour, Average dusts produced by grinding, buffing, material handling and foundries, Heavy dusts such as sawdust, sandblast dust, boring dust and lead dust, Heavy and moist dusts such as most cement, asbestos and lime dust. This website uses cookies to improve your experience while you navigate through the website. How can I tell if a duct isHow can I tell if a duct is poorly made? Theres a lot of factors that play in my part. Thus, I disagree with Allisons statement that The slower the air moves in a duct, the more BTUs each cubic foot gains or loses., if you are talking about per square foot of duct surface area. I dont see how you can condition a 3-level home WITHOUT zone control. That is why duct heat gain or loss in unconditioned spaces goes up, even if U goes down with higher duct air velocities. I would be curious to get a DB rating effect and Mikea opinion for the various velocities in unlined sheet metal. Because the design air volume is now trying to be pulled through a smaller cross sectional area, the air velocity would have to increase through that section of duct to maintain the design flow. In residential applications, you will want to see 700 to 900 FPM velocity in duct trunks and 500 to 700 FPM in branch ducts to maintain a good balance of low static pressure and good flow, preventing unneeded duct gains and losses. Upon a final understanding of the exhaust volume, the contaminants being collected and the velocity requirements, sizing the ductwork for an industrial ventilation system can be determined with the Flow Rate Equation Q=VA. There are a several methods for controlling conveying velocities the first being good initial duct design. 2022 Donaldson Company, Inc. All Rights Reserved, Understanding Conveying Velocities to Save Money on Dust Collection. Please read Google Privacy & Terms for more information about how you can control adserving and the information collected. He put this into a chart that allows you to find the duct diameter that gives you the right velocity and air flow rate(cfm). Reduce energy costs & improve air quality. Much appreciate this and theMuch appreciate this and the other arrivals you put together. Sustainable Economics. Only emails and answers are saved in our archive. The wires should be appropriately sized, have an appropriate level of tension, and should not be vulnerable to chafing or abrasion. Air is also required to overcome the resistance of the duct and other parts of the industrial dust collection system. Notify me of follow-up comments by email. If ducts are inaccessible, theres a technique called AeroSeal that seals ducts from the inside, but its expensive. What Happens to Air Flow in Ducts When Size Changes? Especially with the little port & small ducts their connected to with a turn or two in the line/path, exiting the building through the roof. This category only includes cookies that ensures basic functionalities and security features of the website. I just watched Joe LstiburekI just watched Joe Lstibureks presentation on YouTube To vent or not to Vent. Danko, thats a great point. You can revoke your consent to receive emails at any time by using the SafeUnsubscribe link, found at the bottom of every email. We also use third-party cookies that help us analyze and understand how you use this website. The cookie is used to store the user consent for the cookies in the category "Performance". When you put the ducts in an unconditioned attic and have the minimum insulation allowed, you want to move the air at a higher velocity, pushing it up near the maximum recommended by ACCA Manual D, 900 feet per minute (fpm) for supply ducts and 700 fpm for return ducts. You see your smoke being thrown back into the bathroom. 10 Building Science Lies to Watch out for, My Undersized Mini-Split Heat Pump in a Heat Wave, Make It a Mechanical Room, Not Just a Closet, Introduction to Whole-House Electricity Monitoring, Air Conditioner Sizing: Load Calculations vs Rules of Thumb, Five Fun Facts About Dew Point Temperature, Efficient Hot Water Delivery With a Simple Tool, Insulation level (as given by U, the heat transfer coefficient, or R, the resistance to heat flow). Based on the performance characteristics of each material type, it seems there should be two charts, one for flex and one for sheet metal (also assuming nobody is using alumaflex ducting). Bruce, yes, thats why when the ducts are exposed in an attic and have the minimum allowed insulation (R-8 usually), you want to move the air faster. I would be happy to share those if you would like. This can be measured in either feet- -per-minute (FPM) or meters-per-second (m/s). Basement and main floor are served by the basement trunk. He uses the following ranges of velocity for ducts in different types of space: 600 to 750 fpm Exposed ducts in unconditioned attics, 400 to 600 fpm Deeply buried ducts in unconditioned attics, Less than 400 fpm Ducts in conditioned space. Whether you are dealing with light or heavy particulate, you must design the ducts to prevent material from settling out of the air steam inside the duct. IVI can assist in the design of a new system or the redesign of an existing system. AddThis use cookies for handling links to social media. The third law of thermodynamics addresses the absence of heat and what that means for entropy. Of course that means lower velocity when more than one zone is calling, thus a bit more heat gain in attic runs during multi-zone calls. This website uses cookies to improve your experience while you navigate through the website. Although not related toAlthough not related to velocity per se, it bears mentioning that the A (area) in the heat transfer equation is not just a function of a ducts cross-section but also of its length. When its humid the house is like a oven after 3pm. The cookie is used to store the user consent for the cookies in the category "Other. More mixing will occur via entrainment, but the register will also be noisier. These cookies do not store any personal information. If it were, youd always try to get the lowest velocity possible by using the biggest ducts that fit the space without blowing the budget. Go inside your bathroom, close doors leave them open it doesnt matter. To reduce air velocity for a given fixed airflow rate (cfm), you do have to increase the duct surface area significantly. One of the challenges in practical applications is that systems may not operate at their ideal design air volume at all times. As the conditioned air moves through a duct, it gains or loses heat in proportion to those three factors above. But we as an industry cant change the design without consequences. System Replacement The full chart covers duct sizes ranging from 4 to 18 and air flow rates from 0 to 1,200 cfm. More typically, the fan will not have sufficient static capacity, and, as a consequence, the total air volume will drop resulting in reduced capture performance at the hood and an overall reduction in the systems performance. Pressurized Plenum Air Conditioning & Ventilation, Air Tower Air Handling Units (Upflow & Downflow AHU), The Complete Guide to Raised Access Floors, The Complete Guide to Underfloor Air Distribution, ASHRAE: Stratified Air Distribution Study, Who Pays for Green? If the ducts are all (or mostly) metal, removing & replacing all that insulation may be cost-prohibitive, assuming you have no recourse with the installer. The dominant term in the above equation is area (A) which is the duct surface area. At times it gets so humid I feel the heat is coming from the ground, I understand where on a living organism thats needs to breathe too, especially when where suffocating it with concrete & asphalt all over this God given land. Air is not like water where water takes its course of path. Thats horrible. My friend Mike MacFarland of Energy Docs in Redding, California is a home performance and HVAC wizard. However, the damper method is only effective if employees monitor the situation. You also have the option to opt-out of these cookies. The second law of thermodynamics says that when you have objects at different temperatures, heat flows from the warmer to the cooler object. It is common to keep main duct velocity above 20 m/s (3940 ft/min). Yes, zoning may help the comfort issue, but now the downstairs stat satisfies and Im putting most of my air in a smaller duct to the upstairs, now my 900 fpm is up to Too much! So check with your building inspector before doing anything else. I agree with many of theI agree with many of the overall conclusions, but I do have to nitpick about some of the specifics. These cookies ensure basic functionalities and security features of the website, anonymously. Thus, the overall R-value of the duct (wall insulation plus surface convective resistances on each side) goes up with decreased air velocity, but this effect is small compared to the wall resistance if you have insulated ductwork. Sample Reports. Return grilles themselves should be sized as large as possible to reduce face velocity to 500 FPM or lower. Why? As service technicians, we are often expected to understand a bit about design to diagnose a problem fully. (Figure 1). This resistance is referred to as the total system static pressure (SSP). The humidity left for a couple hours, but later after 9pm returned for the rest of the week. The way we spec flex duct and the way Mike MacFarland installs it, theres not a lot of difference between metal and flex. Need to see it better, turn off lights light a candle, now watch your smoke as you blow it into the vent, some will leave into the vent but not all of it, as I pointed out before blowback will be seen more clearly with lights out, candle on & smoke source of your choice! Blow cigarette smoke? But thats not the end of the story. But I dont really want a huge amount of noise from my ducts, plus that whole article about the effective length goes up as velocity increases. We also use third-party cookies that help us analyze and understand how you use this website. Just hard to put it to practical use being that duct system are bigger & youll need a partner with a big enough thumb! I achieve higher velocity with smaller ducts and therefore smaller fittings. Instant leak finder. The duct selection process should ensure proper conveying velocity based on the design air volume. Doubling the duct diameter reduces the friction loss by factor 32. The ductwork of an industrial dust collection system connects the systems hoods, air cleaning device(s) and fan. I asked him no to install the insulation before I inspected the seams and he packed up and left the job. But opting out of some of these cookies may have an effect on your browsing experience. Download Mike MacFarlands Duct Sizing Tool. This reduction in air volume is often corrected by opening an outlet damper on the fan to compensate for the resistance developed as the filters build up a dust cake. Likewise with your requested pre-insulation inspection. Is Low Velocity Bad for Air Flow in Ducts? Air volume through the system may be fairly dynamic. Wouldnt the dwell time inWouldnt the dwell time in the attic pick up more heat with slower air flow. It also helps get blower door numbers like I do at .36 ACH-50. Abrasive dusts will wear away at duct material, eventually requiring replacement, and excessive velocities can accelerate that wear. Logical Fallacies False Cause & Strawman, Tips for Learning in HVAC/R w/ Trevor Matthews, Giving Kids The Tools For Any Job They Want, Refrigerant Changes Coming & The Helix w/ Emerson, Something to Watch For When Replacing a Thermostat, High air velocity at supply registers and return grilles resulting in air noise, Low velocity in certain ductsresulting in unnecessary gains and losses, Low velocity at supply registers resulting in poor throw and, therefore, room temperature control, High air velocity inside fan coils and over cased coils resulting in higher bypass factor and lower latent heat removal, High TESP (total external static pressure) due to high duct velocity, Single vs. multiple drains with other appliances, Drain pitch and if there could be too much fall. We can think of the condensate assembly as its own system. The key to this is when youThe key to this is when you wrote If you put ducts in conditioned space, you can move the air as slowly as youd like. Get rid of ridge, soffit and gable vents. It is critical to understand conveying velocities in your dust collection duct system. First, you must realize that residential, commercial, and industrial spaces tend to run very different design duct velocities. Amer Conf of Governmental. The optimum conveying velocity may vary depending on the type of dust in the application, but there are several design practices and devices you can use to select and maintain the optimum air volume and resulting velocity. Condensate systems also consist of pans and switches. Brian, thats a great question. Too far to go with too small of ducts. The cookies is used to store the user consent for the cookies in the category "Necessary". These cookies help provide information on metrics the number of visitors, bounce rate, traffic source, etc. As for evaluating the ducts, most jurisdictions now require a duct leakage test (unless ducts are fully within conditioned space). I dont know where that term started, but it is meaningless to me. Because U includes the convective resistance of the indoor air stream, and it goes up as the air velocity goes down. Data Collection In summer, the cool air gains heat from the surroundings. Your air duct velocity should be kept within set limits in order to lower noise and excessive friction loss. While they definitely contain some familiar operating principles to an air-to-air heat pump, there are some striking differences in how the heat is transferred and how [], In HVAC and electrical school, one of the first things you learn about electricity is Ohm's law: Volts = Amps x Ohms Pretty simple, right? The cookie is set by the GDPR Cookie Consent plugin and is used to store whether or not user has consented to the use of cookies. (You can download the full chart by clicking on the image below or the link at the bottom of this article.) Carefully planning the size of the ductwork will assist in reducing waste, reducing costs and improving efficiency. If you use closed cell foam under the deck, you get the added benefit of increased structural integrity by gluing the deck to the rafters/trusses. Your point about velocity with air-zoned systems is manageable. I would however not say: If you put ducts in conditioned space, you can move the air as slowly as youd like. If you slow things down too much, the thermal losses defeat the purpose of the the ducts, which is to get the heating or cooling distributed to particular rooms.