The answer to 'when to replace emergency HVAC service' is that you should replace your emergency HVAC service when your existing system is no longer able to adequately heat or cool your home. This is typically when the system is 10-15 years old and has become less efficient, requires frequent repairs, or is unable to maintain a comfortable temperature in your home.
As HVAC systems age, they become less reliable and more prone to breakdowns, especially during extreme weather conditions when the system is under the most stress. If your system is frequently needing repairs or is not keeping your home at a comfortable temperature, it may be time to consider replacing the entire system rather than continuing to repair it.