Do You Need New Ductwork for a 6–10 Ton Commercial Unit? Tony’s Rules for CFM, Static Pressure & Air Balance
When a business upgrades to a 6–10 ton commercial packaged unit, they almost always ask Tony the same question:
“Do I need new ductwork, or can we keep the old system?”
And Tony gives the same answer every time:
“Your ductwork decides how well your new unit performs — not the equipment.”
He’s seen brand-new 10-ton units cool like 5-ton units because the duct system was choking airflow.
He’s seen perfectly good systems burn out compressors, overheat gas furnaces, short cycle, roar like jet engines, and cause constant hot/cold complaints — all because the ductwork was wrong.
This is Tony’s complete, field-tested guide to whether your building needs new ductwork when installing a 6–10 ton commercial rooftop or ground-mounted packaged unit.
1. The First Rule: Commercial Ductwork Determines 80% of Comfort
Ductwork either delivers the airflow your new system needs or strangles it.
For a 6–10 ton system to perform correctly, it must move:
350–450 CFM per ton
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6-ton: 2,100–2,700 CFM
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7.5-ton: 2,600–3,400 CFM
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10-ton: 3,500–4,500 CFM
Tony doesn’t care what the brochure says.
If the ducts can’t move that airflow, the equipment will never hit full capacity.
This is why “duct evaluation” happens long before installation day.
(Reference: [Commercial Airflow])
2. Old Ductwork Usually Can’t Handle the CFM of Newer 6–10 Ton Systems
Most older buildings were designed for older equipment with:
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lower airflow
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lower efficiency
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weaker blowers
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lower external static pressure
Newer rooftop units push much higher airflow, especially with variable-speed blowers.
If your old duct system has:
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small trunks
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narrow branches
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crushed flex
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leaky seams
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long runs
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multiple 90-degree bends
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undersized returns
…it cannot support a modern 6–10 ton system.
Tony sees this constantly:
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restaurants with 30-year-old ductwork
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retail stores with undersized returns
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offices with duct systems designed for a 5-ton but using an 8-ton
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strip malls with long, narrow runs that kill airflow
Old duct systems + big new units = nonstop problems.
3. Return Air Is the #1 Problem in Commercial Buildings
Tony doesn’t start by checking supply ducts — he checks returns.
Most 6–10 ton commercial systems are starved for return air.
Return issues include:
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return plenum too small
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too few return grilles
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long undersized return runs
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returns pulling air from hot mechanical rooms
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leaking return ducts pulling attic/roof air
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returns installed too close to supply diffusers
If the return side can’t breathe, the blower suffocates.
Symptoms of bad return airflow:
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loud blower noise
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extremely hot supply ducts
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coil freeze-ups
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constant hot-cold complaints
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short cycling
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high static pressure
Tony fixes return issues before anything else.
(Reference: [Commercial Return Air System Assessment &
4. Static Pressure — The Invisible Killer of New Units
Static pressure is the resistance the blower must overcome to move air.
Too high = the blower can’t deliver airflow.
Too low = noisy, unbalanced air.
Most packaged units require 0.5 in. w.c. total external static pressure.
Tony often finds older duct systems running at 0.9–1.5 in. w.c. — way above limits.
High static pressure destroys:
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blower motors
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compressors
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heat exchangers
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energy efficiency
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comfort
You can install the best unit in the world, but if your static pressure is high, the unit will never perform.
During assessments, Tony uses:
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static pressure gauge
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CFM capture hood
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probe readings
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supply/return differential testing
This is not guesswork.
(Reference: [Commercial Static Pressure Testing ])
5. Supply Duct Problems — More Common Than You Think
Tony sees supply duct issues every week:
✔ Undersized supply trunks
The most common problem.
A 10-ton system on a 14–16" trunk will fail immediately.
✔ Poor duct transitions
Hard 90-degree turns and sharp metal transitions kill airflow.
✔ K-style tees
These split airflow unevenly, starving half the building.
✔ Leaks
Commercial duct leakage averages 10–30%.
Tony seals every seam.
✔ Wrong diffuser styles
Low-throw diffusers in high-ceiling spaces cause stratification.
✔ No balancing dampers
Without dampers, airflow gets pushed into the shortest runs and avoids the long ones.
Tony evaluates supply pressure, balance, and distribution BEFORE deciding if the ducts can stay.
6. Building Size, Shape & Ceiling Height Matter More Than People Think
A 6–10 ton unit can perform beautifully in one building…
…and struggle miserably in another with the same square footage.
High ceilings
Heat stratifies and requires more supply airflow.
Long rectangular buildings
Edge rooms starve for supply unless the ducts are sized correctly.
Multiple tenant spaces
Shared trunk systems rarely work — zoning is mandatory.
Drop-ceiling offices
Ducts may be compressed or hidden behind layers of remodels.
Restaurants
Kitchen heat load completely changes airflow needs.
Tony sizes airflow based on cubic feet, not just square footage.
(Reference: [Commercial Building Envelope])
7. Signs You Need New Ductwork (Tony Sees These Every Week)
If your building has ANY of these issues, duct replacement is very likely:
❌ High static pressure
❌ Undersized return plenum
❌ Hot/cold spots
❌ Supply air barely pushing
❌ Loud airflow “whooshing”
❌ Sweating or hot supply ducts
❌ Rooms never reach setpoint
❌ Compressed or crushed flex duct
❌ Leaky rooftop ducting
❌ Mold or debris inside ducts
❌ Uncomfortable employees/customers
A 10-ton system will NOT fix any of these problems by itself.
8. When Existing Ductwork Can Stay
Tony won’t make you replace ducts unless it’s truly required.
Ductwork can stay if:
✔ supply trunk is properly sized (18–22" for 7.5–10 tons)
✔ return ducting supports full airflow requirements
✔ static pressure is within limits
✔ duct insulation is good
✔ no airflow restrictions
✔ balanced airflow can be achieved
Buildings with newer ductwork (under 15 years) often pass.
Older systems rarely do.
9. When You Only Need Partial Duct Replacement
Often, only certain parts are bad:
Replace:
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return plenum
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main supply trunk
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leaking rooftop duct sections
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old K-style branch tees
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damaged flex runs
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plenum transitions
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undersized elbows
Keep:
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branch lines in good shape
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diffusers that match airflow
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newer insulated ducts
Tony saves businesses money by targeting the worst sections rather than replacing everything.
10. Zoning, Economizers & Fresh Air Change Duct Requirements
Modern commercial units often include:
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economizers
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modulating dampers
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fresh-air minimums
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CO₂-based ventilation
These require more return air, larger trunks, and better balance.
Many older buildings were never designed for:
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outside air
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mixed air
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damper controls
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free cooling
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staged heating/cooling
Without duct upgrades, these features perform poorly.
(Reference: [Commercial Ventilation, Economizer])
11. Restaurants Need Different Duct Rules Entirely
Restaurants have:
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massive heat loads
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constant door traffic
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kitchen infiltration
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humidity spikes
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poor roof insulation
They often require:
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oversized returns
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high-throw diffusers
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extra supply to dining zones
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improved makeup-air balance
Tony almost always modifies ductwork in restaurants during a 6–10 ton upgrade.
12. Gyms, Medical Offices & Retail Stores Each Need Unique Airflow Plans
Gyms
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high humidity
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high occupancy
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heavy return requirements
Medical Offices
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precise temperature
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clean airflow
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quiet diffusers
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high static sensitivity
Retail Stores
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long room shape
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varied ceilings
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sun-exposed glass
Airflow planning must match business type — not just tonnage.
13. The Math Tony Uses to Decide if Ductwork Must Be Replaced
Step 1: Total CFM required
350–450 CFM × tonnage
Step 2: Duct size charts
He uses metal duct sizing rules based on friction rate.
Step 3: Static pressure test
0.5 in. w.c. max allowed
Above that → new ductwork.
Step 4: Temperature rise checks
Gas furnace rise must match rating plate.
Step 5: Air balance
Capture hood readings at every diffuser.
Step 6: Visual inspection
Crushed, leaking, moldy, or disconnected ducts = replace.
Tony never guesses airflow.
He measures it.
14. The Cost of New Ductwork — Real Commercial Numbers
Costs depend on:
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footage
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insulation
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roof work
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number of drops
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diffusers
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sealing requirements
Typical ranges Tony sees:
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Partial duct replacement: $2,500–$8,000
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Full duct replacement: $6,000–$30,000
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Rooftop duct repair: $3,000–$15,000
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New return plenum: $1,200–$3,500
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New supply trunk: $2,000–$7,000
Expensive?
Sure.
But replacing a $10,000 compressor because of bad airflow is more expensive.
15. What Happens If You Install a New 6–10 Ton Unit on Bad Ductwork
Here’s the truth Tony tells every owner:
A new 6–10 ton unit will NOT fix duct problems.
It will make them WORSE.
Expect:
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high bills
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poor temperature control
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short cycling
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blown heat exchangers
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noisy ducts
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cold spots
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hot spots
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early component failure
Tony has seen brand-new units die in under 3 years because of wrong ductwork.
Ducts are not optional.
They are the system.
Tony’s Final Verdict: Your Ductwork Must Match Your Tonnage
Here’s Tony’s real-world summary:
✔ A 6–10 ton commercial unit requires 2,100–4,500 CFM
✔ Old duct systems usually cannot support modern airflow
✔ High static pressure = guaranteed failure
✔ Return air is the most important ductwork factor
✔ Zoning and economizers require upgraded duct design
✔ Partial duct replacement is often enough
✔ Full replacement is only required when airflow cannot meet demand
If your ductwork can’t breathe, your new unit will never perform — no matter how advanced it is.
In the next blog, Tony will tell how efficient these units are in 2025
