Water Heater Won't Stop Draining
The Short Answer: A heater that will not stop draining is still connected to the city main through a path that will not close: T&P stuck open, drain valve left cracked, a hose still on the spigot, or cold water siphoning out a failed dip tube. It is not a “clogged drain” problem. Close the cold inlet. If it still dumps, the tank is emptying — kill energy.
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Search Intent & The Real Problem
You opened the drain to flush it, or the T&P started running, and now it will not quit. Fifty gallons plus whatever the street will feed. This page is uncontrolled emptying. A tank that will not empty when you want it to is a different job (sediment-packed drain). Tankless “always draining” is condensate or freeze ports — excessive tankless drain.
What other guides get wrong
They send you to “clear the clogged drain valve” — that is the opposite symptom. They also say let it finish. It will not finish while the cold inlet is open. And they treat a T&P that keeps discharging as a drain problem. Different pipe, different valve.
Step-by-Step Troubleshooting
Step 1: Name the pipe that is running
| Running fitting | What it is | First move |
|---|---|---|
| Plastic/brass hose-thread valve at the bottom, hose attached | Drain valve open, packed open, or handle broken | Close it; if it will not, cold inlet off |
| Brass T&P with a test lever, pipe to pan or floor | Relief stuck open or still lifting | Cold inlet off. Never cap. |
| Cold or hot nipple / flex | Fitting or gasket, not “draining” | Paper-towel; bottom leak |
| Tankless condensate / freeze ports | Not a storage tank | Tankless draining |
| Hot faucet in the house will not quit running | Mixing valve or a fixture, not the tank drain | Fixture off; then the heater |
- What to look for: Which hole is actually moving water.
- Required tools: Flashlight, paper towel. No cap on the T&P.
Step 2: The isolation test — this is the whole diagnosis
- What to look for: Close the cold inlet (usually the handle on the pipe into the top or side). If the stream dies in a few seconds, street water was feeding the leak — drain valve or T&P or a fitting. If it keeps running until the tank is empty, you are watching stored volume leave; same two fittings, but now you know the inlet is shut.
- Required tools: The cold shutoff. If that valve will not close, shut the house main.
Open a hot faucet after the inlet is closed so the tank can breathe while it dumps. Do not run an electric element dry. Breaker off first.
Step 3: Drain valve left open — the save
- What to look for: Garden hose still on the factory plastic 3/4” drain. Handle at 45 degrees. Sediment chunk holding the seat open. Cracked Rheem plastic drain that will not shut.
- Required tools: Two wrenches with a backup on the tank bushing if you are replacing it, 3/4” brass ball drain, bucket. No floor drain? Drain with no floor drain.
If the tank bushing spins with the valve, stop. That is a replacement heater, not a $12 valve. Do not gorilla plastic “snug.”
Step 4: T&P that will not quit
- What to look for: Stream from the discharge opening, not a weep at the threads. House static over 80 psi. Waterlogged expansion tank. Lever that someone tied or that did not reseat after a “test.” Streaming vs dripping: stuck open vs leaking after testing. Typical stamp 150 psi / 210°F.
- Required tools: Hose-bib gauge after you have isolated. Do not plug the pipe.
A T&P that is the drain path is doing its last job. The repair is pressure, expansion, or the valve — not a cap. If the tank ran low on an electric, treat it as a dry-fire.
Step 5: Inlet siphon, dip tube, and “it drains from the faucets”
- What to look for: Cold inlet off, drain and T&P dry, but a hot faucet still runs a long time — that is the tank emptying through plumbing (normal if a tap is open). Cold inlet on, water pouring from an open drain you thought you closed, plus lukewarm showers for months: broken dip tube can mess temperature; it does not usually siphon the tank by itself. A hose on the drain dropped into a sink below the tank will siphon if the valve is cracked.
- Required tools: Eyes on the hose end. Lift it out of the standing water.
When DIY Ends: Calling a Licensed Pro
Stop if the cold inlet will not close, the bushing spins, the T&P is streaming, the jacket is wet, or the closet is at 240 volts and flooded. You may close the inlet, kill energy, and shut a drain valve that still has a handle. You may not cap a T&P or re-energize an empty electric tank. A licensed plumber isolates, replaces a brass drain or a T&P, sets the PRV/expansion tank, and tells you if the shell is the real leak.
Frequently Asked Questions
Q: Why won’t my water heater stop draining? A: The cold inlet is still feeding a path that is open: drain valve, T&P, or a hose siphon. Close the cold shutoff. If it still runs, the tank is emptying through that same path.
Q: Should I cap the overflow so it stops draining? A: No. Never cap a T&P. Close the cold inlet and shut energy off. A capped relief valve is a rupture and scald risk.
Q: Can I keep the heater on while it drains? A: No on electric — a 4500 W element dry-fires in seconds. Gas: kill the control so you are not firing a falling waterline. Fill completely before you restore energy.