Furnace Replacement Sizing and Permit Review

Existing output is 80,000 x 0.80 = 64,000 BTU/h. Proposed output is 90,000 x 0.96 = 86,400 BTU/h, well above the 48,000 BTU/h Manual J loss. Even the low stage of a two-stage furnace at roughly 60% to 65% of high fire would be about 52,000 to 56,000 BTU/h, still above the calculated loss. The proposal is oversized and will cycle more, reduce comfort, and may worsen duct static. A smaller condensing furnace, such as a 60,000 BTU/h input unit at 96% (about 57,600 BTU/h output), or a properly selected two-stage unit with a low stage near the load, is more defensible if the duct system can handle the airflow.

The measured 0.80 in. wc static is already high on the existing furnace. A 20 by 8 in. trunk has limited capacity, and the return path and filter may be restrictive. Before selecting an airflow setting, compare the blower table to the measured static and required CFM, check trunk and branch velocities, and verify return capacity. If the new furnace requires more airflow than the duct system can deliver, correct the ducts or select an airflow setting the ducts can support. Do not use a higher blower speed to overcome a restrictive duct system without confirming the motor and static limits.

The changeout requires the permit and inspection required by the local jurisdiction, and the work must be performed by the properly licensed contractor. A 96% condensing furnace cannot reuse the existing natural-draft vent connector; it needs the manufacturer-approved PVC or other listed venting, proper slope, condensate disposal, and combustion-air provisions. The permit application and final documentation should show the Manual J load, equipment capacity, venting details, and static-pressure verification. The rebate file should include the equipment model, AFUE, load calculation, and permit/inspection record so the installed system matches the claimed efficiency and size.