Oversizing a Furnace and AC in Albuquerque at 5,300 Feet
A homeowner in Albuquerque, elevation about 5,300 ft, has a 2,100 ft² single-story house built in 1998. The existing 80,000 Btu/h, 80% AFUE furnace and 3-ton R-22 air conditioner are 22 years old. The west rooms are cold in winter, and in July the house never gets below 76°F. A contractor proposes a 100,000 Btu/h, 96% AFUE two-stage furnace and a 3.5-ton air conditioner, reusing the existing duct system. The ducts are R-4 flex in a vented attic. Measured external static pressure is 0.95 in. w.c. at 1,200 cfm. The return is one 20 in x 25 in filter grille. The contractor says the extra capacity will 'fix the hot rooms' and that altitude 'doesn't matter for gas.' The homeowner asks for a written opinion before signing.
1. What load, duct, and altitude information must be gathered before accepting the proposal? Explain how Manual J, S, and D apply.
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Before accepting, the contractor should perform a room-by-room Manual J load calculation using the actual envelope: window U-factor and SHGC, insulation, infiltration, orientation, duct location, and Albuquerque design conditions. Manual S then selects equipment using manufacturer expanded performance at 5,300 ft and the design indoor and outdoor conditions, not nominal tons. Manual D sizes the ducts for the selected airflow and checks available static.
The contractor should measure return and supply static, filter pressure drop, coil pressure drop, and room airflows. The complaint about west rooms may be a balancing or duct problem, not a capacity problem. The existing 0.95 in. w.c. external static at 1,200 cfm is already high; adding a larger blower without duct work will not fix it.
2. Evaluate the proposed 100,000 Btu/h furnace and 3.5-ton AC at 5,300 ft. What are the likely consequences?
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At 5,300 ft, a natural gas furnace must be derated for altitude, roughly 4% per 1,000 ft above sea level according to the manufacturer and code, so a 100,000 Btu/h input unit does not deliver sea-level output. Even after derate, the 100,000 Btu/h proposal may be oversized. Manual S requires matching capacity to the calculated load; oversizing causes short cycling, wider temperature swings, and poor part-load performance.
A 3.5-ton AC is also likely oversized for a 2,100 ft² 1998 house in Albuquerque. Oversized cooling runs briefly, removes less moisture, and may leave the house clammy. The correct answer is not more capacity but a load calculation, duct correction, and equipment selection within the load. The contractor's statement that altitude does not matter is wrong.
3. What duct and return-air corrections are indicated by 0.95 in. w.c. external static pressure, and how should the contractor proceed?
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0.95 in. w.c. external static is about double the 0.5 in. w.c. rating of many residential air handlers. That indicates a restrictive filter, undersized return, tight flex duct, or a coil and filter combination that is too small. A single 20x25 return grille is only 500 in²; at 1,200 cfm it is likely noisy and restrictive.
The contractor should use Manual D to resize the return and supply ducts, reduce fittings and flex length, and select a filter with acceptable pressure drop. After duct corrections, verify the air handler fan curve and set airflow to the required cfm. If static remains high, the equipment selection is wrong. The contractor should not simply increase equipment size; that would make the static and noise worse.