Sunroom Addition: Rule-of-Thumb Sizing and Duct Capacity
A homeowner with a 2,000 sq ft, 1998-built house in Houston has a 3.5-ton split system and wants to add a 300 sq ft sunroom with 12 windows and no shade. The existing trunk is 20 in. x 8 in. galvanized, the return is one 20 in. x 20 in. grille, and the measured external static is 0.80 in. w.c. at 1,400 CFM. The contractor proposes a 4-ton unit because 500 sq ft per ton gives 4.6 tons, rounded down. The existing contract is a one-page bid with no load calculation, no permit, and a 50% deposit. The homeowner asks whether the 4-ton unit will solve the problem.
1. Evaluate the contractor's sizing method and recommend the correct approach.
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The 500 sq ft per ton rule is not a load calculation and should not be used to select equipment.
- The sunroom adds 300 sq ft of highly glazed, unshaded space. Its sensible and latent load per square foot is much higher than the existing house average, so area alone says little about tonnage.
- A proper Manual J load calculation is required using Houston design conditions, actual window U-factor and solar heat gain coefficient, orientation, insulation, infiltration, and duct location. Equipment selection then follows Manual S.
- A 4-ton unit may be oversized even if the total square footage suggests 4.6 tons by rule of thumb. Oversizing causes short cycling, poor dehumidification, and higher humidity in a Houston summer.
- The existing 3.5-ton unit may already be oversized. The correct answer may be a load calculation showing that the existing capacity plus modest duct and envelope improvements is adequate, rather than a larger unit.
2. Evaluate the duct and return capacity for a 4-ton system.
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The existing duct system is not adequate for 4 tons at the measured static.
- 4 tons requires about 1,600 CFM. A 20 in. x 8 in. trunk has 160 sq in., or 1.11 sq ft. At 1,600 CFM the velocity is about 1,440 fpm, which is too high for a residential supply trunk and will add pressure drop and noise.
- The 20 in. x 20 in. return grille has 400 sq in., or 2.78 sq ft. At 1,600 CFM the face velocity is about 576 fpm; with filter grille free area around 70%, the actual velocity is closer to 820 fpm, which is marginal to high.
- Measured external static is already 0.80 in. w.c. at 1,400 CFM. Adding 200 CFM without enlarging the trunk and improving the return will push static higher, reduce airflow, and repeat the low-capacity symptoms the homeowner is trying to solve.
- Perform a Manual D duct design for the new airflow and correct the trunk, branches, and return before changing equipment size.
3. What Texas licensing, permitting, and business-practice issues does the proposal present?
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The proposal has compliance gaps that must be corrected before work begins.
- The contractor must hold the appropriate Texas air conditioning and refrigeration contractor license through TDLR and use licensed technicians as required. Verify the license and insurance before signing.
- A mechanical permit is required for the equipment replacement and duct work in most Texas jurisdictions; the contractor should pull the permit and schedule inspection. Doing the work without a permit exposes the homeowner to failed inspections and resale problems.
- The one-page bid with no load calculation, no scope, and no equipment model numbers is inadequate. Provide a written contract identifying equipment, SEER2/HSPF2 ratings, duct scope, permits, warranty terms, and payment schedule.
- Do not rely on a large deposit to start work without a clear contract. Address the sunroom load with a Manual J before accepting the 4-ton recommendation, and document the final sizing and duct design in the contract.