Water Won't Drain From the Water Heater
On this page (6 sections)
The Short Answer: When a tank heater won’t drain, or drains to a trickle and stops, the usual cause is sediment packed into the drain valve opening. The other common cause is a vacuum: no hot faucet is open, so air cannot enter the tank as water leaves. A kinked or uphill hose and a broken plastic valve stem are next. Clogged valves can usually be cleared or replaced without replacing the heater.
Set up before you decide it’s clogged
Several simple things make a healthy drain look blocked:
| Problem | What happens |
|---|---|
| No hot faucet open | Vacuum in the tank; flow slows to nothing |
| Cold-water inlet still open | Tank refills as fast as it drains; never empties |
| Hose runs uphill, kinks, or ends in a full drain | Little or no flow |
| Plastic valve handle turns but stem is broken | Handle moves, valve stays shut |
| Hard-water tank never flushed | Sediment plugs the valve opening |
Before draining at all, turn off the energy (breaker off for electric, gas control to OFF for gas) and let the water cool for a few hours if you can, since water from the bottom of the tank can scald. Close the cold-water inlet valve and open a hot faucet somewhere in the house to let air in.
Sediment in the valve
Scale and grit collect at the bottom of the tank, right where the drain valve is. Factory drain valves often have a small internal passage, especially plastic ones, so a few pieces of scale can block them. Signs include a burst of gritty water followed by a trickle, or flow that starts and stops.
What you can try safely: close the drain valve, wait a minute, and open it again. Sometimes a piece of scale shifts. If water still won’t flow, stop. Forcing a plastic valve or pushing wire into it with a full tank can crack the valve or push the blockage tighter.
What the plumber may do: with the tank cool and depressurized, they may briefly back-flush the drain, or remove the valve and clear the opening, then fit a full-port brass drain valve that passes sediment far better than the factory valve. They will keep the tank fitting from turning while removing the old valve; a damaged tank fitting can mean a new heater. If you need to drain to a place other than a floor drain, see how to drain a water heater with no floor drain.
When it’s not the drain
- Water comes out of the relief-valve pipe instead. That is pressure or temperature, not a drain problem. See overflow keeps discharging.
- You’re trying to drain because the tank leaks. Draining limits damage, but a leaking tank needs replacing. See leaking from the bottom.
- Indirect water heaters heated by a boiler have both a domestic tank and a boiler loop. Make sure you are draining the domestic side; the boiler loop should only be drained by a heating technician. See Weil-McLain indirect problems.
Refill before turning it back on
Close the drain valve, open the cold-water inlet, and leave a hot faucet open until water runs steadily without air. Only then turn the breaker back on or relight the gas following the label. Powering an electric heater that is not full will burn out an element; see turned on without water.
Repair or replace
A clogged or broken drain valve is a minor repair. A heater only needs replacing if the tank leaks, the tank fitting is damaged, or the tank is at the end of its life anyway. If the tank is so full of sediment that it rumbles constantly, see sounds like rocks.
Frequently Asked Questions
Q: Why does my water heater drain so slowly? A: Usually because no hot faucet is open to let air in, or because sediment is partly blocking the valve. Garden hoses also slow the flow, especially long ones.
Q: Should I replace the plastic drain valve with brass? A: Many plumbers recommend it, especially in hard-water areas. A full-port brass ball valve is less likely to clog or crack and makes future flushing more effective.
Q: Is it safe to leave the heater empty for a while? A: Yes, as long as the energy stays off. Make sure no one turns the breaker or gas back on until the tank is refilled.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- A. O. Smith technical bulletin 51: puddles around a heater (opens in a new tab)Common water-leak sources and the distinction between fittings and a failed tank.
- A. O. Smith: standard electric no-hot-water diagnostic chart (opens in a new tab)Professional diagnostic context for standard electric storage heaters; not instructions to open or test your appliance.
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