Yearly water heater service like flushing your tank once a year to get rid of the mineral sludge that accumulates at the bottom is recommended.
The sludge that builds up inside your water heater can cause a knocking/bubbling sound when the water heater is firing. This sludge settles at the bottom of the water heater where the gas control valve is. Attached to the gas valve is a thermostat that sticks inside the tank about 5 inches. This probe gets coated in this sludge and causes it to read temperatures incorrectly. The water heater can overheat itself causing the temperature and pressure relief valve to open and leak.
While your tankless water heater is being flushed we remove and clean filters and remove and clean flame sensors. All piping and valves are checked and exercised. The unit’s venting is checked as well as gas lines. f you have a recirculation pump attached and the connecting piping is galvanized it should be checked for blockages or replaced with brass fittings.