# Thread: Cold air down large staircases

1. Originally Posted by hvacrmedic
It's premature to blame the capacity issue on infiltration. Have the unit checked out, then take it from there. There are several different issues that can result in low capacity.
This. As much as I appreciate building science, the first question I would be compelled to ask the OP is if the upstairs system NEVER kept that area comfortable. If the answer is no, it once did keep things comfy, even on hot days, the system needs attention. If the answer is yes, it never has made things comfy, ever, the solution may lie between HVAC system design and the building envelope.

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Hi, thanks. Lived in the house 2 years so don't know more than that, and i was told the unit is fine.

I just went through the trauma of having inadequate returns downstairs-- very inadequate (different post)

The 5 return registers are wall/between stud jobs, 4x14, supplying 180 cfm by tables i looked at, so 900 cfm total. I need 3 tons (1200 cfm). I'm guessing it is close enough? If not is there a test to know if the return are is adequate?

3. No "Test" really needed. Just a tape measure and a calculator. For a return plenum you want to be under 700 feet per minute velocity, 500 even better. SO you calculate the square foot cross sectional area, then divide the nominal CFM by the area.

Now, to know exactly what is being delivered, you'll have to measure static pressure across the blower and use a table that shows the performance data of the blower... which is included in the installation manual or data sheets. IN your case, newer limber is not full dimension, so the wall avity I think would be 14-1/2 x 3-1/2. SO that's 51sqin and divided by 144 is 0.354 sqft. 5 of those are 1.77sqft. SO 1200/1.77 = 678 fpm. Not too bad. 1 more retrun would be better. The next question would be the size of the main retrun trunk.

4. Originally Posted by motoguy128
No "Test" really needed. Just a tape measure and a calculator. For a return plenum you want to be under 700 feet per minute velocity, 500 even better. SO you calculate the square foot cross sectional area, then divide the nominal CFM by the area.

Now, to know exactly what is being delivered, you'll have to measure static pressure across the blower and use a table that shows the performance data of the blower... which is included in the installation manual or data sheets. IN your case, newer limber is not full dimension, so the wall avity I think would be 14-1/2 x 3-1/2. SO that's 51sqin and divided by 144 is 0.354 sqft. 5 of those are 1.77sqft. SO 1200/1.77 = 678 fpm. Not too bad. 1 more retrun would be better. The next question would be the size of the main retrun trunk.
That would not include the aK of the grill or filter grill, of course. Take that and then see how well each stud cavity draws. Not to mention what else a stud cavity introduces into the return air stream. And what that does to the pressurization of the house.

Stud cavities and/or panned joists used as supply or return air paths should be against code, period. Doesn't mean nobody will still do it, but regardless.

There have been numerous threads concerning multi-story homes and unacceptable temperature variances between floors. Many with one system serving two floors. The OP's house in this thread has two systems, one per floor, and comfort is still a problem. As in other threads with a similar topic, stud cavity based return paths are involved. I'm confident these aren't the sole culprit, as the only way to really get a grip what's going on is to be at the house and take note of everything in sight relevant to the HVAC and building enclosure. That said, give me a hot day, an infrared camera, a digital psychrometer, a gauge manifold, a flow hood, and a few hours and I'd love to be in a house like that to get data on what's really going on.

Therefore the pros who have suggested an energy audit may be giving the OP the best advice, coupled together with having a competent HVAC pro thoroughly go through the systems to catch any performance robbing problems.

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Mr. Motoguy128—you are the best! Thanks for the very specific help.

The plenum is I believe silvery fiber board (or wrapped duct?) 17x12 inches (or take 1 inch off if some is insulation at 16x11). That works out to 983 – 845 fpm. There is ?5 inch media filter at 16x15 (kind of 444 fpm) just before the furnace

If I have my duct guy expand the plenum to 16x15, and make whatever branches are adequate, would I see substantial improvement in cooling?

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I'm thrilled! I was just up in the attic:

The returns are on a long (t) duct, 14x14, but three on one side, and two on the other. So Mr. Motoguy's fpm are under 600.

Well, they used a three foot section of 12 inch flex duct (fpm 1667?) to connect to the run to the furnace!

So, if I just change the 10 foot or so run up to the attic (to 16x25) and replace the flex with something big (like 14x25) shouldn't that make a big difference. Or am I just being hopeful?

P.S. Shophound—there are bare spots up there, some of the can lights aren't covered. When the ducts are fixed i'll definitely have someone pad things up. Thanks for the comment (as depressing as it was). Steve

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