Author [expand]	[Modern View]
TimMiller 11-01-2009 21:10:21 70.242.4.236 812090 Report to Moderator	I have been looking online trying to figure out how to size ductwork. We are adding onto the house (5 extra rooms, one being a large upstairs area). I have not been able to find much info online besides higher a pro. My gf's dad is a mechanical engineer with an EPA certification. I'm going out to see if I can find a how to book tomm. Any tips would be great. [Reply]   [No Email]
bc 11-03-2009 10:41:31 69.148.145.104 812505 Report to Moderator	Re: Installing duct work in reply to TimMiller, 11-01-2009 21:10:21   Hi Tim. What T-Bone said. As I reread your post, it appears you are adding a large addition on to the house. It also appears you are wanting to add on to the end of the existing system. Is that all correct? If that is the case, it won't work without replacing and resizing the air handler, plenum, and all main ducting including the returns. I don't thing any existing system can handle a 5 room increase. You can probably keep the end run ducts in the original part of the the house. One of the tricks is maintaining the right flow and right pressure. Just as well start getting bids from the pros for the whole system. And to get the rough in details right, you better start doing that before you start laying down joists and preferably have a set of plans before you even start building. I'm not sure any quality building contractor would work without some plans to lay out the foundation and on up. He has to know where all the plumbing and electrical is going as well. You can't run a duct through a drain pipe or some romex and vice versa. If one of those rooms is a bathroom, then you have lots of design issues to deal with. Depending upon the brand of air handler and system you prefer, you can discuss the bids with the HVAC companies. Then you can negotiate with them on doing a turn key job or how much you are going to DIY. If you are going to home cheapo or lowest and buying everything, then you can ask them to price out the design. Think about a hepa or electronic air cleaner and humidifier as well. Besides sizing a placing ducts and registers, another trick to flow design is getting the returns properly sized and located to run through and stud wall and floor joist cavities and then have it all get to the main return ducting. I have one bedroom at the end of my pro designed system that just doesn't get the air flow like the others. Always cold in the winter. Pinching down the other registers doesn't help. Guess I'll try the tissue test on the return duct although I've done it before. For all I know, they may have forgot to cut off that part of the bottom plate or cut the hole in the floor. Good luck Tim. I don't mean to write a book here or offend the readers that only want simplistic and direct answers when the question doesn't have such an answer. As an aside, I'm in the middle of the project from heck (or Oklahoma, same difference). What started as new vinyl siding and some windows for my mother in law's almost 100 year old farm home has grown. Tore off 3 layers of siding and replaced 1 door and 1 sliding patio door. Found some termite damage and have replaced about 25' of stud wall and sheeting. Got half of the 26' of the north end torn off including the lath and plaster interior walls this last weekend and next weekend will replace the new stud wall along with a set of french patio doors. Have it boxed in with plywood and tarps for now. Then do the other half of the north end. Have ordered all new new construction style windows that will require every window being removed and reframed for the new ones. Since I'm going that far, I just as well pull off the remaining 1 by 6 sheeting and replace all the sheeting and insulation while I'm at it. Replaced the galvanized plumbing in the kitchen wall and all new wiring per code along with a new panel in the basement when I'm done. I hoped to have a siding party at Thanksgiving but it may be Christmas now.   [Reply]   [No Email]
TimMiller 11-03-2009 09:44:49 70.242.1.46 812478 Report to Moderator	Re: Installing duct work in reply to TimMiller, 11-01-2009 21:10:21   Personally if it were me, i'd get someone to do it, but they dont. I know all of the insulation values, and no ductwork will be exposed to the sun. We are planning to use flex ducts. I do have the whole other side of the house to use as a guide, since it was all installed by a pro. One idea I had was I will try to figure it out as best as possible, put it on paper, then he can run it by one of his HVAC guys, or I can mention the possibility of him just giving the guy some $$$ to design it. I can draw up a rough blue print of everything, with the insulation values. Over the entire roof (up stairs is a vaulted ceiling) it is R-13, between the walls it will be R-30 along with the ceiling. Her mom wants EVERYTHING insulated, including the closets. She also wants ducts run to the closets, to just provide a little air to "keep moisture down."   [Reply]   [No Email]
T_Bone 11-03-2009 08:22:00 64.57.205.153 812447 Report to Moderator	Re: Installing duct work in reply to TimMiller, 11-01-2009 21:10:21   Hi Tim, Well I can shed some light on your question but your not going to like the answer. Anyone that states designing air flow is simple has another think coming. Air is very difficult too control therefor takes alot of engineering to make it do what you want. There's only two well recognized authorities on HVAC&R. One is SMACNA (Sheetmetal Air Conditioning National Assoc.) The other is ASHREA (American Society of Heating & Refrigeration Engineers Assoc.) Those two produce 99% of the engineering info that's accurate and there lies the kicker, " that's ACCURATE". Since that is true, then they want who ever uses that info to be qualified to do so to produce quality engineered systems therefor they require a background check for schooling and experience before accepting one into there associations. After ones accepted, there's a huge cost too owning there publications, typically $50 upward with most being several hundred dollars a text for the most common text references that a engineer will use in designing quality HVAC&R systems. From the above info, you would be better off finding a company that will engineer a quality system design for you. I would think you could get a local complete quality design for under $1000 tops. Why pay the engineering cost? You will have a lower utility bills, feel more comfortable, suggestions for the most efficient system units being overall cost effective for your area. Example: (David, I'm not picking on you but you posted the info for my example) Air flow above 500fpm is very noisy and makes it difficult to control human comfort. Humidity plays a huge roll in human comfort. Designing the wrong sized duct work, will cause the fan motor too fail because of too high of static pressure. That's just some of the many considerations I use for designing systems. If your wanting a good "generalized" text reference, but by no means enough info to design a complete system the I would by: Modern Refrigeration and AC GoodHeart & Wilcox ISBN:0-87006-642-0 About $60 or so and at many places, WW Grainger, Johnstone Supply, bookstore, on-line, etc; This one text reference is a great bang for the buck. T_Bone<---45yrs ASHREA HVAC&R Design Engineer, Indoor Air Quality Engineer, Energy Management Engineer, AWS-CWI, Retired, Ranked in the top 60 of 150,000 of my peers, designing & troubleshooting multi-million dollar control systems.   [Reply]   [No Email]
David - OR 11-03-2009 10:16:05 208.67.204.213 812491 Report to Moderator	Re: Installing duct work in reply to T_Bone, 11-03-2009 08:22:00   With all due respect to TBone's experience, I have to dispute two points: 1) 500 FPM in a metal branch duct is NOT excessively noisy. A very typical room application might be 100 CFM through a 6 inch metal duct, giving a flow velocity of 533 fpm. I live with such a system every day and it is fine from a noise perspective (very conservative actually). The typical "ductolator" allows 700 fpm and up through the ducts. 500 FPM at return grilles and registers MAY be excessively noisy, which is why they are sized to a lower flow velocity. But 500 fpm in a DUCT is fine. 2) Excessive friction loss in a duct system will NOT "cause the fan motor too (sic) fail because of too high of static pressure". An ordinary centrifugal fan or pump draws LESS power, not more, as flow resistance rises. There are many reasons to be concerned about excessive flow resistance, such as heat exchanger temperature, duct temperature, and overall SYSTEM efficiency, but failure of the fan motor due to too high of a flow resistance is not one of them (variable speed drives excepted). Look at any text on "fan curve" or "pump curve". Here is a quote from one of them: "As the static pressure imposed on a blower driven by a PSC motor increases, the airflow decreases, following along a pressure-flow line on a fan curve. Because the power consumed by a fan is proportional to the cube of the airflow, power consumption also decreases with increasing static pressure when PSC motors are used." one reference on fan curves   [Reply]   [Send Email]
TimMiller 11-02-2009 23:00:13 70.241.25.46 812378 Report to Moderator	Re: Installing duct work in reply to TimMiller, 11-01-2009 21:10:21   so i calculate the total load based on the rooms, do i need to find square footage of each room, or just figure out how many ducts i will end up having, then add max cfm for each room to figure out what size unit I will need. I looked at our local bookstore that normally carries all of the code books but i was not able to find much in the way of HVAC books, only books that have the charts you can find online, and most of it was on the electrical side of it. It amazes me that there is very little printed material that i have been able to find in the way of HVAC. I'm thinking about going down to my local college bookstore and seeing what they have there. I know they offer HVAC classes.   [Reply]   [No Email]
bc 11-03-2009 08:15:25 67.66.111.187 812446 Report to Moderator	Re: Installing duct work in reply to TimMiller, 11-02-2009 23:00:13   Tim, I haven't done the calculations in years but its not just square footage. Its amount of insulation, glass size and insulating value, sun exposure, etc. A lot of things go into the calculation. Use the total load to size the system. Use the room load to size the ducts and piping. You can find the tables for what the various size round duct piping and registers allow. 6" is around 105 cfm give or take 5 cfm. You have to match the registers. The home centers may have a chart. The trick is in figuring the size of the square ducting coming off the air handler. For long runs you may have to reduce the size as you get close to the end depending upon the number of ducts. To get even distribution at all the ducts, you need to maintain an even pressure going to the vents. You can install manual round duct valves off of the main duct to tweak the system a little. A multiple thermostat system uses electric valves to open and close the ducts as needed for air delivery. There are some general rules of thumb for different areas of the country that apply to a new tight energy efficient house based on the size. Your needs may be more or less depending upon your house. Try some of the HVAC forums. I forget your original post but you size separately for the furnace to get the air handler and heating needs. Then size the tonnage of the AC based upon the total load. When you have a size for the furnace, bigger may be better. When you have size for the AC, bigger may NOT be better if you are between sizes. Depending upon your location, the bigger AC won't run as long which reduces your comfort level. For example if you calculate needing 33000 btu of cooling, that puts you half way between 30,000 (2 1/2 ton) and 36,000 (3 ton). The best option may be to go with the 2 1/2 ton AC. If all else fails, you don't have to reinvent the wheel for HVAC sizing. Go look at comparable houses to get some ideas.   [Reply]   [No Email]
john hoffman 11-02-2009 17:24:21 67.236.78.91 812290 Report to Moderator	Re: Installing duct work in reply to TimMiller, 11-01-2009 21:10:21   David OR has it right. It may sound like a lot of work but its well worth doing the sizing to get it rignt. I can tell you that a lot of the "pros" end up oversizing the the whole system and your stuck with an inefficient system.   [Reply]   [No Email]
David - OR 11-02-2009 09:50:10 208.67.204.213 812200 Report to Moderator	Re: Installing duct work in reply to TimMiller, 11-01-2009 21:10:21   The industry standard for ductwork design is published by the ACCA and is entitled "Residential Duct Systems Manual D". The book can be bought over the internet or from a good technical book store. After reading the book, I personally reduce the methodology to this: Do a Manual J analysis of the heat and cooling load of the whole house, and for each separate room. Size the equipment appropriate to the total load. Divide the load of each room by the total load, then multiply this ratio by the total CFM of the equipment air handler. This gives you the CFM you need for each room. Choose the larger of the heat load or cooling load to size the duct. Size your branch ducts to achieve less than 700 fpm flow velocity if metal, less than 500 fpm if flex. Consider running two smaller ducts rather than one large one if a room is particularly large. Choose a register to deliver about a 400 fpm face velocity to achieve adequate "throw", and place the register near windows or doors if possible (where the highest losses are). Be sure each room except bathrooms and kitches has a return duct back to the air handler. Alternatively, use a "pass through" to connect rooms which otherwise wouldn't have a return. Size your main ducts to achieve around 600 fpm flow velocity. Size your main return ducts for around 500 fpm. Double check that the airflow resistance of the overall system is within the fan curve of the air handler. (It should be, with these velocities, but if you have too many elbows, etc it is possible to mess this up). Try to run ductwork within the conditioned space of the house, rather than attics or crawl spaces. If you must send your warm air "outside" (e.g. to an unheated attic),be sure to seal each joint very well with mastic or metal tape (ordinary duct tape is worthless), and insulate to code. Nothing is all that complicated, but overall there is quite a lot to this. I designed the ductwork for my own home, and am quite pleased with how it turned out, but it took a great deal of reading and study -- maybe 100 hours or so. OK, so I'm a nerd and it was a hobby activity.   [Reply]   [Send Email]
dave2 11-01-2009 21:14:57 139.139.35.70 812092 Report to Moderator	Re: Installing duct work in reply to omahagreg, 11-01-2009 21:10:21   Maybe try here.... http://ths.gardenweb.com/forums/hvac/ Can't see it being rocket science. have fun. Dave   [Reply]   [No Email]
HENRY E NC 11-02-2009 20:11:31 67.142.163.21 812355 Report to Moderator	Re: Installing duct work in reply to dave2, 11-01-2009 21:14:57   Go to a HVAV supply house and spend about 30$ for a ductolator. Will provide all the calculati0ons you need. Be sure that all duct connections are done with zip strips. aluminum tape and covered well with what we call duct butter( More commonly called mastic). Each ton of air should provide 400 Cft of air and the ducts need to reflect that. Return air should also reflect that value. A ductolator will provide all the info you need or ask me again hmmh@gmail.com Henry   [Reply]   [No Email]