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designing my OWN HVAC ductwork

8973 Views 9 Replies 5 Participants Last post by  rightit
Hi Forum, long time lurker, first time poster.

I have some ideas about a much "cooler" way to cool my home... I'm the kind of guy that thinks he can redesign all the wheels in my reach, and somehow make them rounder. ;) so if you guys would be so kind as to set me straight on a couple of ideas i've had rattling around in my head... my wife and I will appreciate it!

Right now, I have a 3 ton goodman condenser, 3.5 ton lineset (i know, weird right?), furnace in the hallway, and flex duct connecting to rigid duct connecting to more flex throughout. It's a one story 1950's ranch home, you know, the kind that has the hallway ceiling a foot lower due to the rigid rectangle above it! Also, it's a 3 sided central return, it has a return register in two rooms and one in the hallway.

So, my plan is to run a main 14" trunk of spiral pipe through the attic, strapped to the rafters, then feeding the rooms air. Also, maybe 16" spiral pipe next to it grabbing air from the ceilings of the rooms and feeding the return, replacing the ugly central return system it has now?

I hear usually it's very expensive to buy and install rigid ductwork... But at $7 a linear foot for 16" spiral pipe in 10' lengths... that really doesn't seem expensive to me... considering the entire house is so small! I know I'd have to tape and seal the joints, then also insulate the outside of the supply pipes...

So unless right off the bat I'm missing something really obvious... here are some questions that would help me get the right parts I plan to install:

1) What is the disadvantage to running a huge main trunk line the length of the house, just 40ft of 16" spiral pipe, then each room tapping into it with, say, a 10" branch VS. dropping down after each delivery of a pipe to a room, like the contractor wants to do?

2) I read it's best to suck return air from rooms from the ceilings near the inside walls... does that mean the best place to put the registers for supply air near the outside walls of the rooms, thereby making a sort of wind tunnel effect?

I can manage some serious picture overload if you guys think it would help to size up my situation? Thanks!
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You're getting good advice here. Taking care of the hot air infiltration should be your priority. If, after resolving these issues, your system still doesn't cool the house, you should have the system checked to verify that it's in good operating condition. You should also find out what size system your house requires for your climate. I wouldn't re-design the duct system until you know what your needs are. Also, duct sizing is important, especially for A/C. Installing 4 ton ductwork on 3 ton equipment will unacceptably reduce air distribution. Undersizing it will increase static pressure, often causing condensating ducts and grills, also causing liquid to flood back to the compressor. Not good.

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