What must be done to the water tanks before testing water heaters?

Before testing water heaters, the water tanks must be filled. It is essential to have sufficient water in the tank to prevent damage to the heating elements and to ensure accurate temperature readings. When the water heater is filled, it allows for proper functioning and testing of the system's capabilities, including its ability to heat water to the desired temperature within a specific time frame. This practice also helps avoid potential hazards linked to running a heater without water, which can lead to overheating and component failures.

Draining the tanks or cleaning them are steps that might be relevant at different stages of maintenance or troubleshooting but are not prerequisites for the testing process itself. Insulating the tanks is typically done to improve energy efficiency and retention of heat, not for the testing phase. Therefore, filling the tanks is the critical action required before conducting tests on water heaters.