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Old 06-18-2010, 02:15 PM   #16
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AC Temp drop over plenum run length


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Originally Posted by beenthere View Post
The supply section that is reduced to 12X8 should be larger. Probably 16X8.

The return ell boot that is attached to the furnace. Was meant to be attached to a return air filter bracket. Not directly to the furnace.

And the return drop, should be 25X10. Here is a PDF that will show you what you can have them do to make your return drop better. Which includes a better air filter set up. Go to page 7, look at #1221. You would want 1222. Since it has a larger filter.


I can't make out the sizes of your joist returns. Can you just post the numbers.

It looks like adding another return in the central area, would be a boost to your overall air flow problem also.
The returns that they embedded into the joists are 15x9 (joists are 16 OC so that is about the size they would be...... what is your opinnion on ditching the foil faced cardboard and putting in actual duct work on the joist return cavities they have? I've heard good and bad about the cardboard return ducts... hard to seal up also...

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Old 06-18-2010, 02:28 PM   #17
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AC Temp drop over plenum run length


Thermopan(cardboard duct) is ok. But it should be caulked to the joist also.

If you want to switch to sheet metal panning. Its ok, but same inherent problems. if its not caulked to the joist.

3 return bays is not enough. The resistance to air flow from the when you cut out the 2X4 plate in the floor. is a lot more then some think. And really cuts down on the air that can be drawn in.

A 2X4 plate, cut out full length on a 16" OC wall, can only move around 180 CFM max. So you would need basically 10 wall cavities to get enough air for a 4 ton unit.

If they're floor returns, what size are the grilles.




PS: The case coil should have been raised off the furnace. And a transition to it made. For better air flow through it. Not always possible in all basements though.
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Old 06-18-2010, 02:31 PM   #18
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AC Temp drop over plenum run length


they are wall returns basically 8x14 I think the grill size is all of them are about 8 inches off the floor...

and the thermopan definatly is not caulked... they just stapled it up
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Old 06-18-2010, 02:36 PM   #19
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AC Temp drop over plenum run length


You need more returns.
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Old 06-18-2010, 02:37 PM   #20
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AC Temp drop over plenum run length


That's what I was thinking a few weeks ago... question is where to put them running out of wall space.....
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Old 06-18-2010, 03:11 PM   #21
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AC Temp drop over plenum run length


Center room. At the closet wall.
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Old 06-22-2010, 09:00 AM   #22
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AC Temp drop over plenum run length


was reading over the furnance installation manual (which when it was installed the installers thankfully left before I bought the house)... it says the furnance has a return opening on the side of 25 1/2" x 16 1/2" and can do up to 2000 CFM but requires 2 air filters unless the bottom return is used at 28x28 (i think it was that size).... I dont even know if this is pushing 2000 CFM at the moment or trying to I guess that depends on the blower speed? anyways... I guess its 1400 CFM with one filter on the side...

should I have two filters / returns? I dont have a 5 ton AC, just a 4 Ton one...


also checked again with the cold air returns on the farthest end it is a 8x8 box... for about 16 ft of the return before that it is 12x8 and that is it
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Old 06-22-2010, 09:08 AM   #23
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AC Temp drop over plenum run length


Return duct is also too small.

Making your furnaces returns on both sides will help to pull more air to it. But you should have larger return duct at/from your returns. Besides adding another as said before. And if there is anyway you can increase the size of the supply duct for those last 3 rooms. That will help greatly.

A 4 ton generally should move between 1450 and 1600 CFM.
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Old 06-22-2010, 09:10 AM   #24
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AC Temp drop over plenum run length


thanks, slowly getting my plan of attack on paper with your help! increasing the return duct is definatly on the top of the list along with a 5" media filter and new return plenum boot

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