Did I do something wrong?

[bbqjason]

Good day,
I just had a new three ton unit installed. The previous unit was cooling about 1100 SF before the compressor went out. We have another 500 SF room that was added twenty years ago, with window units used for cooling. We are in the process of renovating the add-on, so it's gutted to the studs. The AC company told us the three-ton unit would cool 1600 SF, so we decided to include the add-on into our central air system. With a little consultation and short materials list from the AC installers, I decided run the ducts myself (I own a handyman business and have worked in several trades over the course of my life, but my HVAC knowledge isn't much).
I tapped into the main trunk (in the attic) with two 10" flex ducts for the add-on rooms. These 10" ducts both are 12' long, both connecting to junction boxes (10-7-6 and 10-7-4), which then run to cool the add-on room. The 10-7-6 junction box runs 25 and 18 feet, the 10-7-4 junction box runs 25 and 15 feet. Today the 1100 SF area stayed at about 79 degrees all day while the thermostat was set at 73. The add-on room stayed a little cooler. The two 10" ducts that I tapped into the main trunk with are located first on the trunk, followed by the branches down the line which cool the 1100 SF. Is it possible that by locating the add-on room ducting (two 10") first, I've hurt the cooling of the 1100 SF? The reason for my putting them there is location- had I gone further down the trunk I would have been running longer ducts to get to the add-on room. The only return in the house is located in the original 1100 SF structure.
Any help would be appreciated. I have a Manual D and Manual J from my college days, but obviously I was unable to translate the information in them to a good HVAC system. The AC guys who installed it both offered to come out free of charge and put me on the right path, but I hate to bother them because they're very busy. I may yet call on them, but I was hoping someone in the forums here might be able to tell me if I did a bad thing. Thanks- Jason Smith

[bbqjason]

one more thing- I could be worried for nothing- The addition is a flat roof, so I had to cut holes in the ceiling and wall to get the ducts through. Gonna be enclosing it all in a soffit above the kitchen cabinets. I suppose the four holes in my ceiling might have something to do with it.

[tinner73]

you did a very bad thing. sorry no DIY here.

[udarrell]

I believe I would have used a short metal duct at the takeoff from the main trunk with dampers to adjust airflow to the add-on room.

Yes, the four open holes in the ceiling could be messing up the cooling of the main conditioned area.

The linked page below on ductwork is not perfect but will give you some ideas to think about. Filter grille area; boot and diffuser CFM capacities to rooms, etc.

http://www.udarrell.com/proper_cfm_b...g_systems.html

[dash]

Originally posted by bbqjason
one more thing- I could be worried for nothing- The addition is a flat roof, so I had to cut holes in the ceiling and wall to get the ducts through. Gonna be enclosing it all in a soffit above the kitchen cabinets. I suppose the four holes in my ceiling might have something to do with it.

Don't understand this part.


If your not getting proper cooling in either part,the system could be too small,or the extra ducts caused increased static ,and low overall air flow.

Adding them close to the equipment,is good,as the original ducts were not sized for the larger system,how far from the indoor unit are they,that section of duct is likely too small,and the return wasn't incresaed,so too small as wel.

Undersized ducts ,high static ,equals low air flow,and decreased capacity,one possibility.

[bbqjason]

Thanks for all of the reponses. I'm going to try to soak all the information in, then come back and let you know how things are going.
I think I should take care of the holes in my ceiling first and see where that gets me. I'll likely increase the return size, and put a damper on for the add-on room duct, and exactly what else I don't know. There's a 90% chance I'll be getting the AC guys back out so they can look at it on-site and tell me how bad I screwed up.
Thanks to everyone for their help!
Jason

[bbqjason]

Sorry if I insulted anyone in the profession by attempting this on my own- I just love to do things like this and learn how things work. I jumped into this without doing enough research, so I may have created some extra work for myself. You guys have my total respect, as there is no substitute for expertise in HVAC installation.

[bbqjason]

response for tinner-

I don't know why you wasted everybody's time with your reponse. If you think you have knowledge that's worth money, you're probably right. But it's obvious you don't know how to sell it- which is probably why you're so cranky. I don't care what you do for a living; your type's a dime-a-dozen.

[diegocentral]

hey jason, not everyone on this site is a jerk.

Can you tell us the duct runs off of the supply side. Along with the R/A.
Most 3 ton systems are rated at 400CFM per ton. 1200CFMs in a 3 ton system.
I'm not sure if you mean there are only two 10" runs leading off the supply, but if that is the case your ducts are obviously undersized.
You can go to your local A/C dist. and pick up a duct sizing chart and calculate the CFM's per duct and how to distribute it evenly throughout the house. If the ducts aren't restricted visibly (Flex) it might just be undersized.

* I just read through your post. 2 10" runs for the add on.
Like you mentioned by dampers and adjust the flow until you get your desired cooling, it might help.

[Edited by diegocentral on 08-29-2005 at 07:45 PM]

[tinner73]

i'm not cranky.

[Edited by tinner73 on 08-29-2005 at 10:20 PM]

[Chill]

Originally posted by bbqjason
response for tinner-

I don't know why you wasted everybody's time with your reponse. If you think you have knowledge that's worth money, you're probably right. But it's obvious you don't know how to sell it- which is probably why you're so cranky. I don't care what you do for a living; your type's a dime-a-dozen.

yea, take it easy. He only sited the rules of the forum! I don't see how anyone can size your ducts properly with out seeing it. Getting help thru forums often losses something in the translation.

[bretware]

THOSE LINE ARE TO BIG TO BE COMING OFF THE TRUNK, WHO EVER TOLD YOU TO DO THAT IS A RETARD.

[bbqjason]

Ductboard supply trunk, 20' total length, size decrease at 4' increments (labeled A,B,C,D, and E), last section is only 2'. (A-17"x15"x4', B-13"x15"x4', C-13"x15"x4', D-12"x15"x4', E-9"x15"x2')

A- 10" run @ 12' length, to 13"x15" j-box1. From j-box1 are 6" run @ 10', and a 6" run @ 7' to a triangle j-box. From triangle j-box are 4" run @ 5', and 4" run @ 10'.

B- 10" run @ 12' to traingle j-box. From j-box are 7" run @ 25', and 6" run @ 20'.
B- 10" run @ 12' to triangle j-box. From j-box are 7" run @ 25', and 4" run @ 14'.

C- 6" run @ 7'.

D- 8" run @ 5'.

E- 6" run @ 13', 6" run @ 15', 6" run @ 8'.

System is 3 ton, materials are ductboard and flex-duct.
I added section B.

Man it would have been alot easier just to scan my drawing.

[Edited by bbqjason on 08-29-2005 at 10:16 PM]