The answer to 'how often to replace emergency HVAC service' is that there is no set schedule for replacing emergency HVAC service. Emergency HVAC service, such as repair of a broken furnace or air conditioner, should only be needed on an as-needed basis when your system experiences an unexpected failure. The frequency of needing emergency HVAC service will depend on factors like the age and condition of your HVAC equipment, how well you maintain it, and how heavily you use it.
Most HVAC experts recommend scheduling regular preventative maintenance, such as annual tune-ups, to help catch small issues before they turn into emergencies. Proper maintenance can extend the lifespan of your HVAC system and reduce the need for unexpected, emergency repairs. However, even with good maintenance, emergency issues can still arise, and you should be prepared to call for professional emergency HVAC service when needed.