A heater that sits all summer often fails on the first cold night. Scale inside the tubes is the usual reason. That is a professional cleaning, not a thermostat tweak.
Gas, electric, heat pump
Gas heaters need combustion air, a clean heat exchanger, and a condensate path that is not blocked. Electric units fail more from scale and flow switches than from elements. Heat pumps want airflow and a clean coil — leaves against the fan are a San Gabriel specialty. We listen for the inducer, check error codes, and verify flow before we talk about a new unit.
Scale
Hard water plus high pH plus a hot exchanger equals calcium inside the tubes. Restricted flow overheats the unit and trips the high-limit. Acid washing a heat exchanger is not a homeowner job. Annual service before the first cool evening is cheaper than a weekend with a green spa and no heat.