Lights Flicker when Tankless Water Heater is Switched On (What to Do)

You switch on the water heater and while it heats the water alright, one or several lights start flickering.

Why does this happen? What checks can you do and how can you get the lights steady again?

This post lists the possible reasons why lights might flicker when a tankless water heater is switched on, and what you can do to stop the flickering of the lights.

Note: Electricity can kill. Always use the services of a qualified electrician.

Checks you can Do

Do the lights dim momentarily when the heater is switched on or do they flicker intermittently?

Is it one particular type of LED or do all LED types flicker? If one or a few lights flicker while the rest do not then it’s probably an issue limited to an individual circuit or the LED fixtures themselves.

What can Cause the Lights to Flicker

If the lights flicker when a tankless water heater is switched on:

#1. Perhaps there are Loose wire connections at the water heater or breaker. Have an electrician check and confirm that the wire connections at the breaker or water heater are tight and firm.

The power draw of the tankless water heater can be substantial. If any of the wire connections on the heater circuit are loose, there’ll be sparking and flicker in adjacent lighting circuits.

Contact a qualified electrician to inspect and ensure that the wire connections are tight.

#2. Perhaps the electrical circuit supplying power to the water heater is overloaded. Have an electrician confirm this.

If overloaded, it is possible that a drop in voltage may be causing the flicker.

You can confirm if this is the cause by using a good quality voltmeter to measure the circuit voltage before and after switching on the heater. Does the voltage drop?

#3. The service cable supplying power to your house may be undersized

Confirm with the power company that the service cable to your house and the current (amps) are appropriately sized and sufficient to meet the power requirements of the water heater.

Some water heaters draw substantial power when switched on which causes a voltage drop and flicker of the lights.

Closing Thoughts

If the lights in your house flicker, it may be a result of loose wire connections at the water heater or circuit breaker, an overloaded electrical circuit wiring supplying the water heater or a case of an undersized service cable to you house.

Always contact and use the services of a qualified electrician.

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