14 in. return ducts for 4 ton heat pump?

[overbbg]

I'm in the process of getting quotes to replace a 4-ton package heat pump (Heil "horseshoe" 10 SEER unit). I have a 2300 sq. ft., single story house in northern Alabama. I'm going to be putting in a 16 SEER (probably 2-stage American Standard or Trane) heat pump.
So far, one of four estimators has said my 14" diameter return ducts are too small and he wants to put in 18" ducts. I have two return air grills, both are 20"x25". There is a 14" flex duct from one return grill that runs about 30 feet to a junction box. The second return has about 10 feet of 14" flex duct that runs to the junction box. Then there is an 18" flex duct that runs from the junction box to the unit, about 17 feet.

My question is, are 14" return ducts big enough, or should I pay the extra money to expand them to 18"? One estimator said a single 14" duct was good for 480 cfm, another said it was good for 700 cfm, and a third said it was good for 1000 cfm. What's the right answer?

[beenthere]

It depends on the length, as far as how much air it can carry, without adding undue resistance to air flow.

I don't think you need an 18" from either grille to that box.
But I haven't seen it.

The contractor, that said about using 18", may be trying to lower the return resistance, because your supply is a little resistant, for a VS blower.
And by increasing the return duct size, he can compensate for the supply.

[Gunnery Sergeant Hartman]

14" is way too small for a 4 ton.

The only way to know is a manual D calculation.

I would think 18" diameter or bigger depending on the length.

[beenthere]

14" is way too small for a 4 ton.

The only way to know is a manual D calculation.

I would think 18" diameter or bigger depending on the length.

2-14" flex ducts, feeding an 18" is too small???

[jrbenny]

Using the lengths listed, I'd run 2 - 16" to 1 - 20"

[overbbg]

Thanks for the responses. What's the down side to sticking with the existing 14" ducts? Will it require the unit to work harder/use more electricity? Will it impact the comfort level in the house?

[dash]

Thanks for the responses. What's the down side to sticking with the existing 14" ducts? Will it require the unit to work harder/use more electricity? Will it impact the comfort level in the house?

First ,what you havwe meay be okay,can't tell without testing the static ,or seeing the job,or as built prints.

The unit you are buying is most likely a VS indoor motor,more resistance in the ducts causes it to use more electricity,so larger ducts will likely pay for themselves in time,and you may really need them,or not.

[scottsacavsfan]

I have never seen a unit that had a problem with a oversized return

[graybones]

Assuming 1600cfm for 4tons of cooling and .1 fiction loss per 100ft of ducting a 14" duct handles about 1000cfm air volume and a 18" duct handles 2000cfm. This should answer your question.

[dash]

Assuming 1600cfm for 4tons of cooling and .1 fiction loss per 100ft of ducting a 14" duct handles about 1000cfm air volume and a 18" duct handles 2000cfm. This should answer your question.

The junction box mentioned could be 150 equivalent feet of duct,plus the turns,elbows,return box ,etc.The loss could be for 300 feet of duct not 100.

[mark johnson]

For what it's worth, I have a system running at 1600 cfm (~ 4 tons), with two 16" flex duct returns (each about 10' long) from two 20x25" filter boxes, and I have a fairly high static in the returns. I plan to add another 16" return and filter box, and expect the static to go down by about 0.1".

[beenthere]

For what it's worth, I have a system running at 1600 cfm (~ 4 tons), with two 16" flex duct returns (each about 10' long) from two 20x25" filter boxes, and I have a fairly high static in the returns. I plan to add another 16" return and filter box, and expect the static to go down by about 0.1".

I thought you said you have a 5 ton, in your other thread.

[mark johnson]

I thought you said you have a 5 ton, in your other thread.

You are right, my system is 5 ton, but I have the blower set to 1600 cfm in order to reduce the static. And this kind of airflow is typical of a 4 ton system, isn't it?