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Thurgee 04-02-2007 02:56 PM

Modifying existing duct work
Hello all - I've managed to mangle a drawing together of my existing duct work along with approximate lengths of each run from the plenum to it's respective outlet. The redzone indicates the area that I will be framing and finishing. There are two long duct runs that are in the way currently. All runs are 7" except for the one 10" that is split out way down the line to service to 7" outlets. The two rear duct runs (bottom) go to the garage, which is uninsulated currently. I'm planning to remove these runs from service and use at least one for the red area. Another thing that bothers me about this setup is they have the return about 1' from the blower which makes for blower noise as well as noise from turbulence. There is a couple of feet that I could move this unit backwards if that would help alleviate some noise. The return is a high placement, which in the living room which really aggrevates me when you are trying to watch television.

There are 4 ducts run in between the floor joists for the 2nd floor. The remaining ducts are open and lay on top the insulation.

It really would be convienent that instead of homerunning all those ducts to the plenum if there was a way to craft two main supply trunks, one that would service the 4 joist ducts and another to service the other northbound runs.

One thing to note is that the one 10" duct that runs in the floor joists actually comes back out in open area where the Y-duct is. So it would be possible to not have to use the Joist for that run to begin with.

The A/C work that I'm looking at re-doing was pretty shoddy. The coil/plenum isn't completely sealed and there are numerous other leaks going on around the edges. The duct work seems excessively long to me as well as a ton of static pressure from the bends and kinks.

I'm not too sure if this A/C shouldn't have been located in a better place? Perhaps when they built the house (I'm 2nd owner) it was located there for convience.

Any ideas?

KUIPORNG 04-02-2007 03:08 PM

I just want to comment on your drawing... it is so great... although a bit scarely like those monstor robot which has a bunch of arms to kill people... and the red zone is like someone trying to aim at shooting off two of his arms using some night version device....

Hogan's Hero 04-03-2007 09:34 PM

Why Don't you just put in a smaller furnace for the second floor. You can put them right next to each other same gas line, electricial. Lower Btu can be use for simple fact you will loose about 25% of heat do to it rising up stairs.

pjpjpjpj 04-05-2007 09:35 AM

Without seeing the actual system and knowing what kinds of ducts you have (hard metal or flexible, etc.), it is hard to say exactly what you should do. Static pressure drop and airflow (in cubic feet per minute, or CFM) is all related to this. You would need to do a heat loss calculation on your home as well (including wall insulation info, windows, doors, direction the rooms face, where you live, etc., etc.) to determine how much air you need in a room.... or, you could "dumb it down" and just approximate how many CFM per square foot you need in certain rooms (depending on room usage and wall direction, whether you had big windows, etc.). Residential usage is typically anywhere from 0.4 to 1.0 CFM/sqft.

All that being said, from your sketch (well done, BTW), some basic ideas:
-Make sure that if you change around your ducting, you keep about the same cross-sectional area within the ducts... i.e., if you want to combine two 7" round ducts to be one main duct, those are each about 38.5 sq. in. of area, so two would be about that would come out to be about 77 sq. in., which is approx. a 10" round, or, say, a 10x8 rectangular).
-Make sure, if you have flexible duct, to straighten out any kinks and make any turns as long and smooth as possible.
-Obviously, if you can seal any leaks, that is good too.

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