Compare behavior, not just the bill
Weather, rates, occupancy, thermostat settings, and equipment runtime all affect energy use. Look for changes in system behavior at the same time as cost changes.
Simple checks
Review thermostat schedules, filter condition, blocked vents, and whether doors or windows are creating obvious heating or cooling load.
When HVAC performance may be involved
Long runtimes, inability to maintain temperature, short cycling, or new comfort problems can justify professional evaluation.
Safety note: If you smell gas, see smoke or fire, suspect carbon monoxide, or believe there is an immediate safety danger, leave the area as appropriate and contact the relevant emergency or utility service. Do not treat an active safety emergency as a routine HVAC appointment.