Goodman Furnace short cycling because of high limit . Cant figure out why?
How many square feet is your house? When was it built? And where do you live? You should never alter the gas pressure from 3.5 inch water column. It has more to do with the flame speed then the capacity. If the pressure is too low then your flame will ride on the burner faces and destroy them. NG burns at about 12" per second and 3.5 inch NG comes out of the orifices at about 12" per second. Without the requested information above answering your question is essentially impossible. Duct size may be an issue, but it's more complicated then it's 8x24. Also if you have air conditioning make sure the evaporator is clean.