Duct work, myself or hire?

[energyStarBoy]

Hey, i am a 2nd year apprentice, commercial hvac. Installing new heatpump in my attic (yah i know it sucks!, but i put a windowshaker in for comfort ). Well seems i have backed myself up against a wall here. The duct work.... the trunk line anyway, is boggling me.

It is being installed in my home which never had central air of any kind before (old boiler will be st 1(in duct) and st 2 em heat).

I've done the manual D, the trane ductolator, and all that good stuff.

My problem is when i go to the supply house to get parts, nothing matches up to what i need. Need 13"x12-1/2" supply off of unit, and need 22x14 off of unit for return, or there abouts. Also it is going in the attic of a cape-cod, so it is real tight in there.

Should i just hire a residential company to do the trunk? (keep in mind that i'm about broke and saving any $$$$ at all is a good thing) Or should i just wait and tough it out and get it done on my own?

Any help, opinions appreciated!

[Dirty-SMW265]

...my opinion is...

a: you should resize the sheet metal to accomodate standard "supply house" sizes...(use your ductolater again)

b: double check your calculations with your instructors (at apprenticeship school)....during my apprenticeship I asked my instructors ALOT of questions and they were always very willing to help, and it shows that you care enough to do it right and that you thirst for knowledge...

[energyStarBoy]

Right (i should have mentioned that), i planned on transitioning from the discharge side (12x13) to round (dual 10") on a tee to accomodate for that reason. (round to accomodate for space limitations) (then i'll have 14 5" lines off of that dual 10", which i would transition to 9" half of each run). The return side is only going to be about <= 1 foot away from the suction and straight down through the attic floor.

(btw: the cfm and velocity won't be perfect, but seem they should be "good enough"; i can always fix it later.... i just want some cold air for right now)

I just can't seem to get off of the unit, to something, so i can transition.

Oh, and school is out for the summer, so out of luck there. And with it being the hot season, asking at work is kind of a no-no as we are VERY busy right now.

edit: oh i left ductolator in truck, but seem to remember standard being 20x8 and 14x8, or 18" round and 14" round, suction @ discharge respectfully.

I guess i have 3 options, make my own custom boxes off of the unit; bring specs to a fab shop; or hire someone.

I suppose my main question is:
Comming off of the unit, i use canvas for an inch or two, then i need something the same size dimensionally as the unit, then i need to transition that to "common sizes"; correct?

(sorry for the run-on sentences)

[beenthere]

Get someone to help you, so you don't have to turn the A/C off at night to sleep.

By your duct description, I presume its a 1.5 ton system.

[acmech13]

why would u use canvas? sounds like a hack job. why not give someone at work a few dollars to help ya. your not gonna get alot of air out of a 5" run.

[energyStarBoy]

why would u use canvas? sounds like a hack job. why not give someone at work a few dollars to help ya. your not gonna get alot of air out of a 5" run.

5" run (about 3 feet with one 90 ell on each) equates to my memory as being 84 cfm at 800 fpm (roughly, going off memory here). Multiplying 14 by 84 gives: 1176 cfm total. 2.5 ton system, operating at 950cfm; will/should be overly adequate by my calculations. (remember i'm an apprentice in a different field so if i missed something let me know).

Don't all units have to be isolated from the duct work by a non rigid material (such as canvas?), i figured that was why i've seen so many, and that is why they sell them.

I have no problem hiring somebody, but would just as soon learn my trade on my own home and make my own mistakes on my own time in my own home where i can fix and learn as i go. But if i'm completely wrong, then i just might hire.

edit: i re-read my earlier post, and noticed it might have been misleading. the 5" lines are the ducts going directly to the registers. so 10" trunk, with 5" ducts off of that going to the registers. (space limitations lead me to use 5" ducts, floors are 2"x7", so by the time i got the boot in there i could only use 5" ducts).

[energyStarBoy]

Get someone to help you, so you don't have to turn the A/C off at night to sleep.

By your duct description, I presume its a 1.5 ton system.

By my calculations (which could be totally wrong), a 1.5 ton system would normally be 450 cfm, which would then be a 10" or 12" supply and 14" return. no?

But, no, mine is a 2.5 ton.

(if at any point i sound condescending, please to(edit: don't) take it to be so, i'm kind of a to the point/aggravated individual)

[beenthere]

why would u use canvas? sounds like a hack job

Flex connection is a IMC requirement.

[energyStarBoy]

Flex connection is a IMC requirement.

thank you.

[beenthere]

What FR are you designing for, .15?

2 - 10" round is going to put you over .12 FR.

Its going to be loud using 5" hard pipe supplies.

Nominal 1.5 ton, would be about 650cfm.

[energyStarBoy]

What FR are you designing for, .15?

2 - 10" round is going to put you over .12 FR.

Its going to be loud using 5" hard pipe supplies.

Nominal 1.5 ton, would be about 650cfm.

FR == Friction Loss?????

but i read somewhere that my friction loss would be approximately .2.
Does that help?



perhaps i got my cfm for my unit wrong. it is a 2.5 ton. according to it's literature (install booklet) it is 950 cfm.

should i not be going by that, and rather be going by 'nominal'? and if so, how do i determine nominal?

edit: and could u define loud? i havent' run those lines yet(although i have purchased some of them), as i have no trunk yet. I could use flexible if i have to. Although i had planned on using rigid (i just like rigid, looks better and 'seems' better to me).

[beenthere]

Put your vacuum sweeper in one room. Pull the hose into the other room, and hold it near your ear, that kind of loud.

Hopefully your didn't by the equipment yet.
Do a room by room load calc to know what size you really need./ This will also tell you how many CFM each room needs.

Then draw out your duct work(no sizes), include bends, els, boots.
Convert this to total equivlent length.
Then you can use manual D to find your FR to use on the ductulator.

My guess, is that your going to find you need 2 - 12", or a 10", and a 12" for the supply trunk. And 5,6, and 7" for the supply branches.

[energyStarBoy]

Just for clarification and reference.

My designs:

Unit:
Indoor M:gb5bm-030k-a horizontal application.
Outdoor M:ft5bu-030k
2.5 ton gibson

Discharge of unit:
950 cfm (per manuf.)
12.5" x 13"

Trunk:
canvas isolator: 12.5" x 13" (950cfm @ +/- 900 fps)
first piece of trunk duct: 12.5" x13" to 20x8(???) or to dual 10" round (990fpm @ 500 cfm; times 2 = 990 fpm @ 1,000cfm). ehhhhh, bummer, edit that: 12" round dual, 500 cfm @650 fpm, and times 2 = 1000 cfm @ 650fpm.
90 ell up (due to space limitations) (i didn't relate the ell due cfm greater than needed, also was equated with 100' run, so should be well adequate)
edit: 12" tee (well technically not a tee, but 2 ells of 2 seperate lines) over unit to right, and left away from unit for +/- 8'.
reduced to 8" or 10" or something for remainder of run. (yah, u guessed it, kinda guessed on this part of it) (BUT, will have extra capped takeoffs to move around and compensate as needed in future)
14 5" ducts off of trunk. (approx 84cfm @800 fps, which totals 1175 cfm)
-4 to front of house, approx 18' run.
-10 about 2' below the main trunk.



Suction of unit:
12x22 (basically the unit dimensions)

was thinking:
- use "cool kit" whatever its called backwards straight to the floor. with a louver the same size as the ductwork.


Am i WAY off?


(i reserve the right to edit this as i get help redesigning; for my own purposes)