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Old 08-07-2013, 02:35 PM   #1
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Designing new ductwork. would love some input.


first of all, sorry about the wall of text. ive tried to bold the most pertinent bits for the skimmers.
i am in the preliminary stages of putting together a really nice 'modern' hvac system for my place. the current system is entirely awful. The ducting, i believe is from the 1980's or possibly the 70's. for whatever reason( most likely cheapness) the owner had a brand new system installed, and fitted into the old poor flex duct system sometime in the 1990's. this equipment is in good condition as per the last annual visit i had. however the very old duct is so bad, it deserves a place in inspectipedia as a cautionary tale.
What im proposing to do is design and install a proper well sealed r8 duct board duct system onto most of this existing equipment. where possible very short runs, 5-10 feet of flex duct will be taken off the main trunk duct. as the design stands, most of the registers are serviced by duct board 45 degree mitered takeoffs currently. Im also making extensive use of vaned elbows and tee's to reduce pressure loss. ive gotten all of the pressure losses reletivly close for all of the supply outlets, however ill continue to try and get them as close as possible. currently the highest is at .562 in W.G, and the lowest is at .524 in W.G. you can see all of the duct sizing and cfm and fpm values on the prelim design drawing below.


The method im using is the "modified Equal friction method" , as laid out in the SMACA manual, "hvac systems duct design". I am working from a heat load calc i performed, however i elected to not use the manual D, as i found it to be too simplistic and overall not terribly useful. what im doing is pretty much manual calculations with the help of a spread sheet i found elsewhere. all dynamic loss calculations are based on the ASHRAE engineering data. ill attach the layout information below.
Before i ask anything, i must make it clear im NOT asking anyone to do the work for me... ive spent probably over 100 hours already reading and researching SMACNA, ASHRAE, NAIMA, manual J, manual D etc, as well as drawing , calculating and tinkering with this design.
with that said, what im looking for is more practical insight from professional hvac guys like yourselves. Ive posted a couple of topics here on this forum, and what i really love is how you guys can just ruthlessly dig out things and problems ive never even considered. obliviously this is most helpful information to have prior to actually tearing anything out.
ive got a number of questions id like to bounce off you guys, ill try to get to them all below, but i have many lol.



The existing system is a York Stellar 2000 4 ton unit with a matching horizontal separate cased coil. the furnace is a 64k btu (unused, see below) 1600cfm unit called a Diamond 80. the system currently uses a drywall plenum that extends to the floor where there is then a return air grill. this is suspect is just terrible building practice? i suspect its leaky and contributing to adding dust to the supply side? i really have nothing to back that up, however i want to abandon this drywall plenum (for now, see below), and instead install a ceiling mounted supply grill elsewhere.
now... when the current system fails, i want to convert this drywall plenum into a real hvac equipment cabinet with a real angled media filter deal and condensate pump etc. this is not possible with the current equipment i have. the coil is technically vertical AND horizontal coil, but it is not plumbed long enough to reach at the moment. the drywall plenum is 24x24 at the moment, but i can expand it to be much deeper as it abuts onto another closet.

here is the old layout. its basically a snake pit of fiberglass duct branching off of the main plenum above the dining room. the rectangles in each room represent the supply outlets.

you will have to click these images. they are very large to avoid aliasing... about 20 mega pixles, however they are PNG files monochrome and very small, only about 200k. i assume a 7560 pixel wide image would not fly posted directly on a forum lol.



here is the preliminary layout with the appropriate annotations.



here is an overlay of the preliminary duct work onto the house layout.



Question 1
my biggest design consideration so far is to build the ducting such that new system equipment can easily be retrofitted into the existing hereto unbuilt duct work. with this in mind i want to add enough service access panels to the duct work such that i can have the duct work cleaned (turning vanes as well) professionally and extensively prior to having any new equipment installed some years down the line. with this in mind, im curious as to where you gentlemen would have the service openings installed so as to easily facilitate a real good professional cleaning? also, im looking at duro dyne catalogues for service openings and vanes etc. yay, or ney? is there someone else i should i be looking to for these items?
yes im laboring under the assumption that you can indeed clean matt faced duct board. this as i understand is more than possible, and is done frequently. i can link the NAIMA bulletins if anyone is interested in reading them.



question 2


this is my biggest unresolved issue. when the ac system equipment was replaced, a gas furnace was installed with the assumption that gas service would soon be installed to the house itself. for whatever reason gas was NEVER brought to the house. since then pretty much every potentially gas appliance powered appliance has been replaced with an electric model, so getting gas installed does not make sense at this point. a 60 Amp circuit is still in place from the old ac's electric heater. i would simply need to run a new connection from the box a few feet away from the air handler to any new electric heating.
the question is this ... should i just get a new air handler?
The Goodman modular blowers seems like a prime candidate. i would like to do this for several reasons, firstly i imagine it would have less pressure loss than the current gas furnace, mostly due to the absence of the unused heat exchangers. Secondly this would allow me the opportunity to easily install another electric heater strip for the occasional cold nights here in houston. thirdly this would allow me to easily add a real nice Goodman media air filter which would be a huge upgrade from the current 1" pleated filters we use atm.
is this feasible? i presume it's not terribly hard to get a york compressor and outdoor unit to interface with a more modern Goodman air handler unit.
if this is not feasible, how about if i gut the gas heat exchangers so as to remove them from the air stream? i would then patch and mastic all the openings . remember this gas heater is not going to be used, certainly not in the short term ; there is no gas line even plumbed to the unit, or even the house, stupid yes i know.
Question 3.

my next question is as follows... there were a few areas where i decided to remove a supply outlet from the original duct layout. in my untrained and unprofessional opinion they were unnecessary. what i did is add the cfm allocated to these areas via the room by room manual J, to other adjacent outlets. my opinion is that these minor changes are pretty much non issues? moreover these outlets can complicate the duct construction. i would like to know if the decisions i made to remove them are in error or if it's reasonable.
i removed two supply side outlets from the house. the first is in the supply from the front hallway.(see just below the 141 sq. ft. callout)the volume of air associated with this area was added to the main living room cfm.
i also removed the supply from the very small bathroom in the master bath. this room was literally about 20 sq. ft. (this outlet is located to the right of the master bath call out. ) this CFM was redirected to the master bath outlet.
post is too long.. ill post my last few questions below.

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Old 08-07-2013, 02:36 PM   #2
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Designing new ductwork. would love some input.


question 4.

next question... I moved the outlets for the living room to a single wall mounted outlet on an adjacent wall. (see above living room callout) my intent is to find something like a good wide register with the ability to direct the air downwards away from the vaulted ceiling down to the living space and across the whole room. the previous registers were on the angled ceiling very far from the windows on the other side of the room and failed to mix the air in the room effectively. on warm days one could notice a temperature gradient close to the windows.
was this a good decision? its far far far easier to move the supply outlets to where i proposed in the preliminary layout. the attic is accessible from below via the drywall, and i could do this with a small section of duct board and a mitered take off.


question 5.

given that... theoretically, all of the outlets should be somewhat balanced already should i bother getting proper opposed blade dampers all together? can i depend on just adjusting the registers themselves? many manufacturers seem to offer fancier registers with built in dampers, should i go with these considering i would have no real dampers, instead of using just simple cheap supply side registers?

question 6.

Should i bother at all with volume splitters for the two vaned tees as well as the mitered take offs? it seems like a good idea personally, however it seems like a decent chunk of work to fabricate these things, and could easily just be a waste of my time in terms of additional functionality. would you guys bother? ive sourced the required components required to do this from durodyne already, id jsut need to order them i guess.

this is what im referring to.





i realise how absurdly long this thread is, all i can say is thanks in advance to any advice you guys can offer, i surly do appreciate it.


Last edited by Queequeg152; 08-07-2013 at 02:40 PM.
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Old 08-07-2013, 06:22 PM   #3
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Designing new ductwork. would love some input.


  1. While the return duct could have dust/dirt accumulate in it. If your using a high quality media air filter, the supply should never need to be cleaned.
  2. New air handler would be best.
  3. If the trunk line is sized proper, its ok.
  4. 2 supplies are better for a larger area then 1 is. Use a high velocity register. It will give you better throw.
  5. Check out Hart&Cooley VOH/VH series registers.
  6. Volume splitters should be needed.
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Old 08-13-2013, 11:49 AM   #4
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Designing new ductwork. would love some input.


hey beenthere, thanks for the input. sorry about the late reply, but i HAVE taken your advice to heart. ive been busy at work, and accidentally cut the phone line and DSL at my place digging out a stump... so ive been dealing with that.


1.
im pleased to hear i shouldnt need extensive cleaning with a good media air cleaner. however should i still be placing service openings just at the turning vanes maby?
i suppose if they get fowled i could just install one then and there. perhaps service openings are not worth the potential leakage they present?

2.

i ve been reading, and i think ive settled on a goodman MBR1600 air handler, ill have to do some more research for the correct heating element, however it looks like it will be a paltry 300 bucks or so for the whole deal.

im also looking at the goodman model: GMU2025 20x25 media filter. this filter is rated up too 2000cfm @.207 in W.G, however im going to use only 1600ish of that, so guesstimated the pressure loss at said cfm to be .15 in W.G.? this sound reasonable in your experience?

4&5
thanks for the heads up on those lines of hartly and cooley products. it never occured to me to look into their commercial line of products. ive been using there residential specs for throw and losses etc already.

im looking at the 92 VHO a wall mounted register with the horizontal bars in front i believe. i want to throw 14 feet, thats the entire width of the room is that correct?
sorry but im not familiar with registers at all, is there a rule of thumb for this sort of thing? like 80% of the room width? or something like that?
i ask because its going to be the difference of say 600 and 700fpm for the register.

6.

the volume splitters should be fun to fabricate.
i have a parts cataloge from durodyne, this is what i have in mind. hopefully i dont have to buy 25 sets, because thats a deal breaker if i do. see below



see the bottom right. this look reasonable? im assuming i can make the splitter itself from simple 14 or 12 gauge galvanized steel cut my tin snips or angle grinder.
i have a tig welder for what ever thats worth. i dont see any opportunities to use it however. possible to build the turning vanes? idk

this is the only detail ive actually seen for a splitter, do you perhaps know of others? id love to see them if you have any other designs on hand. ive looked all over the web, and came up short.
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Old 08-13-2013, 04:26 PM   #5
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Designing new ductwork. would love some input.


  1. I wouldn't worry about service openings now. If they are needed later, they can be put in when needed.
  2. 10 to 15 KW.
  3. At 1600 CFM, it should be rough .133
  4. When the register is on an inside wall, you want the throw to reach the opposite wall.
  5. Sorry. Didn't proof read my other post. I meant volume splitter should NOT be needed.
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Old 08-14-2013, 11:20 AM   #6
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Designing new ductwork. would love some input.


1.
ok great.

2.
noted.

3.
.133 is even better, thats going to bring my esp at the worst run down significantly, down to like .525

4.
noted

5.
ok so that's good news.

but the idea of splitters etc got me wondering... is it feasable to set up a zoned hvac system within a residence? like say... during the daytime some PIR sensors detect nobody is occupying the bedrooms so a valve actuator chokes down on the airflow in the branch feeding the bedroom, and lets the temps rise up to 82 according to a remote thermostat in the bed room area?

my freshman dorms worked something like that. each room had these water cooled heat exchanger air handler things above the doors tho, so they would just turn on and off.

the office building i work at, does something similar to what im talking about i think, except it just opens and closes a damper. there's a huge utility closed that houses the air handler.
im guessing its fan speed is varied due to how many of these dampers are open.

sounds cool as all hell, but im guessing this is pointless in a small home like mine?
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Old 08-14-2013, 06:05 PM   #7
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Designing new ductwork. would love some input.


Its possible to zone a house to do what you asked about. If zoning the first floor from the second floor. then its best to make a separate trunk line for each floor. And install the zone damper in the respective trunk line.
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Old 08-30-2013, 03:55 PM   #8
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Designing new ductwork. would love some input.


thanks again beenthere, lol youre being so helpful i wish i could buy you a drink or something.

so ive got a nice stretch of free time on my hands, thought id plug at this duct work a bit more.

ive designed an ideal'ish assembly for all of the equipment and transitions required. here is a crude 2d drawing ive come up with.

sorry about how large the image is... the problem here is the text.
in order to avoid nasty aliasing you have to push up the DPI to 200+
pdf's can handle the text and linework much better with vectors... however im a noob and dont know how to share those.




where im stuck now is... how should i approach this?
i drew the fittings as r8 ductboard, however all of the dimensions and angles are taken inside, such that they can be easily adapted to sheet metal.

option one is
i pay local sheet metal guys to build the fittings individually. surely this would be the best way quality wise.

to that end, do you have any ball park numbers that you could toss at me? if i could get these parts made for like 200 bucks id be ecstatic. but at like 300+ im starting to feel stingy and DIY'ish. lol.


Option 2 is
build it myself out of duct board. OR sheet steel.

im imagining duct board transitions would be a nightmare of carefully sculpted miters and ship laps etc. keep in mind im proposing to use r8 duct board. all the tools i have( as far as i know) will only work on 2" duct.

another concern is moisture. like i said before, ive read ALOT. one of the problems ive read about is how horizontal coils like mine tend to "spit" little drops of water that get carried to the down stream surfaces. because of this ive read that steel, speficically galvanized steel is recommended for like a foot or so down stream. additionally this galvanized steel would also be sloped to return the moisture to the coil, then ultimately to the condensate drain.

it should be noted that the duct board im proposing to use is mat faced, and apparently treated to repel moisture. im wondering if this is not enough protection to last a paltry 5-10 more years? however perhabs this water repellency is overstated?


OR

i could build it from WELDED sheet steel... 18-20 gauge sounds about right to me? then paint it well to hopefully avoid rust...
im thinking id just add small 3/8" extended tabs to where ever i need to join panels together and kinda fake spot weld with plug welds. im assuming i can bend and form these tabs to what ever angles i need with simple hand tools and things im familiar with. i do not have any cool stuff like bending breakes, punch presses or w/e real hvac guys have.

this is the detail im going by for the ductboard to equipment connections. where the steel duct would mate with any duct board i would do something very similar.



so then id have to insulate the fittings as well which im guessing is not terribly hard?
ive seen insulating blankets used for spiral duct. im guessing i wold just wrap up the steel nicely, then just staple with the same outward clinch type staples id be using on duct board?


in either case, should i need anything special. like perhaps a flexible joint? or maby something like vibration absorbing pads for the air handler? maby special gaskets for the equipment to steel duct joints? i already have a a new all plastic condensate pan lined up, but beyond that, is there anything else needed?

for the equipment assembly i had planned on a simple 2x4 setup resting on the ceiling joists. maby with little adjustable feet or something to correct any problems with slope.
where feasible id hang the smaller duct by the roof rafters with channel strut sections and threaded rod, this hopefully would allow me to finely adjust the height of the duct where ever id need.
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Old 08-30-2013, 06:02 PM   #9
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Designing new ductwork. would love some input.


Horizontal coils can have the water blown off them if the velocity through them is too great. If the velocity through them is keep low/practical, no problem.

A 2 foot piece of sheet metal is always a good thing on any horizontal application, even heat only apps. makes a better seat to the furnace or coil.

If going with duct board, no flexible connector/joint is needed to prevent/minimize vibration to the duct.

Yeah, using 2X4's to raise the pan so you can drain it if it ever needs to be drained is important really bites if its not raised). The furnace and coil should also be raised out of the pan far enough that you have room under it to seal the ducts to it.

With some practice. You can cut R8 with a knife. Yeah, takes a bit of time.

Best thing to do, would be get a price from a sheet metal shop, and see what they will charge. Sheet metal won't crush as quickly as duct board if you accidentally kneel on it.

Its mostly a personal choice, that you have to make.

the matte surface holds up pretty well against small amounts of moisture. And as long as you keep your coil clean so the velocity stays down at a proper level, it should last 20 plus years.

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