Why would you leak antifreeze only when using the heat never leaks in the summer using the air?

Leaking antifreeze only when using the heat may indicate a problem with the heater core, which is responsible for transferring heat from the engine to the cabin. When the heating system is activated, coolant circulates through the heater core, and if there's a crack or a bad seal, it can leak. In contrast, when using the air conditioning in summer, the heater core is not engaged, so no coolant flows through it, preventing any leaks. Additionally, temperature fluctuations may exacerbate existing weaknesses in the cooling system, leading to leaks only when the heat is in use.