Modifying 2nd Story Vent

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Justintime

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I'm looking for general direction on placing a new trunk line to the second floor and what you guys think.

Location: Zone 5B
House: 2x4 construction, poorly insulated, r13 throughout, single pane windows
Some background:
I've got a basement furnace I'll be upgrading to a heat pump servicing a home a total of 1400-1500 sq ft. I got a quote for $16,000 min ($21000 for mid-grade) for 2.5 ton heat pump, that's if I do the electric. I'm very comfortable with all things electric, plumbing, copper work/brazing, welding blah blah blah etc. Given the cost, I'll probably be installing this one on my own and hire someone to help me charge and balance the 410a heat pump with the right digital gauges to get the subcool and superheat right.

1. Top floor is 450 square feet, bottom is ~ 950
2. The plenum feeds two trunks that look very similar to this:
Font Rectangle Line Parallel Slope


3. The left feeds ~550 sq ft and is good for up to 950 CFM, the right feeds ~400 sq ft and is good for 950 CFM (the house is noisy with the HVAC running). Total 1st floor vent CFM is ~765 and I'm sure they're seeing more than that
4. There's a 10" duct to the second story that terminates in the middle of the second floor to a single 10x10" vent, no option for adding branches
5. My intent is to run a new 12 vs 14" round solid pipe w/ damper off either the plenum or the larger of the two trunks, up to the second floor inside a 1st floor closet, into the kneewall attic
6. This will split into all flex duct with a 6 inch single run to the bathroom, and a 12 inch flex duct trunk from which (2) 8" vent supply pipes and a single 6" supply pipe at the end (to maintain pressure)
7. Everything will be R8 covered in unconditioned attic, likely with r30 cellulose blown in over that
8. I'll turn the 10" current supply duct into a return duct


My questions are:
1. Any general concerns?
2. Does the duct sizing seem appropriate proportionally? It's sized a little big for slanted attic walls, but I can damper it down, I'm more worried about cooling capability in the summer. A variable speed blower and 2 stage heat pump for winter with semi-closed upstairs vents I believe will be ok if the effective heating area is reduced
3. Would you take off from one of the two trunks in the basement, or the plenum itself?
4. Would it be better to run a 8x14" duct vs 14" round duct vs 12" round duct up through the closet to a stack head

Here's a diagram of the upstairs HVAC plan:
1669083920512.png


LoadCalc of the upstairs (room 1) and downstairs (room 2)
1669084011805.png
 

Fitter30

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12" round is only good for 525 cfm 113sq"
14" 154sq" round pipe cfm 750
Plenum would be best. Cooling is 400 cfm per ton
Mini split would be alternative two systems always work out better.
 

Breplum

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I strongly recommend you get a load calc design for the entire dwelling done and duct design with it.
Wrightsoft is the software we subscribed to when we were doing HVAC before I retired.
rule of thumb air duct sizing CFM
6" 125
8" 225
10" 350
12" 450
14" 650 -750
16" 900
18" 1,200
but we always want to be sensitive to velocity, which originates with CFM.
There is a lot to duct design.
My favorite systems: Bryant/Carrier Evolution/Affinity models, with variable everything, dampers everywhere you can zone for.
The system itself balances room loads without bypass on return air and adjusts airflow accordingly (as long as you sized each room properly and have a damper and system remote tstat on as many relevant locations as possible.
 

Justintime

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My downstairs is fed by two 960 CFM (1840cfm total) trunks with actual total vent CFM at only 765, though it's pushing more than that given the size of the system.

I guess let me ask it this way, for a 420 sq foot upstairs, 960 sq ft downstairs, and the BTU loadcalc I've included above with the given trunks feeding the bottom floor, are you recommending I push closer to 800-1000 CFM? Like go for a 10x18 rectangular duct feeding the upstairs and put a damper on that so I can better balance heating vs cooling? Total CFM with the 10x8 registers I have currently above would be ~450 cfm, so there'd be some pressure in the system still, presumably.

Fitter, where'd you get the 4:1 CFM:SqFt ratio? I haven't seen that before.
 

Fitter30

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Don't know about any ratio but 9148 CFM is over 2 tons. 400 CFM is a ton. Your figgers are wrong. Without changing windows and doors you have no idea how much money your blowing out the doors. Ask your utility companies is they offer a energy audit that includes a blower door test. Mine cost $50 and they gave me $100 worth of stuff. My current home 1900 sq. ft with a loft 550' no insulation in walls new windows and doors can heat and cool in southern mo. 35 miles from Arkansas middle of the state with 2 ton mini split. Gets below 10° turn on a LP stove.
 

Justintime

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Don't know about any ratio but 9148 CFM is over 2 tons. 400 CFM is a ton. Your figgers are wrong. Without changing windows and doors you have no idea how much money your blowing out the doors. Ask your utility companies is they offer a energy audit that includes a blower door test. Mine cost $50 and they gave me $100 worth of stuff. My current home 1900 sq. ft with a loft 550' no insulation in walls new windows and doors can heat and cool in southern mo. 35 miles from Arkansas middle of the state with 2 ton mini split. Gets below 10° turn on a LP stove.

That number is BTUs, not CFM. That's a loadcalc. 400CFM per to ton is the general rule, but not what those numbers are depicting.

I'm just wondering what CFM I should try to attain for a 2nd story 420 Sq ft finished attic with r13 insulation and kneewall with a 1st level of 960 Sq ft with capability for 1820 CFM and vents rated up to 765 CFM. Thanks!
 

Fitter30

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3/4 ton of cooling, 300 cfm
This is for sheet metal not flex duct flex should be run more than 5 - 6' but people do it.
 

Justintime

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Any other thoughts on this? I'm thinking of going with 12x16 duct from the basement plenum, then using a head stack rectangular T in the attic going to flex duct. 14 inches feeding the longer run, 8 inches feeding the smaller bathroom. Total vent CFM will be ~500. This will roughly match the downstairs proportion of trunk to vent airflow so they'll be some retained back pressure and velocity. A damper vent with electronic control will be used upstairs. I'll mate the entire system to a 2 stage variable air speed heat pump/air handler negating need for a bypass.
 
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