The 4-Ton Swap in Baton Rouge: Load vs. Duct Reality
Single-story 2,650 sq ft slab house in Baton Rouge, built 1978, 8-ft ceilings, original single-pane windows, ducts entirely in a vented attic that hits 140°F in July. The 4-ton condenser failed and the homeowner wants it swapped, same size, by Friday. Your Manual J run at 75°F DB / 50% RH indoors and 95°F DB / 79°F WB outdoors returns 32,400 Btu/h total: 24,900 Btu/h sensible and 7,500 Btu/h latent. The existing duct system is one 20 x 8 trunk with 18-in flex runouts and nine 6-in supplies; four of the supplies have 20-ft flex runs. With a flow hood, the supplies total 1,150 cfm. Total external static measured at the air handler is 0.92 in. w.c. with a clean filter. The homeowner says, 'Don't put another weak one in - go to 5 tons, I want it cold.'
1. Size the replacement. Justify the number and explain to the homeowner why bigger is worse here.
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The load is 32,400 Btu/h total, so the equipment is a 3-ton (36,000 Btu/h) nominal selection - and even that must be checked against expanded ratings at 95°F outdoor and 75/63 entering wet bulb, because in Louisiana the sensible capacity falls off at design ambient while the latent load does not.
Why not 4 tons, and certainly not 5:
- Latent control. At 7,500 Btu/h latent, a 3-ton with roughly 0.75–0.8 SHR removes the moisture if it runs long enough. An oversized 5-ton short-cycles, the coil never gets cold-soaked, and you get a cold, clammy house at 72°F with 62% RH - which is exactly the complaint that leads to a callback.
- Airflow. A 5-ton needs 2,000 cfm. The existing ducts deliver 1,150. You would be forcing 2,000 cfm through a 1,150-cfm duct system: static goes through the roof, the blower curve falls off, and you get neither the airflow nor the capacity you paid for.
- The measured 0.92 in. w.c. is already excessive. The duct system is a 4-ton-era system that cannot even support 3 tons as it sits.
Present it as a package: 3 tons of correctly rated equipment plus duct corrections, or the homeowner is buying a bigger box that performs worse.
2. Identify the duct deficiencies and specify the fixes, with numbers.
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Deficiencies and fixes:
- Trunk undersized. A 20 x 8 trunk is 160 sq in. of free area, ~1,400 cfm at 0.1 in. w.c. friction, but the long runouts and fittings drop the available pressure. Replace with an appropriately sized trunk - a 24 x 10 or an equivalent rectangular plus takeoff layout - and support it independently of the flex.
- Runouts too small for the length. A 6-in flex at 20 ft with a compression-fitting loss is roughly 60–70 cfm, not the 100+ assumed. Four supplies in that condition are starving. Either upgrade those runs to 8 in. or shorten and straighten them; eliminate the sharp 90° turns and the sagging loops.
- Static reduction. Target 0.5 in. w.c. total external static at design airflow. That means filter grille area, an appropriately sized return, and a clean transition - the 0.92 you measured is mostly the duct and return, not the coil.
- Attic heat gain. The 140°F attic adds 15–25% to the effective load on the flex runs. Buried or R-8 ducts and a radiant barrier are cheaper than a ton of equipment.
Verify the fix with a flow hood afterward: nine supplies at 3 tons should total about 1,200 cfm, no single supply below its design, and static at or under 0.5 in. w.c.
3. The homeowner still wants the 5-ton. What do you put in the proposal, and what do you refuse to do?
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You put the recommendation in writing with the numbers, and you refuse to install a 5-ton on the existing duct system without a signed acknowledgment that it is a customer-directed deviation - or you decline the job.
What belongs in the proposal:
- Manual J summary: 32,400 Btu/h total, 24,900 sensible, 7,500 latent, and the selected 3-ton model with its expanded rating at 95°F/79°F WB showing it meets the load.
- Existing conditions documented: 1,150 cfm measured, 0.92 in. w.c. static, attic duct in a 140°F space.
- Duct scope with quantities and a post-install airflow and static verification.
- If a 5-ton is installed, the required duct system and the price to build it - never a 5-ton on the old ducts.
What you refuse: installing oversized equipment with no duct work and no humidity control plan. That configuration guarantees short cycling, elevated indoor RH, and a callback you will own. Note in the file that the homeowner was advised of the latent and airflow consequences; that note is what protects you when the complaint comes in August.