How to Turn On a Water Heater After a Shutoff
How to turn on a water heater safely comes down to one rule: fill the tank completely before you turn on the heat. Open the cold supply and a hot tap, let the tap run until the water flows steadily with no air bubbles, then switch the breaker on for an electric heater or turn on the gas and relight the pilot for a gas heater.
Skipping the fill is the classic mistake. A.O. Smith warns that turning on an empty electric water heater can damage the heating elements.
Step 1: Refill the tank (every heater)
Close the drain valve
if the tank was drained, and remove any hose.
Open a hot water tap
somewhere in the house. A tub spout works well because it runs fast and shows air clearly.
Open the cold water supply valve
at the top of the heater.
Wait for steady flow
At first the tap will sputter and spit air. When you get a steady flow without air bubbles, the tank is full. A.O. Smith says to let it run for 3 minutes to purge all the air.
Close the tap
and check the heater's fittings and drain valve for drips.
If the tank was never drained, for example after a short vacation, you can skip this step. Just make sure the cold supply valve is open.
Step 2a: Turn on an electric water heater
Confirm the tank is full
(see the refill steps above).
Turn the breaker back on
at the main panel.
Give it time
A.O. Smith says the first tank of water takes 1 to 2 hours to heat. After everyday use, an electric tank's recovery runs about 60 to 90 minutes.
If the water's still cold after a couple of hours, check the reset button. A.O. Smith says to turn off the power, then press it firmly until it clicks. If it trips again, the heater has a real fault and needs a professional. Our reset button guide explains why it trips.
Step 2b: Turn on a gas water heater
Confirm the tank is full
(see the refill steps above).
Open the gas supply valve
on the pipe to the heater, if you closed it.
Find the lighting label
on the front of the tank. It gives the exact steps and waiting times for your model. Follow it, not a general guide.
Relight the pilot,
if your heater has a standing pilot. The usual sequence on the label is: turn the control to OFF and wait for any gas to clear, turn it to PILOT, hold the knob down while you light the pilot, keep holding for the time the label specifies, then release and turn the knob to ON.
Watch the burner light
Once the control is on ON and the temperature is set, the main burner should fire with a steady blue flame.
Many newer gas heaters have electronic ignition instead of a match-lit pilot. On those, you turn the gas control on and the unit lights itself, or you press an igniter button. Your label will say which you have.
A gas tank recovers faster than an electric one, about 30 to 40 minutes after normal use, though a first fill takes longer. For step-by-step pilot help, see how to light a water heater pilot. If it won't light at all, see pilot light won't light.
Step 2c: Turn on a tankless water heater
Open the hot and cold isolation valves
if they were closed.
Turn the gas back on
on gas models.
Purge the air
A.O. Smith's tankless guide says to run water for at least 3 minutes before turning the unit on.
Restore power
at the breaker or the unit's switch, following the manual.
Run a hot tap
until the air clears and the sputtering stops. Rinnai says that can take 2 or 3 minutes.
Tankless units heat on demand, so there's no waiting for a tank. If you get an error code on startup, look it up in the manual before resetting it.
Set the temperature
A.O. Smith recommends 120°F as a good balance between energy savings and hot water supply, and the same setting appears in its troubleshooting guide as the balance of heating speed, cost and safety. You can go up to 130°F if you need more hot water, but hotter water scalds faster. Most gas dials have no degree marks, so check the tap with a thermometer after the tank recovers.
If something's wrong after startup
- No hot water at all: Electric, check the breaker and reset button. Gas, check that the pilot is lit and the gas valve open. Then see no hot water.
- Air sputtering at the taps: Normal for a few minutes. Run each hot tap until it clears.
- Pilot lights but won't stay lit: Usually the thermocouple. See pilot light won't stay lit.
- Water dripping from the relief valve pipe: Some discharge can happen as the tank heats in a closed system. A steady drip isn't normal. See relief valve leaking.
Mistakes to avoid
- Turning on power before the tank is full.
- Relighting a gas heater from memory instead of the label.
- Cranking the thermostat to make it heat faster. It doesn't. It just makes the water hotter when it gets there.
- Ignoring a gas smell because the pilot lit. Shut it down and call the utility.
FAQ
How do you turn on a gas hot water heater?
Fill the tank, open the gas supply valve, then follow the lighting instructions on the heater's label to light the pilot and turn the control to ON. A.O. Smith suggests checking gas connections for leaks with soapy water after any work on the line.
How do you restart a hot water heater?
Make sure the tank is full, then restore the heat: breaker on for electric, gas on and pilot lit for gas. If an electric heater won't heat, press the reset button with the power off.
How long after turning on a water heater do you get hot water?
A.O. Smith says the first tank takes 1 to 2 hours to heat. Normal recovery after use is about 30 to 40 minutes for gas and 60 to 90 minutes for electric.
How do you turn on a tankless water heater?
Open the isolation valves, turn on the gas, purge the air, then restore power. Run a hot tap until the sputtering stops.
Sources
- A. O. SmithWater Heater Installation Guide (opens in a new tab)
- A. O. SmithWater Heater Not Working? Troubleshooting Guide (opens in a new tab)
- A. O. SmithHow to Drain and Flush a Water Heater (opens in a new tab)
- Rinnai13-Step Tankless Water Heater Flushing Procedure (opens in a new tab)
- A. O. SmithTankless Water Heater Buying Guide (opens in a new tab)