What builds up in the tank
The City of Houston’s 2025 water quality report puts hardness on the main system at about 135.5 mg/L, roughly 7.9 grains per gallon. The USGS classes that as hard, and explains that when hard water is heated, calcium carbonate can form solid deposits. In a tank, those minerals settle on the bottom as sediment.
What sediment does
Sediment takes up room meant for hot water and sits between the burner or lower element and the water. Common signs are popping or rumbling while the heater runs, hot water that runs out sooner than it used to, and grit in faucet screens.
How often to flush
Rheem recommends flushing once a year for most households and every six months where the water is hard. Our water heater page says once a year as a minimum. Your heater’s manual has the final word for your model.
Gas and electric are different at the start and the end
On a gas heater, set the gas control to OFF or PILOT before you drain. On an electric heater, switch off the breaker, and don’t switch it back on until the tank is full again: an element powered with no water around it can burn out in moments.
A note on the relief valve
Rheem’s steps open the temperature and pressure relief valve so air can get in and the tank drains freely. On an older tank, a valve that hasn’t moved in years may not close tight again. Opening a hot-water faucet in the house also lets air in. If the relief valve drips after a flush, call us; never cap or plug it.
Tankless units: descaling, not flushing
Rheem notes that tankless water heaters need periodic descaling instead of flushing. A pump circulates a descaling solution through the heat exchanger between the service valves, then the unit is rinsed and its inlet filter cleaned. It’s a pro job on most units; see tankless water heaters.
Sources: Rheem, how often to flush your water heater (and how to do it) · City of Houston 2025 water quality report · USGS, hardness of water. Checked November 2026.
