[darctangent]

Dang, Scott, your avatar makes me want to move to Colorado

It's really something on a good day. I have a story or two about that kind of thing.... Let's just say there's nothing like waking up on the side of a mountain with nothing but more mountains in every direction.

 

[darctangent]

Ok, on the return air drop, yes you can get bigger, and yes, IF DONE WELL, it's better. First I'll post some collages of RA (return air) drops that pose more resistance than absolutely necessary, then I will post the best, least restrictive version of what I have come up with, and explain a little bit about how it works. Please keep in mind that if a contractor does not already do this kind of work you will have very little luck in getting them to do it this way. It takes a little bit of experience.

OK, here's the first batch. These all have points of restriction that don't need to be there.




Those last three are all the same job and I wanted to show why the most common EAC/media filter install is done poorly,or in this case horribly wrong. notice the two close up photos that show about 25-30% of the air path blocked by sheetmetal? That setup creates absolutely horrible static pressure drop and filter performance, and yet it's very common. There are a bunch of different ways that restrictions are introduced into the system, and most of it has happens because the contractor just does what everybody else does and hasn't started thinking out side the box- and thinking about where the restrictions are in the system.

see the next post for "my way" of doing it.

 

[darctangent]

Ok, something I should have said at the end of that first post- I took more than a couple of those photos off HVAC-TALK, and many of the photos I've selected to point out restrictions were actually off the "wall of pride". I'm not trying to trash anybody elses work- I'm just trying to say that there's a better way to do it, that's all. When I started out in the industry I didn't have nearly enough training and I did just about everything wrong, so please don't think of me as putting myself above everybody else's work. It's just a part of our trade I've spent some time and money trying to find a better way, and I think I've done that. Ok, so here we go...

This is a fairly typical install of a 100K Furnace and a 3 ton AC. Brand doesn't matter, SEER doesn't matter. I do about the same thing for three tons and up. 2.5 tons and below I generally do without the return air box unless I really need the extra capacity, like a large furnace /small AC or I'm trying to make every last .001 of a IWC count.
At the top the duct gets flared at a 45 degree angle or so in the direction of air travel and if the trunk is wider than the drop then I flare that out to the sides are well. I like to put the filter about half way through the return air drop for a few reasons but the one that customs pick up on is that it makes filter changes really easy. Now after the air passes through the filter it has two paths into the furnace- one through the side of the furnace, the other through the bottom. It's REALLY important with this design to seal everything very well to prevent air & dirt from getting into the system after the air filter.

I've been able to cut the total external static pressure by over 50% on some systems when I optimize the return and the supply. There are a couple of variations I haven't tried yet, but this outlines the basic design. I will note that getting all the elements of this design to mate up takes some work and planning, so anybody that thinks this just slaps together should use some caution. The return drop generally matches whatever the air filter opening is, plus a little bit.




For you guys that insist that every thing goodman sucks, here's another one for you:

 

[beenthere]

Vane that return, and you can drop another .1" on the return static.

 

[54regcab]

Love the combo side/bottom air returns !!

 

[blg002]

I understand some of that, but man do i wish i understood more of it, this is all very interesting to me now and it seems like you've done something very cool.

 

[darctangent]

Vane that return, and you can drop another .1" on the return static.

On the bryant that was true, but once I went to the single piece duct underneath I used on the Amana that went away. I forget exactly what It came to on that section for the last job but it was barely there. Part of the problem on the bryant was how much I had to hold back the return drop to give clearance for the condensate & natural gas. Another thing.... well, I guess I'll leave that for the pro side too. like I said there are a couple details I'm not covering in the open forum but I can tell you that it's a very low static pressure set up, at least for the drop and the plenum, and I also I haven't run a five ton system on this design as it's tough to get the recommended airflow for 5 tons when we correct for altitude. That being said this is the setup I will use when I need to run over 2000CFM if I'm not able to run dual RA drops.

One last thing- people are always saying- "now if you just radius that return..." LOL. the drag at that point isn't as high as people think, and a radius on the heel of a fitting won't do jack. people are lucky if they don't reduce the volume of the duct too much and increase static that way because the throat didn't get a radius too. It's better to drop the velocity and provide as straight of a path as you can in a RA drop.