Why Do the Lights Dim or Flicker When the Tankless Water Heater Turns On?

You switch on the hot water and the tankless water heater starts up. You’ve got hot water at the sink, but the lights start flickering. Why does this happen?

Your lights may flicker when the tankless water heater runs because of the amount of power required to bring the water up to the desired temperature. Home lights may dim momentarily, but will return to normal quickly. If a problem with the lights continues, there may be a loose wire somewhere, or the circuit or household power may be insufficient.

Read on to find out why the lights in your home or apartment may be flickering when the electric water heater, and what you can do to fix the pulsing of the lights for good.

Why your house lights may flicker when the tankless water heater is running

Tankless water heaters, also called electric water heaters, take up less space and are more efficient than traditional water heaters. Water runs over a super-heated element, heating the water immediately before use rather than storing it for later; however, this efficiency does require an influx of energy upfront.

If your house lights flicker when the tankless water heater is running, it could be because of:

1. Extra power draw on start up
2. Temperature set too high
3. Incoming water is unusually cold
4. Overly sensitive LED lights
5. Loose neutral or ground wires
6. Inadequate power from the circuit
7. Inadequate power from the utility company

Some of these potential reasons are much easier to identify and fix than others. Keep reading for what exactly causes each potential issue, and what you can do to solve the problem.

Extra power draw on start up

If the power only flickers or dims for an instant when your water heater comes on, it’s probably not a big problem.

Tankless water heaters draw extra power when they’re first starting up than they do throughout their run cycle. This can cause the lights in your home to fluctuate momentarily.

A flicker when the water heater starts up is likely the result of the circuit being at capacity when the water heater initially pulls power.

If this seems to be your issue, I wouldn’t worry about it unless it gets worse.

Temperature set too high

Your tankless water heater doesn’t need to be set higher than 120°F.

If the water heater’s temperature is set higher than necessary, it will be expending extra energy to heat the water. The additional electricity used here could mean the difference between a steady power draw and flickering lights.

Depending on what the water will be used for – sink, shower, or washing machine, for example – and how far the heated water needs to travel, you may be able to reduce the output temperature of the water and save some energy.

Incoming water is unusually cold

Tankless water heaters heat water to a set temperature, usually 120°F.

Unusually cold water will require more energy than usual to reach the same output temperature. This additional energy draw could cause your lights to dim or flicker while the water heater is running.

Depending on how often this happens through the winter, it could be a sign that you need to look into the circuit that the water heater is on.

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Overly sensitive LED lights

LED lights are longer lasting and more energy efficient than traditional incandescent bulbs, but they’re also more expensive. This tradeoff makes sense as you’re not having to buy them as often or pay as much to keep them on. 

Imagine this, though: light bulbs are on the list, you don’t have time to stop at Home Depot, and all that’s available is the big box brand. Surely they’re good enough, right? Not always.

Poor-quality LED lights can flicker with even minor voltage changes. Swap out one or two led bulbs (that flicker) with a name brand and check to see if the flickering clears.

Since lower-quality LED lights are more susceptible to power fluctuations, replacing the flickering lights with higher-quality bulbs may make all the difference.

Loose neutral or ground wires

A loose neutral wire or poor ground connection can cause the lights to flicker. 

Pick up an outlet tester from your local hardware store and try plugging it into the water heater’s receptacle. It will be able to tell you if the outlet is wired properly, although a loose wire may register correctly when you’re testing it. 

If that doesn’t work, you may want to consider contacting a licensed electrician to inspect your house’s internal wiring.

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Inadequate power from the circuit

The underlying reason the lights may flicker or dim when your water heater runs comes down to power. 

A water heater requires a 120V outlet that is connected to a 20-amp circuit. If for some reason the outlet is connected instead to a 15-amp circuit, running the appliance may overload the electrical circuit. You may also experience this problem if the 20-amp circuit is also being used for other devices.

If this is the case, you may also find that the breaker trips regularly when the electric water heater is running.

Unless you have a lot of confidence in your electrical skills, you’ll want to hire a professional to upgrade the circuit.

Inadequate power from the utility

Depending on your power source, dimmed lights may be a sign that your entire electrical flow is lacking.

A tankless water heater requires a 20-amp circuit to function properly, but that doesn’t mean anything if the household power supply can’t support the household power usage.

Contact an electrician or your utility provider to determine if there’s an underlying problem.

Why your house lights dim when the electric water heater turns on

Power use surges may cause your lights to flicker, but you may also experience either a momentary or a more extended dimming.

Dimming of the lights is usually a result of the drop in available voltage at the lights which can be detected by a good quality meter. Reasons for that drop include:

1. Loose wire connections
2. Inadequate power from the circuit
3. Inadequate power from the utility company

Keep reading for how to find out your specific problem, and what to do about it.

Loose wire connections

The most common reason that your lights will dim while the electric water heater is running is a loose connection.

Loose wire connections at the water heater connection box, sub-panel, or main panel. Inspect the wire connections in these places and confirm that they are securely fastened with no loose wires or sparking connections that can lead to the dimming of the lights.

To do this, you’ll need to turn off the main breaker. Check all points of connection to the live, neutral, and ground bars in the main panel and or sub-panel. Tighten any that are loose. Once everything has been returned to normal, you can power back on the main breaker.

Inadequate power from the circuit

The underlying reason the lights may flicker or dim when your water heater runs comes down to power. 

The electric circuit connections may be overloaded or undersized for the current draw by the water heater. The water heater should have a dedicated circuit.

Most water heaters require 120V from a 15- or 20-amp circuit, although you should consult your owner’s manual for the specifications of your particular unit.

You can check the actual voltage of your water heater using a multimeter.

Inadequate power from the utility company

Depending on your power source, dimmed lights may be a sign that your entire electrical flow is lacking.

It could be a problem too with the supply voltage. Perhaps your house is at the end of the supply line, or there may be a problem with the line.

Does the dimming happen at all times of the day or only during the night during peak hours? Do other appliances with a large power draw such as an AC, or microwave cause the lights to dim?

Contact an electrician or your utility provider to determine if there’s an underlying problem with your power supply.

Is it normal for the lights to flicker or dim when the water heater is running

It may be surprising to find your appliances affect your lighting, but is it actually a problem?

It’s not uncommon for lights to flicker momentarily when an appliance that uses a lot of energy when it starts up first comes on line, but the lighting should not be affected beyond that. If the lights flicker constantly or lights dim significantly, there’s a larger issue at hand.

It should be possible for a water heater to turn on without noticeable dimming of lights for as long as the heater is on. 

Do I need to fix it

Minor flickering of the lights when a large appliance starts up isn’t necessarily a concern, but significant dimming of the lights should be investigated and rectified. 

Loose sparking connections or undersized wires can cause wires and or insulation to burn – potentially causing a fire, injury, and damage to property.

Contact a qualified electrician to examine the wiring to your electric water heater. It’s possible that the wiring used isn’t strong enough to carry the power that the water heater requires, or there may be underlying damage that you wouldn’t be able to find yourself.

Final thoughts on lights that flicker or dim when the tankless water heater is running

Along with the HVAC, oven, dishwasher, washing machine, dryer, and even microwave, your water heater is one of the larger power draws in your home.

The power surge that happens when your electric water heater is activated can lead the lights in your home to flicker or dim. This may be caused by:

• Extra power draw on start up
• Temperature set too high
• Incoming water is unusually cold
• Overly sensitive LED lights
• Loose neutral or ground wires
• Loose wire connections
• Inadequate power from the circuit
• Inadequate power from the utility company

Depending on your specific cause, the solution may be as simple as replacing some light bulbs, or you may need to contact an electrician.

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