5 ton AC ducting questions

[Hookah79]

I am having issues with my ac unit.
A 5 ton ac in which HVAC guy installed 4x10 inch 90 degree elbows that can be pointed to the desired direction.Problem with that is the back part of the basement barely gets any cold air.So i ended up running a about a 15 foot round ducting across,but this setup is still not practical,because the plants in front of the blower get blasted.The guy installing it said that no ducting is necessary because the basement is not long enough(22'x'20).

Is it practical to run 4 long round 10" ducting with 4" elbow every 4 to 6 feet to accommodate the room?

I spoke to another HVAC tech and he said that to run standard duct work it would have to be 8"x24" because it's a 5 ton ac and can't be less than that.I've seen units like this with smaller ducting ,but he swears that that's incorrect because it's constricting the air flow.Problem with this is the basement is not high enough and taking 8" of the ceiling is going take off a lot of head room from the reflectors .

I 'd really appreciate the help.

 

[queequeg152]

ur existing setup is quite undersized...

unless my math is wrong 4 -10" dia. elbows yield just over 2 sq.ft. and 900+ fpm.

ideally u want no more than 700 fpm in a supply duct. and 700fpm is quite high and noisy for duct work.

without pictures or a drawing its hard for me to reccomend anything in particular.

what people ususally do in basements is fit the duct work inside the joist gaps.

 

[Hookah79]

The vinyl duct runs about 15 feet across.

 

[queequeg152]

yea thats a prety shitty setup.

u should probably just run a single reducing trunk down to the center line of your back wall there, make the turn into the center of the room, and come across the center of the room .

u could ether do runouts form this trunk to where ever u want the air, or just install some louver registers directly into the trunk line.

but yea you will loose some ceiling space.

also consider some turning vanes, media air cleaner, and a cheapo manometer to monitor static pressure across your filter.

 

[Hookah79]

yea thats a prety shitty setup.

u should probably just run a single reducing trunk down to the center line of your back wall there, make the turn into the center of the room, and come across the center of the room .

u could ether do runouts form this trunk to where ever u want the air, or just install some louver registers directly into the trunk line.

but yea you will loose some ceiling space.

also consider some turning vanes, media air cleaner, and a cheapo manometer to monitor static pressure across your filter.

The AC points to the center of the room,so all it needs is some straight ducting all the way across.What i am curious about is the size of the ducting(8"x24")that the guy is insisting on using.I've seen smaller ducting than that for 5 ton AC's.I don't know what type of formula/calculation is used for ducting/ac size.

 

[OG Tree Grower]

Cheap and dirty- run a 14" flex duct down the room and cut small slits in it near the unit and make the cuts larger as you go. The more cuts the more even your temp will be

The best way- put your air handler in a lung room and redesign everything.

 

[queequeg152]

The AC points to the center of the room,so all it needs is some straight ducting all the way across.What i am curious about is the size of the ducting(8"x24")that the guy is insisting on using.I've seen smaller ducting than that for 5 ton AC's.I don't know what type of formula/calculation is used for ducting/ac size.

like i said earlier, 700cfm is about the max you would go for residential hvac. this is due to nosie AND the static pressure limits of the blower. if you take very high velocity air... then try and turn it this way and that way, you loose lots of static pressure. so generally speaking, lower velocity is always better from the standpoint of efficiency.

it looks like a goodman AHU u have there... probably a PSC fan motor so you really should not exceed .5 inches of static pressure... .6ish is ok but above that and u loose lots of cfm and wast electricity.

i mentioned moving the duct along the walls because i thought you would want to take advantage of the joist spacing which is what most folks do.

sure the duct would still stick out... but assuming 2x10 joists, you could get atleast 8 inches of the duct innto that cavity.

the question you need to ask yourself right now is... do i want to pay like 500 bucks for a ductboard trunk, or do i want to do the fastest cheapest thing (flex duct + runouts)

the price honestly might not that different material wise, but the duct board will require an hvac tech to fabricate.

honestly i do not know where the 8x24" number came from. thats even smaller than your current setup.

8x24 was probably his proposal for a bullhead tee with two branches of 8x24... 8x24 is WAY WAY small for a 2000 cfm air handler, there is no way it would work properly.

 

[Hookah79]

like i said earlier, 700cfm is about the max you would go for residential hvac. this is due to nosie AND the static pressure limits of the blower. if you take very high velocity air... then try and turn it this way and that way, you loose lots of static pressure. so generally speaking, lower velocity is always better from the standpoint of efficiency.

it looks like a goodman AHU u have there... probably a PSC fan motor so you really should not exceed .5 inches of static pressure... .6ish is ok but above that and u loose lots of cfm and wast electricity.

i mentioned moving the duct along the walls because i thought you would want to take advantage of the joist spacing which is what most folks do.

sure the duct would still stick out... but assuming 2x10 joists, you could get atleast 8 inches of the duct innto that cavity.

the question you need to ask yourself right now is... do i want to pay like 500 bucks for a ductboard trunk, or do i want to do the fastest cheapest thing (flex duct + runouts)

the price honestly might not that different material wise, but the duct board will require an hvac tech to fabricate.

honestly i do not know where the 8x24" number came from. thats even smaller than your current setup.

8x24 was probably his proposal for a bullhead tee with two branches of 8x24... 8x24 is WAY WAY small for a 2000 cfm air handler, there is no way it would work properly.

The joists run across the air handler like the picture shows so that's out of the question.The 8x24" is just one main trunk that would run for like 20 ft with 8x8" elbows sticking out ,4 on each side.

I've seen units with one main duct running across with dimensions smaller than the one i mentioned,so the curiosity.

Money isn't an issue but i am interested in a setup that would be shorter but wider.

Thanks again.

 

[Hookah79]

Cheap and dirty- run a 14" flex duct down the room and cut small slits in it near the unit and make the cuts larger as you go. The more cuts the more even your temp will be

The best way- put your air handler in a lung room and redesign everything.

I seen that somewhere online but can't remember the exact details.I wonder if i run 14" flex duct 20 feet across with 4 inch elbows on both sides.

 

[queequeg152]

The joists run across the air handler like the picture shows so that's out of the question.The 8x24" is just one main trunk that would run for like 20 ft with 8x8" elbows sticking out ,4 on each side.

I've seen units with one main duct running across with dimensions smaller than the one i mentioned,so the curiosity.

Money isn't an issue but i am interested in a setup that would be shorter but wider.

Thanks again.

yea i understand there is no clearance over head. what id reccomend is that run the trunk along the back wall perpendicular to the joists, then turn the trunk at the center of the room allowing it to fall into the joist space.

a single 8x24 is TOO small man trust me.

900 fpm is MAX for a supply trunk, 900 fpm will be noisy as shit and very lossy... 900 fpm is considered unacceptable my todays standards, hell 700 fpm is considered shoddy by alot of people myself included.

with an 8x24 trunk your blower will be trying to push air at like 1500 fpm.

u need to get the ducting right more than u need the extra inches of clearance.

here is how u do the math... 5 ton air handler is pushing 2,000 cfm.

take the cross sectional area of the duct... 8x24 = 192 inches squared.

192 inches squared / 144 inches squared per ft squared = 1.3333 sq.ft

2000 cfm / 1.333 sq.ft. = 1538 fpm.

cfm = area X velocity . its that simple

if you want to get into dynamic and static pressure losses across fittings and lengths of duct it gets more complicated, but just looking at the velocity u will see how insane what your guy proposal is.

i understand that u want to keep your duct thin... thats fine, just make the duct wide enough to accomidate the 2000 cfm.

after u start dumping air out of the trunk via some registers, you can start to reduce the width of the trunk to save money on duct material and space.

this is called a reducing plenum or reducing trunk. if you size the reductions properly u can maintain a near constant velocity across its length and each register will flow approximately the same volume without agressive dampering.

the last register on a dead end trunk or plenum usually wants to flow more than its fair share of air... for these last registers id reccomend getting a register with opposed blade dampers.
u can usually pull out the opposed blades if it turns out you dont need them.

i would not reccomend dumping the air out of elbows or taps or what ever. registers will throw the air more evenly. they dont cost shit... like 20 bucks at most each.

http://www.hartandcooley.com/produc...-blade-vertical-fin-register-ms-or-obd-damper

i would reccomend a register like this... installed directly into the side of the trunk. its not ideal, but it will work. the vertical fins are adjustable and allow you to direct the air straight out or in a fanned shape.

alternativly i could just install round commercial type diffusers under the trunk. but i assume u are more amenable to the above type owing to height restrictions.

https://www.jedcosupply.com/150W-Round-Ceiling-Diffuser.htm

at the ends where u approach the wall u might want to switch to a three way.

 

[Hookah79]

Ok,when you say fpm,is this refereeing to how far the air travels/circulates before it gets sucked into blower/handler thus adding more duct work?

Looking at the picture there is a stairway to the left of the handler when looking at it,and about 8 feet clearance from the wall right of the handler.The basement is 26x20 minus a 5x6 furnace room that's section off.If i understood you correctly run the trunk to the right,go along the wall halfway thru the basement then come back all the way across?

 

[queequeg152]

why dont u sketch out the whole room, to scale if possible?

i can show u what i mean.

fpm = feet per minute. for what ever reason this is the standard in hvac, its the figure used in all the ashrae documents and SMACNA documents as well... idk why. it is what it is.


im strictly referrring to your SUPPLY duct. this is the duct that comes off your cabinet at the top. its the pressurized air thats blowing out through your duct and into your room.

the RETURN duct is what you are referring to... its the duct that feeds back into cabinet at the bottom( probably). your return duct normally is sized 15-30% larger than the return to prevent noise at the return air registers, as well as to ensure 100% the blower is not working hard to "breath"....

imagine having a garden hose that you can exhale through, but you only have a straw to inhale through... if you restrict either end ( return or supply), you are making the blower work harder, consume more electricity, and flow less air.

again i strongly reccomend u get a media air cleaner. they are baiscally the same as the 1" thick filters u buy at home depot except they actually work. they have MUCH deeper pleats which translates to massive surface areas which translates to very small pressure drops across the filters.

a very very good media air cleaner .... say merv 8 might only have .15 inches drop across it, where as a thin 1" filter with a much shittier rating might have .25 inches drop fresh.

on the topic of return air junk... why dont you check out the goodman angled media air cleaner?
its not very tall... so it might fit under your air handler cabient without issue. its made by goodman so i assume its dimensions match your cabinet. i usually reccomend the trion products because the replacement filters are so much cheaper... but in your case i think the goodman will be easier to deal with.

https://www.filtersfast.com/P-Goodman-GRAM-2000-Right-Angle-Return-Air-Cleaner.asp

i also reccomend a manometer. basically a cheap oil filled glass tube mounted on this plastic base. it measures the change in pressure across the filter and tells u when to change the filter.