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-   -   Adding new duct lines to AC (http://www.diychatroom.com/f17/adding-new-duct-lines-ac-200850/)

solislbc 05-15-2014 07:35 PM

Adding new duct lines to AC
 
Hi all,

I'm new to the forum. A lot of great information here. I've been searching but can't seem to find the answer to the question I have.

I had a contractor add an AC unit to my existing central heating system which was only a couple years old. Both the AC and the heater are 3.5 ton. The ducting on the existing system was new, so to save cost, i asked him not to change any of the duct work.

After the install was completed, I realized that 1 12x12 bedroom, 1 12x15 bedroom, the kitchen and the dining room are all splitting off one 12" feed on the box that holds the start collars. The air flow from these 4 vents is about half of the strenght as the other vents which have direct ducting from the box (sorry, don't know what this box is called that has the start collars).

My intent is to separate the 4 ducts that are currently split and install 2 new starter collars on the unit.

Is there anywhere on the box that I should not install the collars? The unit is laid horizontally in the attic. Should i stay away from the end cap of the "box"? The end currently has no ducts coming off of it. They are only installed on the sides?

Thanks ahead.

ben's plumbing 05-15-2014 07:42 PM

Quote:

Originally Posted by solislbc (Post 1351185)
Hi all,

I'm new to the forum. A lot of great information here. I've been searching but can't seem to find the answer to the question I have.

I had a contractor add an AC unit to my existing central heating system which was only a couple years old. Both the AC and the heater are 3.5 ton. The ducting on the existing system was new, so to save cost, i asked him not to change any of the duct work.

After the install was completed, I realized that 1 12x12 bedroom, 1 12x15 bedroom, the kitchen and the dining room are all splitting off one 12" feed on the box that holds the start collars. The air flow from these 4 vents is about half of the strenght as the other vents which have direct ducting from the box (sorry, don't know what this box is called that has the start collars).

My intent is to separate the 4 ducts that are currently split and install 2 new starter collars on the unit.

Is there anywhere on the box that I should not install the collars? The unit is laid horizontally in the attic. Should i stay away from the end cap of the "box"? The end currently has no ducts coming off of it. They are only installed on the sides?

Thanks ahead.

pictures worth a 1000 words....12x8 rectangular duct has a design cfm of about 400 what is the other sizes of rooms ...

solislbc 05-15-2014 07:51 PM

The kitchen is about 12x12 and dining room is about 18 x 12. The only thing I am trying to accomplish is separate the 4 lines. They are currently all tied into 1 12" outlet on the unit. My thought was to leave the dining and kitchen on the existing outlet and 2 more separate 7" starter collars for each room. The end of the box which has all of teh ducting coming out of it has all ducts coming off the side and none off the end of the box. The end of the box is probably getting the highest air pressure from the fan. Should I not install on that end cap?

HVACDave 05-16-2014 07:46 AM

Well, there are different schools of thought on that one, but I would say that the end cap in the plenum (box thing) is where the pressure point for the fan is pushing against, by cutting into the end cap you will certainly get more flow out of the register attached to it, but that really messes up the pressure to the other runs. Best practice is to tie all runs into sides of the plenum so you are working with a similar static pressure to each feed line.

What size are the runs to each of these registers now? All 7"? Is the 12" feed a round pipe or rectangular? If it is round it should be able to feed 3-7" supplies, in which case you may only need to reroute 1 run to the plenum.

solislbc 05-16-2014 09:30 AM

Thanks! The 12" round duct splits into two lines at 8 and 10. Then, those two split again to a 7 and 6. There is hardly pressure on any of these lines. I have confirmed no leaks. I was thinking of leaving the 10 and 6 line that feed the kitchen and dining room on this existing line. Then cut 2 more independent 7 feeds for a 1010 & 1110 bedroom. But runs will be about 28 to 30 ft so I will have to splice those since I can only find 25 ft duct at home depot.

So any of the sides of the plenum will do? Right now, only two opposing sides are cut and there is barely room in either side to add one more 7". Can I start fresh on a new side? The unit is horizontally elevatedn on a platform between the trusses and the bottom side of the plenum is exposed. Can I cut there? And is foil tape all that is nessesary to seal the start collar or should I use that grey mastic as well? I can't be down more than a few hours so I was thinking the mastic might take too long to cure.

HVACDave 05-16-2014 02:05 PM

Yep, you can cut into bottom without problems, most starting collars with flex use some kind of large zip tie type of clamp, then tape around that, but you may use mastic if you like. Obviously you will want to keep your runs as straight as possible with the flex to minimize friction losses. make sweeping turns if possible to keep restriction down.

solislbc 05-16-2014 04:42 PM

Thanks for your help. Zip ties or on the list.

Would it be unwise to use mastic on top of foil tape? If the foil tape fails, then that renders the mastic useless right?

Doc Holliday 05-17-2014 01:18 AM

NO NO No NO and then no again.

Duct sizing is based mostly on AC capability, not heat. Heat uses a MUCH slower blower speed to deliver air (so as not to cool of the air upon it's deliverance).

Total system tonnage allows for said tonnage's air flow measured in cfm, cubic feet per minute.

Each room requires a certain amount of cfm taken from total system allowance and that cfm is traveled to room via duct and that duct needs be able to carry said air flow so sized properly.

Yes, dedicated runs to each room are best, but for efficiency, comfort and reliability reasons you can not simply add ac to an only heating system and expect the best. In other words, heat load and then duct changes as well.

In ac mode now, your system is choking using existing ducts.

solislbc 05-18-2014 12:12 AM

I added two direct lines to the two bedrooms. My gosh what a difference. It must have been 120 degrees in my attic today. It took me all day. I'm beat. Also capped off the y that was supplying the bedrooms previously. More air flow to the dining room now!

Pretty gratifying. ..I just saved over 1000 bucks quoted to do this. I also took the time to over tape the plenum box since I felt areas where some air was escaping.

I can see why hvac contractors charge a premium. I probably lost 5 pounds today.

Thanks all!

solislbc 05-26-2014 12:04 AM

Hi
I'm back. I have another question related to my a/c. When I was adding more tape to the plenum box I noticed that some cold air was escaping the unit from where the copper line comes in. The line comes into the unit about 4 inches from the edge of the plenum box. Should any air be circulating any where else besides the plenum box?

ben's plumbing 05-26-2014 06:04 AM

Quote:

Originally Posted by solislbc (Post 1355363)
Hi
I'm back. I have another question related to my a/c. When I was adding more tape to the plenum box I noticed that some cold air was escaping the unit from where the copper line comes in. The line comes into the unit about 4 inches from the edge of the plenum box. Should any air be circulating any where else besides the plenum box?

tape and seal it shut...:yes:

solislbc 05-26-2014 09:20 AM

That's it? Why is there air circulating in the furnace. To be efficient, I would think that the only place air should be circulating is the plenum box. Is there not a seal where it meets the unit or is this normal?

HVACDave 05-26-2014 10:47 AM

The fan is pushing air through the coil and the plenum, so the air will escape around the line set penetrations, just tape em up or use a duct sealing compound (kind of looks like plastecene or silly putty)


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