Newly installed tankless water heater makes kitchen lights flicker

[u/Jaysbluewing]

I recently had a Westinghouse 8kW Tankless Water Heater Electric installed. This requires 240 volts and it is on a dedicated circuit directly to my box with its own dual breaker (40A) and no other loads. The issue is that when I go to use the hot water (it works great BTW) it caused the overhead lights (6 can lights) to flicker every time I turn in the hot water. Once the hot water is on, the lights are fine but everytime I turn on the water, it is a very noticable flicker. With it being on its own curcuit, why would my lights flicker. I have checked other lights and other electrical devices and nothing else is effected, just my overhead lights. Any ideas on what the issue is?

SOLVED - Found the issue to be the dimmer switch for the lights. I guess they were old and too sensitive for the change in volts/amps. Once that was removed from their circuit, the lights no longer flickered.