Shortening main duct run

[bucks_2002]

Question: My main duct running from my HVAC unit runs right up to the bottom of my stairs. I want to shorten it so people don't have to duck when walking down the stairs to my finished basement.

Ok, I took the end piece off, no problem. Yet, looks like I'll have to do some bends to get it back on once I shorten the rectangular piece. Is there a special tool to do bend the end piece so I can put the cap back on?

Thanks

[coolwhip]

Use your teeth.

[docholiday]

yeah, there is. Its a folding tool and tongs. Together they cost as much as just calling a local contractor to do it.

You might want to know, there is a reason the duct extends past the last run. Messing with that may just mess up the duct static.

[bucks_2002]

Shortening it 5 inches will cause a 'static' problem?? If a non professional can't do this, then there is nothing they can do.

The extra 5 inches will be a world of difference for the stairs.

[dash]

Originally posted by bucks_2002
Shortening it 5 inches will cause a 'static' problem?? If a non professional can't do this, then there is nothing they can do.

The extra 5 inches will be a world of difference for the stairs.

How many inches from the current end of the duct back to the last branch run??

[bucks_2002]

About 8 inches to the next run.

[dash]

Originally posted by bucks_2002
About 8 inches to the next run.

Next run or last run?


Is this correct?From the last run/branch ,to the end of the trunk, was 8" and you want to reduce it to 3".

[Edited by dash on 04-04-2005 at 02:40 PM]

[bucks_2002]

Last run. New house. Furnace is 2 years old.

[bucks_2002]

Yes, I want to reduce the main duct by 5" and leave a couple of inches from the last run.

Yea, na

[dash]

Many here will disagree,but it may not be a problem.

It's common practice,to have the trunk extend well beyond the last branch,and many believe this is needed to turn the air ,reduce static(resistance to air flow),prevent too much air flow in the last branch,I've heard all these opinions.

It was the way I was taught in the field early on,but I've never heard it in a classroom.

I find nothing to support this in Manual D,The Authority residential duct design.In fact drawing in Man. D often show the last branch very close to the end of the trunk.

So at the least be aware,their is a possibilty that the current air flow to the last branch may be effected.You could remove the end cap,and "plug" the duct,at the point you plan to cut it,and test the air flow(feel) for yourself.

[bucks_2002]

Ok thanks. Good suggestion on the test. If it is a problem, my father in law will just have to bend down when enetering the man room.

Where can I get the tool to put the cap back on? The tool to bend the ends so the cap will slip back on. The dudes at Home Depot are clueless.

[docholiday]

Its not that it will affect the last run, it will affect total system static. The last run in this case may actually see an increase in airflow but the total system static may drop as a result which may affect airflow to other runs. There is no promise that it will affect it like adding a run out of the end of the duct would but as Dash said, try it by blocking it back for a few days. I just thought I would point it out.

And BTW, a professional would be able to move the last run if he needed to.

I said it May.... Dash said it May not... No telling without looking at it. (May is not diffinative)

[alexb]

Here's a PIC of the end of one of my duct runs.

I know there's do DIY here, but I'm just stating what I've done. I've cut the end off this run, and I'm putting in a 45-degree bend from the main line, to the line dissapearing off the top of the picture. The small 5" round going to the left will be cut off & capped; it goes to a tiny bathroom.

I'm counting on this providing MUCH more airflow to the 15' of duct that connects to the line that dissapears off the top of the pic. I suffer from v. poor airflow in this room; not meeting CFM requirements..
-Alex

[docholiday]

Why not just use turning vanes?

[bucks_2002]

Nice pink dress.

Mine is not that complicated. Very simple, brand new house. Airflow is more than enough in entire house. I'll block it off for a couple of day before cutting just to be sure.

Thanks everyone for the advice.

[dash]

Originally posted by alexb
Here's a PIC of the end of one of my duct runs.

I know there's do DIY here, but I'm just stating what I've done. I've cut the end off this run, and I'm putting in a 45-degree bend from the main line, to the line dissapearing off the top of the picture. The small 5" round going to the left will be cut off & capped; it goes to a tiny bathroom.

I'm counting on this providing MUCH more airflow to the 15' of duct that connects to the line that dissapears off the top of the pic. I suffer from v. poor airflow in this room; not meeting CFM requirements..
-Alex

The duct that you refer to will get more air with your changes,but I wouldn't think it would be a lot more ,if that's the end of the only trunk line.

Reason is,it's likely that is the longest run, in Total Equivalent Length(TEL),from the indoor fan.So the changes likely need to be made closer to the indoor fan,to increase the Static available at the end of the trunk.


The "takeoff" fiiting is used there is not a great loss,likely a 45° "bend" may be worse.An unknown is what exactly your 45° turn will look like,and the effect of making the branch duct,more of a trunk duct,ie taking a branch out the end of the trunk,will increase air flow.

You will gain all or most all of what went to the bath.

Turning vanes for the turn would be an improvement,but you need to looh at adding vanes between there and the indoor fan,or some other method to reduce the PD and TEL of the duct ,to get much more air at the "end' of the trunk.

[dash]

Turning vanes ,if needed is a good idea.

Doc,I'd say any increase in ESP,would have an effect on the branch in question,as it's likely the longest path,in TEL,in the home.