I wish someone would find an authorative reference to this as well.

It's been a topic here many times,all agree ,not out the end cap,but how far away it all over the place.

If you look in Manual D,you'll see duct layouts,they are not to scale,but the last branch appears close to the end cap.I've not found it referenced in writing in Man. D .

 

What is the issue of a take off coming off the end cap? Does it matter if there is a damper on the take off?

 

If i want a LOT of air in a run, because of lenght or a restriction I can't correct, I take it off the end cap and install a takeoff in the end cap.. Then I use a flow hood and adjust the flow to what I want.

I read one time the trunk should go 18" past the last takeoff.

 

look at TEL --
for wye & tee --

I jerked my tees out & put in wye in my attic!

the whole idea is to help, not hinder air flow?

 

From the sound of it some of these 'members' sound as though they have actually done some of this type work. If so, you should be registered on the pro site. Then a more open discussion can take place.

 

I don't got any guide as to this. But I'll tell you how I do this, beacuse it's how I was told to do as a newb.
Preferred is to stay back from endcap 16". 12" is acceptable if you have to but no closer.
Never take it off the endcap. It destroys duct pressure and airflow to other registers. Almost like a deadhead.
Never take them off the bottom of the trunk. Unless maybe if you do them all like that.
I always cringe when I see the run outs taken off the top of the plenum. I just don't like what it makes me think the air is doing. Now on a horriblezontal install, one of the sides becomes the top, so then it's okay.
Now this ain't the word of Gospel by any means, just what I was told by my far elders at the outset.
So I just noticed this thread and just thought I'd throw in my two cents.
And I hope it's not too much info for the Res. forum

And I hope someone will correct me if I'm wrong on any of these points...so I can be better educated about it.


"Air ain't stupid. And it won't just go anywhere" [size=-3]-Wise old Furnaceguy[/SIZE]

 

I won't even dignify the abomination known as triangle boxes tied together with flex.

Which, strangely enough, I was told was actually a Maunal D approved duct system

But, My God! It gets abused and hacked to death.

 

Pro membership-

I've applied twice - emailed copies of CFC certs, etc - that was weeks ago. Never heard back. Figured just because someone's name says "pro" member doesn't make them anymore knowledgable than some other members.

 

I've applied twice - emailed copies of CFC certs, etc - that was weeks ago. Never heard back. Figured just because someone's name says "pro" member doesn't make them anymore knowledgable than some other members.

You must be patient.

The members of the membership Committee are volunteers, they have jobs or a business that is their first priority and it may take some time.

They will get to you.

 

That all sounds great and all, but last time I checked, runs go on the outside wall of a house, and how exactly are you to extend the ductwork 18 inches past the last joist channel?

 

wow -

wow - didn't realize that static pressure "prefers" takeoffs in some locations better than others. Interesting - even for us "residential" guys -

That abomination of triangles and flex - depends on the "hack" installing it. Installed by a knowledgable sheet metal worker, its plenty capable of working fine.

 

Many moons ago I was taught that your last takeoff should be no closer than 12" from the end of the trunk line, because of back pressure. In simple terms, If no back pressure you will have improper airflow thruout the system and you'll have too much air flow on that run thats been taken off of the endcap. At least thats what I was taught when I was an installer, and thats what I taught rookies, and never had problems. And the systems I've encountered that have runs taken off of the endcap the airflow was incorrect.

 

panhead, I can appreciate the fact that you have made your application. I do however, respectfully disagree with you regarding the credentials associated with the 'professional areas' on this site. You can at least take comfort in knowing those in that area have passed an initial level of scrutiny and in addition, there are subjects that are discussed in the pro forums that really should not be spoken of in a highly visible and accessible forum such as residential.

We are all here to help any HO's we possible can, in the most beneficial way possible. If that is telling them which wire to put where, well, no that is not what this site is for, i.e. no DIY's. However, I hope you can appreciate the fact that there are a lot of folks that are simply looking over the site, and perhaps never even post an inquiry. However, they pick up some little snippit of information here during a conversation such as this one, go home and 'attempt it themselves' and then inadvertently screw up their system.

Then the next thing you know, they are back here lambasting the "pros" that they don't know what they're talking about. Unfortunately, they are basing that opinion on information garnered from the resi site and NOT the pro site.

.

 

HVAC Student

What classifies a member as a professional member? I just thought I would ask maybe I do. As far as the take off from the end cap in my manual d class they explained how air flows and that you would not want to have a take off any closer than 12" from a 90 as there would not be any airflow until you get about 12" from it. Wouldn't that same concept apply to an endcap as the air hits the end of the duct if you get to close there might be dead spots with no flow? Just a thought.

 

What classifies a member as a professional member? I just thought I would ask maybe I do. As far as the take off from the end cap in my manual d class they explained how air flows and that you would not want to have a take off any closer than 12" from a 90 as there would not be any airflow until you get about 12" from it. Wouldn't that same concept apply to an endcap as the air hits the end of the duct if you get to close there might be dead spots with no flow? Just a thought.

As a Pro member you have access to forums that you don't currently have access to.

 

HVAC Student

I know about the other forums. I was talking about credentials what would I need to have or know. I am HVAC certified have my EPA universal card but I have only been working in the field for a few months.

 

I know about the other forums. I was talking about credentials what would I need to have or know. I am HVAC certified have my EPA universal card but I have only been working in the field for a few months.

Read this

 

HVAC Student

So the membership commitee will determine if I am eligiable. Thanks for the information.

 

kroberts: Their site........ their rules! Pretty simple really.

 

Eh,

Originally posted by panhead98
wow - didn't realize that static pressure "prefers" takeoffs in some locations better than others. Interesting - even for us "residential" guys -

That abomination of triangles and flex - depends on the "hack" installing it. Installed by a knowledgable sheet metal worker, its plenty capable of working fine.

I see this thread has been inactive for several hours but I just wanted to shoot back a response on this.

As a sidenote, which doesn't have much to do with anything, my grandfather started in this trade in the mid 1930's. He was followed by my father, two uncles, and three cousins. These are the 'elders' of whom I spoke.

How it was told to me is to think of the plenum as a 'mixing chamber' and that turbulence is desirable but too much restriction is not. Without the turbulence (i.e. some amount of desired restriction--the air bounces around) you have flow but no pressure, since air is one of those which takes the path of least resistance.
So I've kind of forgotten quite what I was getting at with all that but what I'm trying to say is that you want to stay back from the endcap so far so that the air will 'bounce' off the endcap and maintain pressure. Also the top of the trunk is the most highly pressurized region, the bottom is the least pressurized and the sides are in between. So in this sense, there are preferred locations for takeoffs as to pressure.

As for the triangular boxes and flex--I agree with you that it is capable of doing the job, sure. Perhaps 'abomination' was a poor choice of word on my part, but I called it such because the examples I have seen of this type of duct system were thrown in and looked like garbage. Not to mention where someone had ran an 8" flex to one of these triangles, using it to supply four 6" flex run-outs, then on to another triangle, and then they wonder, "Hey...where's the air at?".....that sort of thing.
But in the hands of someone who knows what they're doing, I'm sure that type of duct system works fine.

 

just to be clear-

I see this thread has been inactive for several hours but I just wanted to shoot back a response on this.

As a sidenote, which doesn't have much to do with anything, my grandfather started in this trade in the mid 1930's. He was followed by my father, two uncles, and three cousins. These are the 'elders' of whom I spoke.

How it was told to me is to think of the plenum as a 'mixing chamber' and that turbulence is desirable but too much restriction is not. Without the turbulence (i.e. some amount of desired restriction--the air bounces around) you have flow but no pressure, since air is one of those which takes the path of least resistance.
So I've kind of forgotten quite what I was getting at with all that but what I'm trying to say is that you want to stay back from the endcap so far so that the air will 'bounce' off the endcap and maintain pressure. Also the top of the trunk is the most highly pressurized region, the bottom is the least pressurized and the sides are in between. So in this sense, there are preferred locations for takeoffs as to pressure.

As for the triangular boxes and flex--I agree with you that it is capable of doing the job, sure. Perhaps 'abomination' was a poor choice of word on my part, but I called it such because the examples I have seen of this type of duct system were thrown in and looked like garbage. Not to mention where someone had ran an 8" flex to one of these triangles, using it to supply four 6" flex run-outs, then on to another triangle, and then they wonder, "Hey...where's the air at?".....that sort of thing.
But in the hands of someone who knows what they're doing, I'm sure that type of duct system works fine.

Just to be clear - you're saying that in a trunkline, at any given place, in a residential application, the static pressure of the air is NOT equal - its got the most pressure on the top surface of the duct, least on the bottom, and the sides are somewhere in between? So a takeoff taken from the top of the duct will provide better airflow than one taken from the sides or bottom?

 

Noone has explained how you are to get runs in the end channel and still have 12" of duct past the last takeoff.

 

yelram

yelram - have you done much residential work? Just curious - I see that you are a pro member.
As you can see, no one has been able to quote any printed authority on this subject, but its been well explained by a couple of members. Seems almost everyone is working from a rough rule of thumb that you should allow somewhere between 6" and 18" inches of "bump" at the end of a trunkline. It creates backpressure. You always want to avoid coming directly off of an end cap - sure, airflow can be controlled with the damper, but noise/whistling will become an issue.
Its pretty much something most people learn early in their careers, from their more experienced co-workers (apprentices and journeymen, etc) in the residential market.
As for running heat runs up outside walls, in the end joist space - again, that would only be used as a last resort. Why? Can't get a full batt of insulation behind it, usually gets "tweaked" by the electrician running conduit thru stud spaces, or its on the side of the house that the plumber runs his gas pipe in - making connections even harder. Unfortunately, even if you run a heat in that end joist space, you should never have the takeoff for that run in the last bay without at least 6" inches (some say more) of bump. You would need to come off the side of the duct with a takeoff, etc. to solve this problem. That's only my opinion, though.

 

yelram - have you done much residential work? Just curious - I see that you are a pro member.
As you can see, no one has been able to quote any printed authority on this subject, but its been well explained by a couple of members. Seems almost everyone is working from a rough rule of thumb that you should allow somewhere between 6" and 18" inches of "bump" at the end of a trunkline. It creates backpressure. You always want to avoid coming directly off of an end cap - sure, airflow can be controlled with the damper, but noise/whistling will become an issue.
Its pretty much something most people learn early in their careers, from their more experienced co-workers (apprentices and journeymen, etc) in the residential market.
As for running heat runs up outside walls, in the end joist space - again, that would only be used as a last resort. Why? Can't get a full batt of insulation behind it, usually gets "tweaked" by the electrician running conduit thru stud spaces, or its on the side of the house that the plumber runs his gas pipe in - making connections even harder. Unfortunately, even if you run a heat in that end joist space, you should never have the takeoff for that run in the last bay without at least 6" inches (some say more) of bump. You would need to come off the side of the duct with a takeoff, etc. to solve this problem. That's only my opinion, though.

I ALWAYS put them in the end channel because thats where the windows and outside walls are. I'm 24, but i've been doing this since I was 13. I dont put them on the endcap, because that is obvious, but the back pressure is maintained by reduction. If anything adding extra duct at the end causes excess turbulence. I have never had an issue with what you are talking about NEVER. and neither has my father who has been doing this since HE was 13, and he's now 64. My point is, its more important to have a run where its needed, than to follow some stupid rule of thumb. If theres an end channel I can get into, I use it. Or lets say that the duct lines up so the last piece ends in the last channel with little to spare, you would REALLY add an extra piece? Because thats just stupid. Some of you guys must live on a different planet or something. Your concerns and priorities seem out of whack.

 

I don't think any of us are saying it won't work. I bet when your Dad got his start, the old beer can cold method was the charge method of choice. Did it work? Yeah, sorta. Just because something has been done like that for years doesn't make it the best way to do it.

Now, I can't argue at all, if there's a window (or other need) for a run to be in that last bay, it'd better have a register there. But... they have this new invention out called an elbow. With one of these magical devices you can indeed, stay back 12" from the end of the duct and still get to the end of the house. Heck you could even go MUCH further back than that.

When I run pipe I always try to do this, but I'll admit it doesn't always happen either.

I think with residential air flow is much more forgiving. Sure you still need the proper air flow to make everything comfortable.... but there's much less air, and a drastically different load variables in resi than commercial.

Going back to my commercial days, I don't think I ever put anything closer than 2' away from the end cap (unless it was return). In fact, I think most of the larger jobs I did there was usually 4' of duct designed past the last take off (at least on the big supply trunk lines).

 

I'm going to end the duct where the last run is without having to cut a piece, every time. Doing it another way would be silly. If it goes 12" past before it ends, thats fine, If Its really close, there is just no way I'm going to add an extra piece of duct just so it has extra "bump" at the end. If I have to get it in the last channel I usually take one of the top really close to the end, and one off the side a little further back.

 

Originally posted by panhead98
Just to be clear - you're saying that in a trunkline, at any given place, in a residential application, the static pressure of the air is NOT equal - its got the most pressure on the top surface of the duct, least on the bottom, and the sides are somewhere in between? So a takeoff taken from the top of the duct will provide better airflow than one taken from the sides or bottom?

This is what I'm getting at:

I've seen a very similar diagram somewhere, but I can't remember where.
I don't know that coming off the top will give you better airflow per se, but I have experienced that coming off the bottom, your airflow will usually suck. Unless maybe if you put a scoop in it.
I think how good a airflow you get is dependent on several factors but where you come off at is one. I think it is that the airflow as you get higher up the trunk cross section has more...turbulence, ooomph!, velocity? That's it, I think coming from a higher pressure zone will yield higher velocity. I'm not sure that it's static pressure so much as duct pressure (maybe they're one in the same?) I really didn't get much chance to talk to the old man much about the mechanics of it, other than what I've already posted, and then seeing this diagram. And I seem to recall also seeing similar diagrams for round and oval pipe.

 

This is what I'm getting at:

I've seen a very similar diagram somewhere, but I can't remember where.
I don't know that coming off the top will give you better airflow per se, but I have experienced that coming off the bottom, your airflow will usually suck. Unless maybe if you put a scoop in it.
I think how good a airflow you get is dependent on several factors but where you come off at is one. I think it is that the airflow as you get higher up the trunk cross section has more...turbulence, ooomph!, velocity? That's it, I think coming from a higher pressure zone will yield higher velocity. I really didn't get much chance to talk to the old man much about the mechanics of it, other than what I've already posted, and then seeing this diagram. And I seem to recall also seeing similar diagrams for round and oval pipe.

I've not seen that,and don't see it in Manual D,though they show takeoffs from other then the side.

 

yelram

I don't think Amick is suggesting to add an extra piece. Just to come back with your take-off a little farther. Maybe something like this:

Now I'm not trying to criticize your style.
I'm just...blowing hot air I guess.
Plus I kinda like drawing these goofy little diagrams.

 

I don't think Amick is suggesting to add an extra piece. Just to come back with your take-off a little farther. Maybe something like this:

Now I'm not trying to criticize your style.
I'm just...blowing hot air I guess.
Plus I kinda like drawing these goofy little diagrams.

So add two elbows? That seems like a waste. Especially because I still dont believe setting it back changes things that much.

 

Originally posted by dash
I've not seen that,and don't see it in Manual D,though they show takeoffs from other then the side.

Hello dash, I've been waiting for you to chime in on this, O great ductmaster.
Maybe that diagram was a hallucination or something. I used to have a lot of those.

Let me go see..., maybe I can find it.

 

Nice drawing,what do you use?

However doing so would add TEL to what may be the longest run of the supply(Man D).

 

???

Hey Y'all

"This is what I'm getting at:

http://img257.imageshack.us/img257/5...titled3ge7.png"

That has got to be the ........... (deleted)......... . I recommend that you get a manometer or magnahelic gauge and a static pressure probe and take some measurements on a section of operating duct. You will find that drawing is PURE BS!

While you are at it, get a flow hood and use it on a regular basis! take a run off the end cap, and MEASURE what happens to the rest of the runs as well as overall air flow!

If you aren't measuring, you're just guessing! (From the National Comfort Institute)