Trunks cfm capacity reached. Ok to run ducts off plenum?

[R J Cedar]

Hello,

I am told the max capacity of the 2 - 7 1/4" x 14 1/2" trunks I have, one running North and one running South from the plenum, is 500 cfm each.

The 3 ton heat pump to be installed needs to move 1200cfm. The trunks can move 2 x 500 = 1000 cfm., so I need to move 200 more cfm.

Is it an option to draw off the additional 200 cfm, by adding additional ducts to the basement, coming right off the central plenum? 1 - 7" (135cfm) plus 1- 5" (65cfm)?

In theory, taking 200 directly from the plenum, that then leaves 1000 cfm to go out through the ducts, so the duct capacity is not exceeded.

Is this standard practice? Or should this be avoided for some reason, and ducts should always be taken off the trunks, and never directly from the plenum?

Thank you,
RJ

[lynn comstock]

...I am told the max capacity of the 2 - 7 1/4" x 14 1/2" trunks I have, one running North and one running South from the plenum, is 500 cfm each....

That may or may not be true. The duct fittings, the duct length and roughness, and the system fan matter a lot. Also the return duct and the filter matter as well.

The fan simply creates a pressure difference. Not all fans are the same. Some fans deliver full airflow on smaller ducts than others. We don't have enough information to say with certainty what your results may be.

[hvacrmedic]

If the total equivalent length of the longest run is small, then you could und up with more than 1200 cfm on high speed on the existing duct system. Why are you increasing the unit size?

[R J Cedar]

That may or may not be true. The duct fittings, the duct length and roughness, and the system fan matter a lot. Also the return duct and the filter matter as well.

The fan simply creates a pressure difference. Not all fans are the same. Some fans deliver full airflow on smaller ducts than others. We don't have enough information to say with certainty what your results may be.

What information do you need? The system is a proposal, its not built yet.

The AHU is a 1200cfm unit, max sp is .8, fan is variable speed.

The filter is not yet chosen....I now understand that a 5" media filter has a lower sp drop than a 1" filter due to its larger surface area so I could go to that and size the filter and return duct accordingly to exceed slightly the sum of the area of all ducts out - to ensure the return and filter do not hinder the fans capacity to move the 1200 cfm.

2 different contractors propose 2 different things.

One says stay with the existing 5" duct system and add 2 more 6" ducts then this is a 12 duct system. Here I think he would drop cfm to 1000cfm and give me a higher balance point, than is optimum, given demand, but i guess that is the unstated trade off.

The other contractor proposes upgrading all the ducts to 6" and running at the 1200 cfm. His first proposal was to run 200 cfm off the plenum, as I described above in a 12 duct system. But now he has a revised his proposal to go to a 13 duct system and take all the ducts off the north and south plenum. To me I like the 6" duct plan as I do need the cfm capacity, but did wonder if the idea of taking off the plenum was a valid way to go, as the new proposal, it would seem to me, exceeds the cfm capacity of the trunks. So maybe I would get noise? Or too much SP drop?, just in the beginning of the N and S trunks for a while until the flow hits subsequent ducts and the velocity slows down.

Curiously in the second proposal for contractor 2, he also proposed reducing the size of the trunks halfway down the north and south runs, but did not inc the size of the beginning of these trunks even though he said their capacity was 500 + 500, they are now assigned to carry 600 + 600. I guess he is trying to keep costs down, but basically all the ductwork is being upgraded except for 1/2 of the n and south trunks.....that is all that is left of the old ductwork.

Is this enough information to discuss the question - is it a good design to take off from the plenum?

Thanks!

[R J Cedar]

If the total equivalent length of the longest run is small, then you could und up with more than 1200 cfm on high speed on the existing duct system. Why are you increasing the unit size?

Small, relative to what?

Longest runs are 14 feet, straight off the trunks, there are 2 off the south trunk and 3 off the north trunk.

Do you mean the total equivalent sp of the longest run? I guess you are saying if these are fast, the shorter runs will be faster and exceed 1200 cfm and have higher than planned velocities, and therefore noise? So then I need to balance sp for each run to even out the velocities and cfm delivery?

Why are you increasing the unit size? Not sure what you mean. This is a new install, trying to come up with a duct plan - see post one earlier than this one - unit is sized to meet the heat load of the house.
Thanks!

[Cooked]

The short answer to your question is yes, absolutely you can run a single duct directly off the plenum as long the entire system is properly balanced.

[hvacrmedic]

Small, relative to what?

Longest runs are 14 feet, straight off the trunks, there are 2 off the south trunk and 3 off the north trunk.

Do you mean the total equivalent sp of the longest run? I guess you are saying if these are fast, the shorter runs will be faster and exceed 1200 cfm and have higher than planned velocities, and therefore noise? So then I need to balance sp for each run to even out the velocities and cfm delivery?

Why are you increasing the unit size? Not sure what you mean. This is a new install, trying to come up with a duct plan - see post one earlier than this one - unit is sized to meet the heat load of the house.
Thanks!

Did your existing duct system have a unit attached to it? What size was it?

[second opinion]

Hello,

I am told the max capacity of the 2 - 7 1/4" x 14 1/2" trunks I have, one running North and one running South from the plenum, is 500 cfm each.

The 3 ton heat pump to be installed needs to move 1200cfm. The trunks can move 2 x 500 = 1000 cfm., so I need to move 200 more cfm.

Is it an option to draw off the additional 200 cfm, by adding additional ducts to the basement, coming right off the central plenum? 1 - 7" (135cfm) plus 1- 5" (65cfm)?

In theory, taking 200 directly from the plenum, that then leaves 1000 cfm to go out through the ducts, so the duct capacity is not exceeded.

Is this standard practice? Or should this be avoided for some reason, and ducts should always be taken off the trunks, and never directly from the plenum?

Thank you,
RJ

You will be just over 800 fet per minute. What is your return side size and length?

[genduct]

figure out if your old short ducts will deliver.
You have a contractors ADD Cost and will know what it will cost if their GUESS about low airflow comes true

[hvacrmedic]

The short answer to your question is yes, absolutely you can run a single duct directly off the plenum as long the entire system is properly balanced.

You can, but what will it accomplish? If the area currently being fed by the duct system requires 3 tons of cooling @1200 cfm, then sending 1000 cfm to it won't cut it because it'll only get 2.5 tons.

[genduct]

with sensible heat factors <.80 would be 350 CFMs/ton WOuld low airflow really be that bad? Maybe the OP is in the south

[lynn comstock]

Why are you trying to design the ducts? This is the contractor's job. If you don't trust them, keep looking.

The variable speed fan is a strong fan. Larger trunks and branch ducts are better. The design of the branch takeoff fittings can make a big difference. Lower ESP turns into greater efficiency. We still haven't heard much about a return duct. Both ducts, lengths, fittings and construction details matter. Err on the big side.

The 5" media has more surface area, but the filter "mesh" is tighter. The 5" media will likely have double the static pressure drop of the cheap replaceable filters (.10"wc static drop when clean is typical when properly sized for a velocity of 300 fpm).

[hvacrmedic]

with sensible heat factors <.80 would be 350 CFMs/ton WOuld low airflow really be that bad? Maybe the OP is in the south

I assume by that you mean adjusting the blower output? I've done that on a few systems to get the static pressure down. There will be some loss of sensible capacity, but it isn't that much. 4000 BTU maybe on a 3 ton at 95° ambient.