Changeout on a Tucson House with Ducts That Can't Take the Airflow
You are quoting a changeout on a 1,850 ft² 1979 block-and-stucco house in Tucson (design 104°F DB / 71°F WB). The existing 5-ton R-22 single-stage condenser has had two compressor replacements in nine years. You ran a room-by-room Manual J: 38,600 Btu/h total cooling, with 8,200 Btu/h of that latent during the monsoon. Measured airflow through the existing 20×25 filter grille and duct system is 1,420 CFM at 0.72 in. w.c. external static in high cool. The homeowner says, "Just put the same size back - the last one cooled fine when it worked, and a bigger unit cools faster." Supply registers are 6×10 with 4-in. flex branches serving the two west bedrooms.
1. What size and type of equipment do you propose, and what numbers justify it? Explain what the compressor history tells you.
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Start from the load, not the old nameplate. Design total is 38,600 Btu/h with 8,200 latent, so the sensible requirement is about 30,400 Btu/h.
Then apply the airflow ceiling, which is the binding constraint here. The existing ducts deliver 1,420 CFM at 0.72 in. w.c. - already above a typical 0.50 in. w.c. rated TESP, meaning the blower is off the top of its fan curve. A 5-ton needs roughly 2,000 CFM; 1,420 CFM is only 284 CFM/ton, far under the 350–400 CFM/ton design band. Low airflow means low suction pressure, a cold/iced coil, poor oil return, and high discharge temperature - that explains two compressor failures in nine years far better than bad luck.
A 3.5-ton unit at about 1,400 CFM matches what the duct system can actually move. Verify it with the manufacturer's extended performance table at 104°F outdoor and 75/63 indoor (not nominal tonnage), then run a Manual S. If sensible margin is thin, the correct fix is envelope work - attic insulation, radiant barrier, west-glass shading - because you cannot buy your way past the duct limit with tonnage. Specify two-stage or inverter equipment so long runtimes handle the 8,200 Btu/h monsoon latent load.
2. The homeowner insists on replacing the 5-ton with another 5-ton. How do you handle this technically and contractually?
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Put the constraint in writing and price the alternatives honestly.
- Show the measurement: 1,420 CFM available vs. 2,000 CFM required. A 5-ton on this duct system will run at roughly 1.0 in. w.c. or more, with coil icing, high head pressure, and short compressor life - the exact failure mode the homeowner already paid for twice.
- Offer two legitimate options: (a) 3.5-ton two-stage with airflow verification; (b) 5-ton or 4-ton with a priced duct retrofit (larger return, filter grille upsized, trunk and branch replacement) that gets TESP back under the blower rating.
- Do not install equipment you know cannot be supported by the air distribution. In Arizona that is a workmanship and warranty exposure, and it invites a Registrar of Contractors complaint you will not win.
- If the customer still refuses, decline the install in writing, document the conversation, and leave the door open. A signed waiver does not transfer the code/workmanship obligation to the homeowner.
3. What duct and airflow checks must be completed before and after the new system goes in, and what do you expect to change?
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Before installation:
- Static pressure profile - filter, return, supply plenum, and total - with a clean filter and with a dirty one, so you know the filter's real pressure drop.
- Room-by-room airflow match against the Manual J room loads.
- Filter grille face velocity. 1,400 CFM through a 20×25 grille is about 400 fpm and roughly 2.8 CFM per square inch of filter - too high. Plan on a larger filter grille or a media cabinet with low pressure drop.
- Branch sizing. A 4-in. flex branch realistically supports about 40–60 CFM; the west bedrooms will need 6-in. runs to hit their loads.
After installation, verify: TESP at or below the blower's rated 0.50 in. w.c. (target measurably lower), total airflow near 1,400 CFM, each register within about ±10% of design CFM, and refrigerant charge set by subcooling/superheat per the manufacturer's chart. Record all of it on the invoice - it is your defense and your upsell documentation.