Supply plenum size vs Opening size in air handler

[sb023]

Here's a question:

I have a 5 ton air handler with an opening in the top of it obviously.
That 5 ton unit was obviously designed so the opening can actually push 5 tons of a/c through it.
If my supply plenum is the same size as this opening then it should be adequate I would think since the hole in the air handler is all the air handler can put out anyhow.

BUT, I had a guy come out with a ductulator and measure the supply plenum and said it was too small for a 5 ton unit ! How can this be? It is the same size as the opening in the top of the unit.

[yourairman]

The opening on the air handler is adequate because there is no back pressure(static pressure) to resist the amount of airflow that is required. Once you start adding ductwork with bends and elbows to the equation, the size of the air handler opening is a mute point.

[sktn77a]

????? But once you connect all of that ductwork (with its inherent back pressure) to the furnace, isn't the blower is pushing against it through that opening, just the same?????

[heaterman]

The exit opening of the blower itself is what is dictating the dimensions of the flanged connection on the airhandler. From that point, the duct system must be sized accordingly for the cfm capacity of the unit, the duct system and all of it's branches follow suit. You will notice that the return side is considerably larger.

[yourairman]

????? But once you connect all of that ductwork (with its inherent back pressure) to the furnace, isn't the blower is pushing against it through that opening, just the same?????

Typically the velocity coming off the top of the AH is considerably higher than what the supply duct is designed at.

[sb023]

so, the supply duct that is coming directly out of the air handler should not be used to measure cfm on a ductulator because in my case that shows the opening will only flow 1300, when it needs to flow 2000 for a 5 ton.

which then asks the question: if a 5 ton needs to flow 2000 cfm, where should that measurement be taken on a ductulator ?

[wraujr]

Its probably what comes off the plenum that is the problem.
How many branches come directly off the supply plenum??
What are their sizes??

[beenthere]

Need to know the total equivalent length of the duct.