HVAC-Calc, Ductalator, ManD - duct sizing?
I had some help in another thread and as it got off the original topic, I am starting this related thread.
I am working on a basement design. I am *not* a DIY. I have however had dismal experiences with local contractor's designs. Craftsmanship is OK, and the techs are good, but their designs have not been impressive.
I am a ME, not in the HVAC business, but I have a pretty good understanding of the subject. Fluid dynamics wat my "thing" in college. Not so much compressible fluid flows though
As I work with my contractor to get this basement installed, I want to be sure the ducts are sized properly.
I own HVAC-calc and I have done the Manual J loads using that software. This will be a 1.5 ton unit.
Here are the links to the HVAC-Calc reports:
Sorted by heatgain/summary:
beenthere and some others have pointed out that the duct sizes produced by HVAC calc are, at least in my case, much too small.
adrianf replied and has pointed me towrd doing a full Manual D worksheet to size the ducts. Unfortunately I don't have a copy of Manual D anymore .
I asked about using the Ductalator, and adrianf replied that I need to "determine the design friction rate" before using the Ductalator. Not sure how to determing the DFR, but I'd be grateful for a pointer.
Can I persuade anyone to hold my hand a little to work out the correct duct sizes for this job? I can do the calculations, with some help, if I could have a hand at a start. I am positive the subs who have quoted this job are not doing this level of design, so anything I do in the right direction ought to help.
I'm surprised the HVAC-calc people don't offer any better methods to incorporate frictionlosses into their software, but alas, I don't see any.
Does anyone make a reasonably priced Manual D software calculator? I don't mind the investment to ensure my system is built correctly.
Try goggling ,there are some design firms that will do the Man. D from your sketch/drawing.
One problem is identifing exactly what fittings ,takeoffs,etc. ,will be used,as each type likely has a different value.
You could post a sketch or maybe email to a member for some assistance,email is in my profile.You need dimensions plus the heating and cooling cfms required for each room.
Another option is to use a ductolator and assume a low FR of .05,you may get larger ducts then are needed,but can always increase the static with a damper in the main trunks.
Are you sure No one in your area can/will do a Man. D??
Also be advised that if you use flex duct go up two sizes larger for the same friction loss and flow characteristics as round duct. I have both a flex ductulator and a regular one and that's how they check out. You can also get a fexductulator on line to use. Thermoflex has one on their web site. A 8" flex is about the same as a 6" round and so on.
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Thanks guys. I got access to a couple of software tools to help. I'll see if I can figure them out.
A typical friction loss for a supply duct is 0.08" per 100 feet while a return duct is lower at 0.06" per 100 feet. Are you going to be running a main trunkline the length of the your basement with individual flex run-outs to each room? I would recommend installing a return duct for each occupiable room such as the bedrooms, office, family room, etc. An alternative would be to install transfer ducts and grilles between these rooms and the main hall, or wherever your furnace/air handler is located. This will allow a path for the return air and improve circulation.
FL_PE: Thanks. Yes, mainly one trunk, but the lower is sort of in the middle, and one side of the trunk will have some bends. This main trunk will effectively "split" the basement parallel to the short side, and then have flex out to each register/room. The primary reason for the flex vs rigid is being able to get the duct runs up into the trusses. I will use rigid where it is possible to minimize friction losses and headroom isn't a factor or a big deal.
Originally Posted by florida pe
This basement has a 20" wood truss ceiling. and Most of the rooms can communicate easily via this space to the hall where the main return would be. There are 2 rooms that would be more closed and I had planned to add a passive return in the large home theater/media room. The main issue there is sound isolation both from the outside in, and leaking out of the room. Having a duct communicate freely makes that more difficult and a direct return less practical for that room anyway. Depending on the final choice for ceiling construction in that room (from isolated double drywall on one end of the spectrum, to a drop tile ceiling on the other) will influence that rooms return choice. But in either case a passive transfer cust will likely be preferable.
Since most of the registers will be ceiling mounted, I figured having the retuns as low as possible would be best.
Originally Posted by shorton
Acoustical transfers are pretty common in commercial applications, where plenum return is very normal.
Set the transfer up like a "parascope box" - a long rectangle with an opening on either end, on opposite sides - a long "Z" if you will. Line the transfer with 1" acoustical lining. Size the free area like you would return ductwork.
The open truss joist system will come in handy for your return path(s) - ie, you can use transfers and the plenum space to save some ductwork...
Excellent, thanks. I have seen some designs that even zig zag the air path through a box. But that will take space to make something that does not have a huge friction loss, and I don't have an easy way to model it. I can plan a plenum type space (no interior walls yet). Would be great to use a stud cavity in a 2.4 wall but there wouldn't be much room left after lining (2.5 x 22 or so). And maybe there are other reasons not to do that (moisture, etc.).
As for acoustical lining, what about making such passive periscope returns from "duct board"? Or what kind of lining have you used?
Duct board is OK, not ideal, acoustically speaking. Acoustic liner is something your installer should be able to get. If they do any light commercial work, etc, they'll have it on a shelf, or at teh very least can get it easily.
Originally Posted by shorton
I'm an ME as well - not an installer, so I haven't "used" any, but have speced plenty
Thanks, I'll have them check on it. If it's normal for commercial work it should be trivial to get. Everyone freaks if I mention duct board anyway , so the other woudl be simpler to deal with, or get them to that is.
Originally Posted by larobj63
Originally Posted by shorton
Thanks for spell checking my quote - if I had a dime for every time I typed, "teh" instead of "the", I could retire. lol
I don't have any first hand experience with it but you might have the installer look into the R.A.P. from Tamarack.