The temperature of the water returning from the heating loop in a combination system employing a water heater for domestic hot water and space heating must be:

The condition that the temperature of the water returning from the heating loop must be lower than the minimum temperature required to activate the burner is significant for ensuring the safe and efficient operation of the combination system. When the return water temperature is below this minimum threshold, it guarantees that the burner will activate when needed to maintain the desired temperature levels both for domestic hot water and space heating.

If the return water temperature were to be equal to or above this minimum requirement, it could lead to insufficient heating or cycling issues, which would impair the function of the heating system. Therefore, the specification helps in maintaining the system's efficiency, prevents overheating, and ensures proper functioning.

In practice, ensuring the return temperature is below the specified minimum also aids in maintaining a balanced system where the heater does not overheat or supply hot water inconsistently, ultimately contributing to energy conservation and system longevity.