Cold air down large staircases

[central nj]

Hi, I have 1500 sq ft upstairs serviced by what should be an oversized 3 ton ac. It can't quite keep up when the temp is high 90s inside is over 80.
There are two wide and open staircase going upstairs. Are they significantly contributing to the issue? How much of the cold air is "lost" to the other floor?

Thanks. Steve

[vstech]

the problem is threefold, one, you have cold air falling down the staircases, two, the warm air from down stairs is being pushed above it warming the air upstairs, and three, your attic is pummeling the ceiling with hot air, and radiant surface heating is occurring from the attic high temps as well.

energy auditing is your friend.

[central nj]

Ok but besides adding more insulation to the attic I wonder if there is anything else (1995 house well constructed)

And that also means that the 4.1 cooling load for the 1st floor is too high for my new ac to be installed in 2 days! Any way to estimate the cooling contribution? Any other fixes? Some sort of cross floor air circulator?

[George2]

Hi, I have 1500 sq ft upstairs serviced by what should be an oversized 3 ton ac. It can't quite keep up when the temp is high 90s inside is over 80.
There are two wide and open staircase going upstairs. Are they significantly contributing to the issue? How much of the cold air is "lost" to the other floor?

Thanks. Steve

The way I read your post is that you have a 3 ton A/C just for the (1,500 s.f.) 2nd floor? Do you have a A/C system cooling the lower level?

[danielworkerbee]

were are the second floor returns ?
luck dan

[hvacvegas]

Ok but besides adding more insulation to the attic I wonder if there is anything else (1995 house well constructed)

I just came from a "well constructed" 1999 home. Got up in the attic, There was a 10'x10' space that the insulators didn't blow celulose into, I guess because it was hard to get to. That, and half the attic only had 4" of celulose (which is worth about nothing).

That, and the massive attic (enough to play football in) only had 5, 6" roof vents, so no attic ventilation. It's been about 85-90* for the last 3 days, and about 130* in the attic.

Guess what? The second floor doesn't cool correctly.
Thats todays standard of "well constructed".

Energy audit.

[54regcab]

1st things I would check are the air filters and the condenser (outside part) to make they are clean. These are the 2 most common causes of poor cooling complaints.

[jtrammel]

Just because a house was recently built doesn't mean it was built well

[central nj]

Ok, thanks for the comments so far. 54regcag--had had the unit recently seviced, but it is 17 years old.

Jtrammel, good point. Will have someone look into it.

George2---yes. Two systems. Will be 4 tons downstairs. This great forum convinced me to literally beg the company to downsize from 5 to 4 tons for first floor. I had to sign a waiver. The load was 4.0-4.3.

Mr. Hvacvegas sir (thanks for help on other posts!!!)--got your point. I will have it looked at, good point.

Daniel Workbee—there are 5 low down near the floor, 16x10 return grills, one in each room, and one in the hall. 8 supplies in the ceiling (one in each room, bathrooms, 2 in the hall).

Maybe make the kids close the doors to all rooms? Would that kind of trap the cold air up stairs?

Regardless, how big of a cooling loss are the double open stairways? Will it mess up the load that was done for my downstairs unit?

Thanks
Steve

[hvacvegas]

Ok, thanks for the comments so far. 54regcag--had had the unit recently seviced, but it is 17 years old.

Jtrammel, good point. Will have someone look into it.

George2---yes. Two systems. Will be 4 tons downstairs. This great forum convinced me to literally beg the company to downsize from 5 to 4 tons for first floor. I had to sign a waiver. The load was 4.0-4.3.

Mr. Hvacvegas sir (thanks for help on other posts!!!)--got your point. I will have it looked at, good point.

Daniel Workbee—there are 5 low down near the floor, 16x10 return grills, one in each room, and one in the hall. 8 supplies in the ceiling (one in each room, bathrooms, 2 in the hall).

Maybe make the kids close the doors to all rooms? Would that kind of trap the cold air up stairs?

Regardless, how big of a cooling loss are the double open stairways? Will it mess up the load that was done for my downstairs unit?

Thanks
Steve

It's called "stack effect". Look up in wikipedia for more information. Learn alot about it for air sealing your home.

Your thinking the other way around though. If it's comfortable downstairs, but not upstairs, then keeping it open would help suck the heat downstairs. Heat always moves to cold, more so than "heat rises".

Unless I'm just tired and thinking backwards.

[energy_rater_La]

besides adding more insulation to the attic I wonder if there is anything else

lots of anything elses...adding insulation over air leaks is wasting money.
air sealing goes a long way to allowing insulation to perform.
air moving through insulation reduces insulation values.

living space walls that are shared with attic space have lots
of heat gain from hot attics. insulation alone doesn't do much
to solve these issues. if you insulate and then air seal the wall
with foam sheathing boards, then the living space is easier
to cool. and heat in winter time.

I'm not hearing good things about nj clean energy free audits.
it may be a starting point, but hiring an indpendent rater/auditor
would give you unbiased information that could save you much
more than minimal savings through utility sponsored audits.

best of luck.

[motoguy128]

Just because a house was recently built doesn't mean it was built well

Very sad and very true.

What hasn't been mentioned, is that much of the air movement down the stairs is partly due to reverse stack effect. Air leaks upstairs and cause warm humid air in the summer to leak in, and conditioned air leak out downstairs. The flow is reversed in winter.

The layout plays a major role, but I have a very wide open stairwell and there is cool air rolling down the stairs, but overall the upstairs is mostly isolated from the downstairs.

I would focus on attic insulation and air leaks (can lights, bath fans, ceiling fixtures) and attic venting.

[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.

[Shophound]

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.

[central nj]

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?

[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.

[Shophound]

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.

[central nj]

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?

[central nj]

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