ductwork size 4ton 5ton

[inphx]

As my role of "owner/builder" I need help solving a design problem outside of my expertise. Because of height restrictions on a 2 story build the overhead trusses on 1st floor are 14" deep.

A metal duct installer tells me:

Plans are calling for 16" duct work from a 4 ton on the 1st floor and 5 ton on the 2nd floor. This is too small for the CFM that these two units require to provide the efficiency of the units regardless if its 13 or 18 SEER. For a 4 ton its 18" and 5 ton it’s 20" ductwork.

I assume the 18" and 20" he refers to is the width or height of duct to between beams - it would seem there needs to be two numbers a width and height to know if there is an issue.

How can I get a clue? If I look at the CFM of a unit, lets say its 2160 , and the max physical duct size seems to be limited by the joist width and height... how do I calculate necessary duct width/height... is it 300 sq inches for 2160?

Is there any other influence on the calculation for efficiency? Or simply right sizing to the CFM?

 

[pseudorealityx]

If he's only giving you a single number, he's quoting you round duct, which is typically cheaper. Since you're restricted due to structure, you'll need to go to rectangular, which will likely cost a bit more. Whatever your clearances are, subtract at least 2" to account for insulation. Have a 14" clear dimension? 12" is the largest duct dimension you'll be able to install there.

There's a bit of 'art' to duct sizing. If you're too restrictive in one place, you can add it back in others.

An easy tool to use is a "ductulator". If you do a google search, you can find them online for less than $20. Get one from a known brand... Trane, Carrier, Loren Cook, etc. That will help you find equivalent sizes and whatnot.

 

[Zeke]

Rigid duct is more efficient and with low humidity I'd look into smooth interior duct wrapped on the outside. You can easily calculate the area of a cross section of either but a 20" round flex duct needs to be bigger than a round rigid or rectangular.

I second the calculator but that's one part of the equation. You need to know the energy load of each room to design and size the distribution. This is one service you ought to pay for. I didn't and have had the problem of uneven balance because I let the installer do it from 'experience'. He missed.

 

[RedDirtRoad]

16" round duct will do 1400 CFM at .1 static pressure
A 4 ton unit delivers 1600 CFM. 16" round duct will do this at .12 static pressure.
You should be fine if its a straight duct run. The air velocity will be approx. 1200 ft per minute and might be a bit noisy.

18" duct is fine for a 5 ton unit delivering 2000 CFM
You should be fine using 16" for the 4 ton unit and 18" for the 5 ton unit.
I assume he is reducing the duct too as he takes off branch ducts to maintain velocity and CFM's

 

[DEnd]

Is there any other influence on the calculation for efficiency? Or simply right sizing to the CFM?

There is a whole lot that goes into duct design and getting the desired efficiency. Your best bet is to find an engineering firm to run a manual J calculation and design your duct work for you. It is expensive, but it is your best bet to get a system that performs well.

Also for energy efficiency you want your duct to be in conditioned space. Otherwise you are just blowing expensive conditioned air outside, because all ducts leak.

 

[inphx]

Thank you. I am looking at duculator as an affordable software tool to validate/answer my questions. Seems like the software utility is of value for calculations but their web site seems 1970's....

 

[meathooker]

16" round duct will do 1400 CFM at .1 static pressure
A 4 ton unit delivers 1600 CFM. 16" round duct will do this at .12 static pressure.
You should be fine if its a straight duct run. The air velocity will be approx. 1200 ft per minute and might be a bit noisy.

18" duct is fine for a 5 ton unit delivering 2000 CFM
You should be fine using 16" for the 4 ton unit and 18" for the 5 ton unit.
I assume he is reducing the duct too as he takes off branch ducts to maintain velocity and CFM's

Newer units with variable speed fan motors don't like a lot of static. I like to size supply at .08 static.

 

[inphx]

Appreciate all the replies.

Laying out distribution makes sense to me and duculator , althogh dated as a software application, does provide some good calculations for run segments.


For a return - there must be some science that expects flow back from far end rooms throgh door cracks at the sills or engineered returns. I dont see the consideration in design if you arbitraily place a return near the unit. It seems like some attempt to allow for return from far rooms via ducts or door gaps at sill (if expected to be closed doors in some phases of operation) would be needed.

 

[pseudorealityx]

Returns. Totally depends on the design. Most residential houses use 1 or maybe 2 returns per system. Returns from far rooms are typically handled via undercuts and possible transfer grilles.

 

[pseudorealityx]

16" round duct will do 1400 CFM at .1 static pressure
A 4 ton unit delivers 1600 CFM. 16" round duct will do this at .12 static pressure.
You should be fine if its a straight duct run. The air velocity will be approx. 1200 ft per minute and might be a bit noisy.

18" duct is fine for a 5 ton unit delivering 2000 CFM
You should be fine using 16" for the 4 ton unit and 18" for the 5 ton unit.
I assume he is reducing the duct too as he takes off branch ducts to maintain velocity and CFM's

This is too simplified.

That's 0.1" of static per hundred feet of straight duct. Which in a residential setting, is unheard of. Transitions in residential applications are all over the place, and not typically designed ideally due to structural (will it fit?) requirements.

Even in commercial settings where I typically have more room to run ductwork, I size supply duct at 0.08"/100' and return ductwork at 0.05"/100'. Exhaust ductwork depends on the application.

 

[yeldogt]

I see your location .. but how big is the house? 9 tons of cooling?

 

[91 zeee]

I mostly agree with pseudoreality and red dirt road. I do not think you necessarily need to design at a .05 for the return (if space is limited), but its good if you can and it does quiet the duct system.
However, my .02 is this...if your HVAC contractor cannot run a load and custom design this for you then maybe you should look for another contractor.
It is fine if you want to buy a ductulator, but if I was doing this job for you I'd be using Manual J load calculation software to run my loads (did your HVAC contractor do that?) and then I'd complete a design based off the load. This is not necessarily just reading a ductulator, it's more than that.
9 tons is a lot. I'm also curious as to the sq footage of the house.

 

[pseudorealityx]

Ductulator will help you size the ducts for the airflow you need. Figuring out what airflow you need is where load calculations go.

I still don't like manual J, but that's because I'm so ingrained with commercial calcs, and Manual J just seems silly to me.