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Old 04-18-2011, 07:07 PM   #1
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designing my OWN HVAC ductwork


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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Old 04-18-2011, 07:52 PM   #2
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designing my OWN HVAC ductwork


You'll want to insulate those VERY well though, for when it's in heat mode in the winter. Guess you could wrap batts of fiberglass and somehow interweave them, then wrap the whole thing with vapor barrier (mostly just to contain the insulation, but also in case there are air leaks). In summer it will also help as the pipes might get hot from the heat.

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Old 04-18-2011, 08:03 PM   #3
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designing my OWN HVAC ductwork


1- the air will not have enough velocity on the last runs. Reducing keeps the velocity high enough that it has the correct throw out of the registers.
2- yes

Both supply and return will need to be insulated. 14" spiral will not carry 3 tons of air without very high static pressure but 16" will.
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Old 04-18-2011, 08:09 PM   #4
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designing my OWN HVAC ductwork


Awesome, thanks! Let's say I want to go with a 3.5 or 4 ton unit in the future... Would it be OK to just get 18" rigid spiral ducting for the supplies now or would that be just too big for the current 3 ton I have now?
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Old 04-18-2011, 08:41 PM   #5
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Four tons is a lot of cooling. Planning on adding another 1500 square foot to the house or does 3 tons not maintain a comfortable house temp?
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Old 04-18-2011, 08:46 PM   #6
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designing my OWN HVAC ductwork


Marty, right, 3 tons doesn't seem to keep it cool in here. Of course, it may be that two rooms don't have ceilings or insulation... AND a lot of the windows in the house are the steel kind, single pane, very old... and the filter was very clogged up... so maybe 3 tons should be enough if the house was properly insulated.
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Old 04-18-2011, 08:58 PM   #7
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designing my OWN HVAC ductwork


Fixing those things up will go a loooooong ways. Two rooms with no ceilings or insulation is huge. Might be losing 2 tons right there.
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Old 04-19-2011, 11:55 AM   #8
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Is all spiral pipe created equal? I'm noticing a lot of these pipe shops online are talking about dust collection systems, and only some mention HVAC... Is it that these spiral pipes are not as good for carrying cooled air?
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Old 04-19-2011, 08:00 PM   #9
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As said earlier. using a 40 length of trunk line 40' long, will create air flow problems. Next, 14" spiral is good for about 2 tons of A/C. Also, both the supply and return trunk line need insulated.

Before reinventing the wheel on your duct work. it is far more advisable to make the other improvements to your home first. Insulation, new windows, sealing etc. Those improvements will pay back summer and winter.

When you do redo your duct system. The design process starts with knowing how big of a unit you need, and how much air each room needs.

Most sheet metal shops can fabricate spiral.
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Old 04-19-2011, 10:01 PM   #10
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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.

Mark


Last edited by rightit; 04-19-2011 at 10:04 PM.
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