In a combination system utilizing a water heater for domestic hot water, the temperature of the water returning from the heating loop must be:

In a combination system utilizing a water heater for domestic hot water, the temperature of the water returning from the heating loop must be lower than the minimum temperature required to activate the burner. This is crucial for maintaining efficiency and safety in the system.

When the return water temperature is lower than the activation threshold, it ensures that the burner will fire up to heat the water to the desired temperature. If the return water were equal to or above the activation temperature, the system would not engage the burner, meaning the water wouldn't get heated to optimal levels. Therefore, having the water return at a lower temperature is essential for ensuring that the burner activates appropriately.

This principle helps prevent situations where the system may run inefficiently or not activate at all, ensuring that hot water is available when needed. The design of such systems typically aims for a balance where the return water temperature prompts the necessary heating action from the burner.