HVAC Duct Calulations

[simpler=better]

You guys want to double check my work? This is for the 2nd floor & attic in an old farmhouse. First floor is going to be mini splits. Current system looks like it was designed & installed by drunk raccoons.

The house has no real insulation, single pane windows, balloon framed, etc. Yes I need to fix that stuff, but right now I want to get the ducts sorted.

J-calculation says 30kBTU cooling, the current 3ton unit should be adequate.
Air handler is rated at 1,400cfm unrestricted.

The rooms are 10x10x8=800 cubic feet. 800cuft*10 air changes=8,000/60mins= 133CFM per room.
The attic is 20x20x8 (vaulted ceiling)=2,000cuft*6 air changes=12,000/60mins=200CFM to the attic.

So that's 1,000CFM total-the handler can do that.

To keep the velocity under 500fpm for noise, duct drag calculator gave me a minimum duct diameter of 6.6". I rounded that up to 8".

I'll be replacing the 20x20x1 pleated filter with a Honeywell F100 20x25x4 media filter.

To get air back into the attic, I'm planning on cutting straight down through the floor to put in a 20" grate. The air handler will pull from the top of the room. This is the 20" round hole near the center of the drawing.

These are the rough plans:

I plan on running spiral duct and insulating to R8. The rectangular bends will have vanes in them. All drops will get dampers.
I'll be getting all my parts from Phoenix Metals

Do you guys think my ducts are too big:?

 

[Garage Dog]

simpler,

I'm looking to do something similar myself, so it interests me to follow the thread.

FWIW - I would suggest you move it or get it moved by a mod to "Heating and A/C" instead of Free Parking for a better response.

GD

 

[simpler=better]

simpler,

I'm looking to do something similar myself, so it interests me to follow the thread.

FWIW - I would suggest you move it or get it moved by a mod to "Heating and A/C" instead of Free Parking for a better response.

GD

One day, I'll learn how to read

 

[Stuff]

You have a 3 ton compressor? Normally speced to need 400 cfm/ton so everything is still undersized.

A lot more work but is there any way of running some returns down the 2nd floor walls?

What you are proposing would still be better that 90% of what is out there.

 

[simpler=better]

Does the 12""->10"->8" seem too large or just right? On paper it's okay, but I'm not a pro and have never built an HVAC system.

I can't tear the walls up to install returns in each room (alrough I'd love to). Unico system is out due to budget.

Check out this sweet installation by Tom Sadler from Comfort Temp Heating and Cooling:

 

[danski0224]

That's what one gets when selecting the low bid contractor on a job without any provided specifications

 

[brewchief]

If you're using your existing airhandler and changing it's orientation verify that it can be used in that position, it looks like you would be changing it to a downflow configuration, some units can only be used upflow or horizontal.

Duct sizing looks OK, I'm not a fan of just using transfer grills to get air back to the unit but I understand the limitations you're working with.

You may get some noise with only a single 90 on the return, turning it to the side and then out might help and I would line the return with acoustical liner regardless.

Sealing the duct joints is almost as important as the insulation, either tape or mastic them(I prefer mastic).

 

[simpler=better]

That's what one gets when selecting the low bid contractor on a job without any provided specifications

Yup, I've been finding out that almost everything here was done that way. I didn't have this done.

If you're using your existing airhandler and changing it's orientation verify that it can be used in that position, it looks like you would be changing it to a downflow configuration, some units can only be used upflow or horizontal.

Duct sizing looks OK, I'm not a fan of just using transfer grills to get air back to the unit but I understand the limitations you're working with.

You may get some noise with only a single 90 on the return, turning it to the side and then out might help and I would line the return with acoustical liner regardless.

Sealing the duct joints is almost as important as the insulation, either tape or mastic them(I prefer mastic).

Everything will be sealed with mastic

She's compatible with a downflow arrangement-I just have to rotate the coil and swap the drop pan over.

 

[LS6 Tommy]

Does the 12""->10"->8" seem too large or just right? On paper it's okay, but I'm not a pro and have never built an HVAC system.

I can't tear the walls up to install returns in each room (alrough I'd love to). Unico system is out due to budget.

Check out this sweet installation by Tom Sadler from Comfort Temp Heating and Cooling:

That is pure garbage. We call those "Octopus" installations around here...

Tommy

 

[CNGsaves]

OP . . . . WHERE are you located ?? No insulation and attempting to cool in Phoenix is lot different than Minneapolis !!

UPDATE GJ Profile with a Location.

 

[simpler=better]

OP . . . . WHERE are you located ?? No insulation and attempting to cool in Phoenix is lot different than Minneapolis !!

UPDATE GJ Profile with a Location.

dammit! I'm usually really good about that

Updated, I'm in Balamer, Murrlin.

I've got a change of plan-I tihnk I'll be running individual returns, placed on the opposite side of the room from the supply registers.

Yes, insulation is on the to-do list, but I'd rather get my utilities in place before I load up on roxul.

 

[simpler=better]

Bottom line: Fix your flex duct!

I spent around 12 hours this weekend in the attic. It was nice and warm, I probably sweat out 10lbs or so

Things that were REALLY wrong with the install:
-NONE of the take-offs on the trunk line were sealed. They had 1-3 screws, and were attached to the insulation NOT the sheet metal.
-No ceiling starter fittings-only 90s with fingers cut into the ends and then bent around the plaster.
-No mastic anywhere
-Most ceiling fittings were only partially seated & zip tied
-All flex was loose and excessively long
-Anywhere tape was used, it was only used to kinda hold things in place.

What I did:
-Sealed takeoffs to trunk
-Added scoops to takeoffs
-Eliminated the two runs to the first floor(for now)
-Reinsulated trunk
-Sealed flex duct to takeoffs & ceiling starters(Who would have thought?!?!?!?)
-Caulked ceiling starters as best I could
-Duck taped all any and all joints(It's temporary until I get the solid duct in)
-Reworked flex runs to be taut. That odd one over the ceiling is now 12ft shorter and supported by rigid foam to prevent distortion.

Left to do:
-Seal up main return (floor joice cavity-but it's open to soffits on both ends because of course it is.)
-Rework the whole system with steel ducts(some other day)


The results are incredible-I'm getting about 4x the airflow, it's actually cold, and surprisingly quieter too. It had no trouble getting the second floor down to 76* (The first floor is being cooled by a window unit that does well).