Ducting 101

[jamesaon]

I have a client that recently had new furnaces installed in several rental units. Tenants are complaining about the unevenness of the heat since the installs. I checked the installations and was shocked at the the duct configurations.

On one unit there are 4 supplies and 2 returns (all equally sized). Almost all of the trunk lines have one or two six inch ducts off the ends. In the three systems that I looked at I found only one duct damper, in spite of the fact that duct lengths varied from 3' to 38' with any number of 90's on any given run.

It has always been my understanding that a system should have at least as many returns as supplies, supply ducts should never come off the end of a trunk, and that, in situations where the supply runs are widely varied in length that dampers are adviseable to properly balance the system.

I questioned the project engineer of the installation contractor and was told that the ducting configurations were fine as they are, that my old fashioned theories didn't apply to these installations because the furnaces had "emc drives" and therefore were balanced perfectly, in spite of what I thought.

My question here is: Is this guy feeding me the line of crap that I suspect that he is? I realize that the technologies are evolving, and that maybe I haven't kept up as I should have; perhaps the emc drive may compensate somewhat, but this seems to really be a stretch.

Any comments would be welcome.

Thanks,
Jim

[iwannahelp]

You'd better check the bottom of your shoes, cause what that guy was telling you surely left a pile of bullsh*t ankle deep.

[Special Ed]

Sounds like you been talking to somebody already about your problem, & it sounds like they're sending you in the right direction.

[Special Ed]

It's not a must to have as many returns as supplies although I'm sure it helps. Ideally, you should have a return in every room that has a door that is not a bathroom or kitchen. Again, this isn't a must, however.

[jamesaon]

Sounds like you been talking to somebody already about your problem, & it sounds like they're sending you in the right direction.

I haven't had any imput at all until I posted here. I used to get involved in system design, but it has been well over 15 years since I have done any HVAC work; hence my previous question.

Appreciate the comments,
Jim

[4l530]

Originally posted by iwannahelp
You'd better check the bottom of your shoes, cause what that guy was telling you surely left a pile of bullsh*t ankle deep.
ha ha :D I agree. If you think it was done wrong then it probably was. To my knowledge, the new motor technologies haven't changed the basic laws of duct design, poorly designed ductwork will annihilate the higher efficiencies the ecm motors are supposed to provide:eek:

[iwannahelp]

all the ecm motors will do is "ramp up" to compensate, but if the duct is sized wrong then there will still be no balance of temp. to house.

[jamesaon]

ha ha :D I agree. If you think it was done wrong then it probably was. To my knowledge, the new motor technologies haven't changed the basic laws of duct design, poorly designed ductwork will annihilate the higher efficiencies the ecm motors are supposed to provide:eek:

That was my sense, but I tend to avoid an argument unless I am reasonably certain I can win, and to be quite honest, this bullshoveler caught me by surprise with his line of stuff and I didn't feel comfotable challanging him without having any idea if he might be right.

Jim

[Special Ed]

I haven't had any imput at all until I posted here. I used to get involved in system design, but it has been well over 15 years since I have done any HVAC work; hence my previous question.

Appreciate the comments,
Jim

Regardless, you are right. There's a problem with the design & the ECM motor can overcome only some design flaws, not all of them. In fact, the ECM can only overcome greater than usual static pressures not magically distribute air evenly through poor air distribution designs.

[jamesaon]

Hey guys,

I really do appreciate the comments. This was a pro bono look at these systems that I did for a client that knew that I did this sort of stuff way back. Think I will get more deeply involved now if only to make the smart-a-s engineer's life a bit difficult.

Can anyone suggest some reading material that might be pertinent?

Thank you all so much,
Jim

[iwannahelp]

The yellow pages, and go to the attorney section?

[Special Ed]

Can't help ya there, bud.

But if it's any consolation it sounds as if you got a pretty good handle on things. ;)

[engineerguy]

Not all engineers know what they are talking about, although it would not be productive to say so to that particular idiot engineer. He sounds like the type that you need a baseball bat to communicate with. Here's how:

Arm yourself with OAT, room temperatures, ESP measurements, temperature rises, etc. Talk fast. Use lots of numbers. The heat is uneven, therefore the heating system is NOT WORKING PROPERLY. Ask some questions about air flows vs heat loss per room. Say it real slow. Repeat. Tell his boss.

[dash]

try here;

http://www.acca.org/consumer/design.php