The “suction trick” is a clever plumbing technique that uses a natural vacuum inside the water heater tank to stop water from pouring out of the drain opening while you swap the valve.
By sealing off all upper entry points where air could enter the tank, the water heater acts like a giant straw. If you submerge a straw in liquid and place your thumb firmly over the top, the liquid stays inside—even if you lift the straw out. The suction trick relies on this exact principle.
Using this method, you can unscrew an old, clogged factory drain valve and screw on a new full-port ball valve on a full tank of water, losing only a few cups of water in the process.
Step 1: Lock the Tank Down (No Heat, No Water In) You must completely eliminate any pressure inside the tank before attempting this.
- Turn off the energy source: Shut off the gas control valve or flip the electrical breaker. Never work on a powered tank.
- Shut off the cold water supply: Close the dedicated shutoff valve located on the cold-water pipe entering the top of the heater.
Step 2: Seal the “Top of the Straw”
To create a vacuum, you must guarantee that absolutely no air can slip into the top of the water heater.
- Close every faucet: Ensure every hot and cold water faucet throughout your entire home is turned completely off.
- Leave the T&P valve alone: Do not touch or open the temperature and pressure relief valve at the top of the tank.
- Wait 10 minutes: Let the water inside the tank completely settle and cool down to a safe handling temperature.
Step 3: Bleed the Residual Pressure
The tank is still holding pressurized water from when the main line was open. You must release this pressure safely.
- Go to a nearby sink: Open a single hot water faucet for exactly 2–3 seconds to bleed off the trapped pressure line, then close it tightly.
- Verify the vacuum: At this point, the water heater is completely isolated. The air trapped at the top of the tank will hold the water in place via atmospheric pressure.
Step 4: Prep the New Full-Port Valve
You must have your new valve 100% ready to install before touching the old one.
- Wrap the threads: Apply 5 to 6 wraps of PTFE teflon tape clockwise onto the male threads of the new full-port brass valve (or the brass nipple, depending on your setup). 2. Apply pipe dope: Smear a thin layer of pipe thread sealant compound over the teflon tape for a bulletproof seal.
- Open the new valve: Keep the handle of the new valve in the open position. This allows any trapped air or minor water trickles to slip through easily as you thread it in, reducing resistance.
Step 5: The Swap (The “Straw” Effect)
Place a shallow pan or a few old towels directly underneath the old drain valve to catch minor spills.
- Unscrew the old valve: Use a wrench to slowly unscrew the old factory plastic or brass drain valve.
- Expect a small glug: As the old valve comes out, a small amount of water will splash out, and you will hear a “glug-glug” sound as the vacuum locks into place. The water will suddenly slow to a minor, manageable trickle.
- Thread in the new valve: Swiftly insert the new, open full-port valve into the opening and start threading it by hand. Turn it clockwise until it catches securely.
- Close and tighten: Once the threads are engaged, flip the new valve handle to the closed position to stop the minor trickle. Use your wrench to tighten it completely until it faces the correct direction.
Once the new valve is securely installed, you can reopen the cold water supply valve at the top, open a hot water tap elsewhere in the house to bleed out any trapped air pockets, and immediately proceed with flushing your sediment out through your brand-new, wide-open valve.