Cold air return question

Off of the plenum of my wood/coal furnace If I have a service duct feeding into my NG duct work that is 14*8 for the first 8 feet and then transitioned up to 16*8 for the next 8 feet, what size return should I use? Can I use a 12*8 coming off of my main cold air return that is 16 or 18*8 and not have issues such as whistling or reduced air flow?

 

I have never heard of a trunkline that increases in size as it gets farther from the source. (14X8 into 16X8)

A 12X8 trunkline will handle 500 cfm comfortably. If the blower motor is rated higher than that, there is the possibility that noise (whistling, etc) will increase as soon as the velocity increases.

I hope there is the capability of isolating the NG furnace when the wood furnace is not in use??

 

yooperdave , I have 2 dampers isolating my NG furnace from from the add on. It is done correctly.

 

Typically for residential main air ducts the velocity should not exceed 900 fpm with 700 fpm being the low end. The maximum velocity should NEVER exceed 1200 fpm. Exceeding that speed, you will get whistling and/or oil canning of the duct when the blower starts and stops. If you are lucky enough to have a duct calculator AND you know the cfm of the blower, you'd align say...800 FPM (middle of the recommended speeds) with the cfm of your blower, say it IS in fact only 500 cfm you'd then read the duct size required on the duct dimension scale. Those figures allow a duct size of exactly 8 x 13.

Change the 800 FPM in the calculation to 900 FPM with the same 500 CFM and the duct size required is 8 x 11 1/4.

Change the 800 FPM in the calculation to the low end of the scale at 700 with the same 500 CFM and the duct size jumps to 8 x 14 1/2.

If you can find the CFM of the blower, let us know, and we can let the calculator do its majic for that actual number so there'd be no guesswork.

When sizing return air ducts, it is a good idea to size for 20% more air flow, which increases the size of the duct. This will ensure there is no air flow restriction across the heat exchanger, causing higher than recommended temperature rise and will lower the total system static pressure. High static is a sign of undersized duct as residential systems are typially designed for .5" water column total static at the above recommended velocities.

Hope this helped just a little bit.

 

I have a duct calculator in front of me and it lists recommended residential setting of ".1" not .5...what am I doing wrong?

 

Sounds like you work in the industry. That is spot on as far as what will happen.

 

If I allign 800 fpm (velocity) with 800 cfm (blower motor), my duct size is 20X8...Way too much!

Typically, I use .1 friction and allign required cfm to achieve duct size.
7 runs are placed on either side of the furnace. What size truckline?

14X8 would be the answer if I use .1.
If I use .5, it would be 6X8?

 

You aren't doing anything wrong, yooperdave, but I think what we are experiencing here is most probably a mistake in full explanation on my part as I usually skip the friction/100' scale and go start out on the velocity scale as they work out close enough to the same answer. I pulled the .5 outta my you-know-where not necessarily as a design criteria in mind, but as a never, never to exceed but INCORRECTLY referred to it as the actual design pressure as an afterthought at the end of the post. I think somewhere deep within the wrinkles of memory, I remember something in the fan curves that begin limiting a squirrel cages ability to move a predetermined amount of air at .5"...???? Of course that is just what I'm thinking I remember. Fan curves are even further in the deep recesses and a beast rarely tackled.

However increasing the velocity TOWARDS the .5 maximum nets extreme airflow noise quickly. If you align the velocity at 800 (middle of the recommended branch duct requirement) with the 500 cfm (what we actually started out with) you'll see it works out to 8 x 13 with very near to your correctly suggested design criteria of .1 friction....actually a shade bit less. So airflow noise should be verrrry low to non-existant.

So, long story short yooperdave, you are correct. The design criteria is .1, its just a step I bypass by starting with the known acceptable velocities for either residences, commercial spaces or industrial applications. I should have left the .5 (max) outta the response because admittedly it was not only wrong but confusing.

I'm busted and my hands are against the wall...... That's what I get for being quick on the trigger and not comprehensively previewing the post before posting....but in my defense, I was watching Blacklist at the same time.

rmarlatt4, I HAVE been known to grip a snip a time or two in the past....retired now, as you can tell by the rust in the toolbox and in the memorybox...... Oh, they still get a little bit of use on occasion though. I really wasn't a tinner, just a commercial/industrial HVAC service tech with a smattering of residential tossed into the mix back in the 70's and early 80's.

My apologies for the confusion guys...I'll try to do better from now on.

 

Congrats on your retirement! I'll be along shortly!
Very true on the fan curve charts. We touched on them in schooling, but moved right along. Kinds you'll never need/use them approach.
I have been mostly residential although there is some commercial experience also.

First commercial experience I had was reading the prints, adding up all the supplies (12" down to 6" branches) and pointing out that there is no way the size duct on the print will carry that cfm. Boss and Lead man got a smile out of it and said you have to leave your residential thoughts behind now....commercial duct sizing is a whole new ball game and the velocity is increased accordingly!! I swear some of those systems sounded like you were moving sand through the ductwork!

No need for apologies either, thanks for the reply and explanation!

 

Geeze. That's a lot of info! I do have ductulator hanging around somewhere, but why use it when I have you guys. I'll get the exact dimensions of the duct and report back.

 

Sorry about that.
If you think there may be a chance of air noise, enlarge the size from 12X8 to 14 or 16. I re-read your first post and it seems that you will be coming off an 18X8 R/A trunkline? Kinda a delicate line to toe, but if you take too much air off your R/A. when the NG furnace runs, it could cheat by just pulling R/A from the wood stove instead of the rooms upstairs. Probably another damper in order for the added trunkline?

A rule of thumb for guidance.
12X8 around 500 cfm
14X8 around 600 cfm
16X8 around 700 cfm

It may be easier to use round pipe? Instead of rectangular duct?

8" diameter around 250 cfm
9" diameter (not too common) around 300 cfm
10" diameter around 400 cfm
12" diameter around 650 cfm.

 

Holy cow a lot of great information here!

Maybe you can help me, help my mother. She is having a furnace replaced. The contractor is installing a Lennox El296V HU090XV36C, which from looking on the internet means it's a 3600cfm unit. I was wondering if that is too much flow? BTU wise it's similar to what's there, but wasn't sure about the cfm. Is there a way you can tell me if this is going to be sufficient for her.

The room sizes from the beginning to the end of the supply run are.
Room 1 = 110sf
Room 2= 150sf
Room 3= 156sf
Room 4= 140sf
Room 5= 272sf
Room 6= 112sf
Room 7= 112sf
Room 8= 64sf
Room 9= 500sf

I know the supply duct starts as a 24x12 but I haven't looked what the sized are after that.

 

Just looking at the model number of what you posted, I'm thinking the cfm of that furnace is not 3600. Rather, 1200 cfm.
It would serve the square footage of the room sizes listed well.

Let me see if I can find any more info on it.

 

Odd, thought I saw 3600cfm listed...36 doesn't stand for the cfm either I guess. Do you have a link for that by chance?

Yes 1200 seems a whole lot more reasonable indeed.

Thanks!

 

I have not found anything to verify my hunch yet, but I think that the 36c stands for cooling capacity.
36000 btu's of cooling=3 ton.
1 ton of cooling = 12000btu.
1 ton of cooling requires 400 cfm (rule of thumb; anywhere from 350 to 400 cfm per ton)

 

That makes sense...it's teamed up with a 3 ton air conditioner.

 

Just got off the phone with an old boss.
He is a Lennox dealer and said that is exactly correct. 1200cfm.

 

Wow...thanks for checking for me!

 

You are correct sir. Though they (model #'s) CAN be tricky sometimes...