The answer to 'when to replace heat pump' is that you generally need to replace your heat pump every 10-15 years. Heat pumps have a limited lifespan and as they get older, they become less efficient and more prone to breakdowns. Factors like usage, maintenance, and climate can impact the exact lifespan of your heat pump.
If your heat pump is over 10 years old, it's a good idea to have a professional HVAC technician inspect it annually. They can assess the condition and let you know if replacement is recommended soon. Signs it may be time to replace include reduced heating/cooling performance, increased energy bills, and more frequent repairs.