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

[genduct]

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.

A little more latent, so a couple of hundred CFMs IF it does come up short is not the end of the world as we know it

[hvacrmedic]

A little more latent, so a couple of hundred CFMs IF it does come up short is not the end of the world as we know it

I'm in the South, so FWIW I've adjusted the cfm down on units that didn't have any static issues. When you can get the humidity out you don't need those extra couple of degrees on the hottest days.

[R J Cedar]

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

Existing duct system is for the cuirrent 20KW electric furnace. For size, do you mean cfm? Not sure. Is that on the blower label plate usually?

[R J Cedar]

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

I read that a rule of thumb for rigid trunk ducts is 900fpm/supply side and 700fpm/return side. So I guess you are saying - over 800fpm, meaning in the trunks and that is ok v for the trunks? Here is my calculation:
7x 14.5" = 105.5sq in x 2 = 203 sq in/144sq in/sq ft = 1.41sq ft area
1200cfm/1.41sq ft = 851 fpm. yes, just over 800 fpm on trunk velocity. And that is under 900, so ok, right?

What is return side and length....now whole return horizontal measures 14 x 14" and is 97" long, with a drop in the center of 3 feet, and drop is 12" x 18.5" BUT in going to the new supply/return system, a 3rd cold air return input is planned but not yet designed. I guess your point is that the return air velocity has to be within the rule of thumb for velocity?

So if trunk velocity is ok, and return velocity is ok, then its ok to ducting right off the plenum?

[R J Cedar]

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

I have 2 trunks which I call north and south. south can take 490 cfm; north can take 460 cfm, then I have to get AHU set to 1000cfm I think, instead of 1200 cfm as I cant put the 200 cfm more throught the trunks. So that is why I asked about putting out the extra 200 from the plenum, so I can run the AHU at 1200, design speed.

So if it is ok to run 200 off the plenum, I would like to know theoretically if that is ok. If it is, I would change the duct plan right away, upsizing off the north and trunks duct runs - some of the longer runs to go to 6" from 5" on longer runs to get my trunk cfm close up to 500 each.

Then the extra 200 off the plenum, that I think would be optimum...just trying to check here if that generally sounds ok.

[R J Cedar]

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.

Is this the correct way to explain my context....I am just looking at heating, not cooling. When I said 1200 cfm, that was for heating mode. 1200 cfm then and I am asking for that taking 200 off the plenum is ok so as to keep the 2 trunks cfm to 500 plus 500 max rated capacity.

[R J Cedar]

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

op is on the canadian west coast
I dont know what sensible heat changes.
What I have as givens is 38000 btu system
rule of thumb 12000 btu/ton, so I have a 3 ton system.
The unit is rated at 1200cfm.
However I have been told that the system runs at 350cfm/ton so x3 = 1050 CFM. Why would this be in practice, if the manufacturer rates at 1200cfm? Does your sensible heat factor factor in for this and for this location, for heating mode?

Would lower airflow be that bad? Well, does that not cause an efficiency impact....lower the system BTU output and raise my balance point, thus causing auxillary electric heat to engage more often?

I was after a quiet sytem, and was told that 5" ducts would have higher velocity than 6" ducts, and heat pumps need to move volume and my 5" ductwork would not handle what was needed, so I am trying to come to some reasonable duct plan for resizing up.

[hvacrmedic]

Is this the correct way to explain my context....I am just looking at heating, not cooling. When I said 1200 cfm, that was for heating mode. 1200 cfm then and I am asking for that taking 200 off the plenum is ok so as to keep the 2 trunks cfm to 500 plus 500 max rated capacity.

You stated that you were installing a 3 ton heat pump. Are you never going to run it in cooling mode? In any case, the cfm requirement may still be 1200 in heating mode. Can you get by with 1000 cfm? Sure. It won't be as efficient, but it'll most likely work.

Keep in mind that it isn't a given that you'll actually get 1000 cfm out of the system. The cfm that you get will depend upon the fan curve of the unit vs. the pressure drop of your duct system. When we say that a duct size is good for XXX cfm, that just means that you can push that much air through it without generating bothersome noise. It means nothing more and nothing less than that.

That being said, your velocity will still be too high even at 1000 cfm, so I'd say yes, you need to enlarge your return. Another tap would be a good idea.

Have you mentioned anything about the supply? What are you planning there?

[R J Cedar]

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).

Not a matter of trust, more a matter of me trying to understand 3 different proposals with 3 quite different approaches on duct sizes.

Larger trunks and ducts......that is what I gather....keeping the existing trunks seems to be reasonable up to what I am told is their cfm limit, then going to larger 6" ducts also seams reasonable.

branch takeoff design (and plenum to trunk takeoff) design is completely up to the contractor....I have no idea of what they are planing, but I take your point on keeping sp low, through good design.

Return duct...details....up to contractor. I am looking to see if they have at least the sq ft area of the total sum of all supply ducts out. Hopefully a bit more. And will check velocities.

5" filters more resistance?...the IAQ thread on filters has some comparissoins on 1" and 5". Many say that going thicker gives more surface area so lower resistance.... so going thick and more area, should help? OK, velocity target 300 fpm.

[R J Cedar]

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.

This reply was referring to

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

Not sure how to relate this to my heating context, perhaps it is totally not applicable to heating? But for heating, I am looking at contractor who will set the AHU to 10000 vs the rated 12000cfm because of restrictive ductwork and noise concerns, so in that case....lower airflow in the heating sense, I understand the impact is that lower airflow changes my 3 ton capable system to a 2.5 ton capable system, raising the balnace point. If I am off on this concept please advise?
thanks.

[hvacrmedic]

This reply was referring to

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

Not sure how to relate this to my heating context, perhaps it is totally not applicable to heating? But for heating, I am looking at contractor who will set the AHU to 10000 vs the rated 12000cfm because of restrictive ductwork and noise concerns, so in that case....lower airflow in the heating sense, I understand the impact is that lower airflow changes my 3 ton capable system to a 2.5 ton capable system, raising the balnace point. If I am off on this concept please advise?
thanks.

In heating mode you'd only lose about 1000 btu/hr. Less than that at lower ambient temps.

[genduct]

We haven't.
The main indicator of the future is how id the system delivering NOW!

The duct is a little more complicated than using the ductulator to predict whether or not you will get enough airflow. Perhaps you will get the preferred airflow but duct leakage will put it places you don't see or need.

Maybe if the duct runs are short and the duct is somewhat sealed then you will see some higher velocity. The thing is a good deal of the noise you are concerned about is not air noise but the insertion noise that the fan motor produces. Only duct liner or duct board will absorb these sounds and if your duct is not lined or board, you have to live with what you have.

So my original thought still stands.
Understand how the system is working NOW If the master BDM is not getting enough air now then a new unit will probably not fix that know problem
Why not install the new unit with a firm price for an EXTRA$ if you do need your ducts modified which could be enlarging one or more branch/ runouts from the trunk or from the plenum.
Unfortunately Airflow and duct design are not as well understood by the average HVAC contractor as we would like because the Training has traditionally focused on the BOX / the unit for troubleshooting and repair and not for proper design and installation.
The results of the NATE testing show this to be true.

[Freeagent]

In heating mode you'd only lose about 1000 btu/hr. Less than that at lower ambient temps.

I am putting in same system, so this comment interests me.

Are you saying that we could run our heat pump (that is designed to put out 38,000 btu/hr at 1200cfm) at 1000cfm and have the heat output only drop by 1000btu/hr?

If inlet/outlet temperatures to/from the air handler stayed the same, then our heat output would be (1000/1200)x38000 = 31,667 btu/hr or a drop of 6333 btu/hr.

Looking at it another way, if design temperatures in/out were, for example, 70/100F for 30F delta, then delta would have to be 35 and outlet temperature 105F at 1000cfm in order to achieve 37,000 btu/hr

Would outdoor unit be able to provide a higher temperature to the coil in order to achieve the higher exit temperature?

BTW, RCedar and I are both using the Mitsubishi Zuba 3 ton unit designed for operation down to -30C.