HVAC HELP PLEASE

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ennio

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Good morning. I need help figuring out how long to make my supply trunk. im installing a 3 ton system
with 5 drops. 2 10in flex duct runs and 3 7in with a 14in return. i know i need my trunk will be 18x10. that will
give me 180 fpm. not sure of the length. also is a 14x30 return grill sufficient enough? thank you
 

Dana

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Good morning. I need help figuring out how long to make my supply trunk. im installing a 3 ton system
with 5 drops. 2 10in flex duct runs and 3 7in with a 14in return. i know i need my trunk will be 18x10. that will
give me 180 fpm.
not sure of the length. also is a 14x30 return grill sufficient enough? thank you

Some simple arithmetic:

An 18" x 10" is 180 square inches= 1.25 square feet in cross section, so you'd get 1.25 cubic feet per foot of length. Assuming the air handler is delivering something like 400 cfm/ton at the specified nominal static pressure that would be 1200 cfm.

1200 cf/m divided by 1.25 cf/foot is a velocity of 960 feet per minute, not 180 fpm.

The trunk is undersized- shoot for a velocity around half that (450-ish feet/minute in-duct velocity, give or take).

If the return grill & duct is 14" x 30" that's 420 square inches, or 2.9 square feet, for 2.9 cubic feet per foot of return duct length. The velocity at the grille/duct is 1200/2.9= 414 feet per minute, which is fine. What is the grille manufacturer's recommended velocity (usually more important for supply grilles than returns, but it does affect the static pressure induced by the return grille)?
 

ennio

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Some simple arithmetic:

An 18" x 10" is 180 square inches= 1.25 square feet in cross section, so you'd get 1.25 cubic feet per foot of length. Assuming the air handler is delivering something like 400 cfm/ton at the specified nominal static pressure that would be 1200 cfm.

1200 cf/m divided by 1.25 cf/foot is a velocity of 960 feet per minute, not 180 fpm.

The trunk is undersized- shoot for a velocity around half that (450-ish feet/minute in-duct velocity, give or take).

If the return grill & duct is 14" x 30" that's 420 square inches, or 2.9 square feet, for 2.9 cubic feet per foot of return duct length. The velocity at the grille/duct is 1200/2.9= 414 feet per minute, which is fine. What is the grille manufacturer's recommended velocity (usually more important for supply grilles than returns, but it does affect the static pressure induced by the return grille)?
 

Dana

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in your opinion what size trunk do i need?

Something with a 2-2.75 square foot cross section is great if it fits. Some charts call out 20" x 10" for a 1200 cfm system, but that's still only 1.4 square feet, and over 850 feet per minute- that's going to be noisier and have a higher static pressure than something north of 2 square feet.

At 2 square feet of cross section with a 1200 cfm air handler you're still looking at 600 fps, at 2.5 square feet it's 480 fps, with much lower static pressure drop, and quieter. A 28 x 10 (1.9 square feet) or 30 x 10 (2.1 square feet) would be fine, 36 x 10 (2.5 square feet cross section, 480 fpm velocity @ 1200 cfm) would be closer to the ideal oversizing factor, if it fits.
 

ennio

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ok that sounds great. But my original question is how long should it be 60 inches ?
 

Dana

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BTW: Using tapered reducers after successive sets of flex taps to keep the velocity above 350 fps even at the end is recommended. If you have the different cfm requirements for each tap figured out you'll be able to work that out.

To keep the static pressure drops on the flex low and the system quiet, stretch the flex fully out, and use only radiused hard-piped ells (no sharp throated ells).

Seal all the seams and joints of the hard piping (including register boots) with duct mastic as you go, even the lengthwise seams of the trunk. Every tiny pinhole of a leak has a hiss & whistle potential, and collectively it can add up to a lot of mis-directed air, lowering the net efficiency of the system.
 

ennio

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i plan on connecting 2 7inch taps and a 10 inch tap on the left hand side and a 10 & 7 on right hand side.
i also plan to use the 30x10 that you recommended
Im still not sure how long it needs to be. please advise
 

Dana

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i plan on connecting 2 7inch taps and a 10 inch tap on the left hand side and a 10 & 7 on right hand side.
i also plan to use the 30x10 that you recommended
ok that sounds great. But my original question is how long should it be 60 inches ?

It should be as long a it needs to be to comfortably add the necessary connections. If it's only 60" long it's not a trunk duct- it's really just a short plenum feeding a Medusa of flex ducts, which is fine.

size is not an issue its going in my attic.

If size isn't an issue, and 18" x 18" would have about the same cross sectional area as a 30 x 10 but would have less surface area. If it's in an attic it needs to be insulated duct (R8 minimum on supplies, R6 on returns to meet IRC 2018). Burying the duct with at least 10" of blown cellulose over the top is even better, if done correctly.

Installing it in an unconditioned attic above the attic insulation is about the worst possible location. That punches a lot of holes in the ceiling which have to be meticulously air sealed to avoid 24/365 stack effect driven air infiltration, resulting in wet insulation in the summer, and poorer cooling performance in the summer, and adds at least a half-ton of cooling load.

In the winter cooling ducts in the attic will convect cold air into the house through the registers. Even if the ducts perfectly sealed, the supply registers aren't designed to be air tight, and there is usually no air-flow control at all on the return duct to stop it from parasitic convection loops while off.

Any other possible location that is fully inside the conditioned space pressure & insulation boundary would have far fewer issues.

How far along are you on this design? Is three tons even the correct size- did anybody run an Manual-J type load calculation on the place?
 

ennio

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i had an hvac guy give me an estimate on the job. It requires a 2.5 ton but he said you have a big family that likes to entertain so we decided on the extra 1/2 ton. The attic is the only place for it.
 

ennio

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my living room is 400sg ft
kitchen/ dining room is 385
my bedrooms are 180 110&150
the house is stucco with no isulation in the walls. the living room is the only room that has sun all day with about 5 crappy windows. the rest of the house is shaded for the most part. the upstair has windows with black out shades pulled down all day.
 

Dana

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A load of 2.5 tons for 1225 feet of conditioned space isn't very likely even if there is no insulation in the house anywhere. A 3/4 ton ductless mini-split in the living room and another 3/4 tonner upstairs would probably work out better.

A consultant in the Atlanta Georgia area plotted the square feet per ton against house size for dozens of Manual-J load calculations done for clients to come up with this graphic:

square-feet-per-ton-air-conditioner-sizing.png


Note that even the WORST performing house in the sample was a ~1500' house that came in at a ton per 600', and that's in the steamy hot & humid south where many houses have no insulation and are fitted with single pane windows/no-storms, not Westchester County New York.

Before going any further you may want to do your own Manual-J using the fairly simple but reasonably accurate loadcalc Manual-J tool. It tends to overshoot reality by double-digit percentages unless you pretend the house and ducts are all very air tight, and it doesn't have a lot of nuance for the shading factor inputs. Run separate load numbers for just the upstairs and just the downstairs, since it usually is more comfortable and efficient to zone it by floor.

For the locations on the loadcalc pull down menu list the closest to you are NYC/central park, and White Plains, but the cooling load numbers won't be very different selecting one vs. the other- there will be a bigger difference in the heating load. The 1% design temp for Central Park is 89F, to White Plains' 87F.

Oversizing the AC results in lousier humidity control- bigger is definitely not better in a summer climate as sticky as yours.
 

WorthFlorida

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Hello ennio. Do as Dana suggest that you need to do an Manual J. 3 ton is way overkill for ~1200 sq ft home even with poor insulation. I assume it is a single story home. You're going to have three problems when oversized. 1) High humidity in the home. It will feel cold and clammy, 2) condensation forming on the grills and possibly inside the duct work and 3) temperature of the home will have large swings from warm to cool.

The air temperature coming out of the registers will be too cold for comfort, particularly for women (the wife) they will complain. Your system will short cycle, run less and not be able to dehumidify the air. With very cold air you'll may notice that the windows will have condensation and any condensation on the grills/registers will cause water stains on the ceiling and dripping water on the floor. You don't want a environment for mold to grow in.

I now live in Orlando area and during the summer months, if it doesn't rain, it is 90-95 degrees everyday with humidity at about a low of 60%, morning hours it is about 80%. I have a two story 2400+ sq ft home that runs on a 3.5 ton heat pump. If I had a ranch it would be a 3 ton, double the size of your home. I'm in Florida now for 30 years and no one complains about the temperature, only the humidity. You want a dry home and it will feel cooler with less humidity.
 

Dana

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By the statement response #12 "the upstair has windows with black out shades pulled down all day." makes me believe it's a 2- story. From the room size listing I'm gussing it's 800-850 square feet downstairs (assuming there's an entry hall or hallway/staircase) and about 600-650 square feet upstairs (there has to be a bathroom somewhere!), so maybe it's really ~1500 square feet total, not just the room sizes listed. In Yonkers that would normally be a 1-1.5 ton load, and with favorable shading factors often less than 1-ton.

The local 99% design temp in Yonkers NY is about 13F. Convection from idle but open air conditioning ducts in the attic are usually a comfort and energy use disaster when the temps drop below 30F, which is pretty much every night from half-past December through the first week of March in Yonkers. It will make what's probably an already drafty feeling house even draftier.

If the house has a reasonably tight full basement installing AC ducts in the basement would be OK.

I've seen houses that size in the northeast where a single 1 ton 4-way ductless mini-split ceiling cassette mounted above the top stairwell landing was sufficient to cool the whole house, but without a floor plan and more information it's hard to say if this is one of those houses.
 
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