Water heater runs while using cold water

Have Z3100 -0132 and have noticed that while running cold water in the toilet or bathroom sink the water heater will fire up. Does not seem to do this when using kitchen faucet . Any ideas

Tankless water heaters have a small wheel inside that measures the velocity of the water passing through it and based on that trigger the burner to come on. The only way that wheel would turn is if hot water was flowing thought the water heater.

I would assume that the hot and cold lines are reversed to the faucet but you say the toilet is also making hot water?

If it was just the faucet I would assume it is swapped water supply lines. If it is also coming from the toilet and you are sure you have warm water in the toilet then I would look in the wet bay and trace the water lines. Brinkley color codes them blue (cold) and red (hot) for the lines so fairly trivial to make sure the one coming from the toilet does not join somehow to the hot water side.

This seems something that the factory or dealer should have picked up on during the PDI; crazy it was not found.

I totally agree with what you have said, but am not sure that this has been happening since new. Never noticed it until a couple of weeks ago
Don’t get hot water at the toilet but can hear the water heater fire. Did a Quick Look in the bay but nothing looked out of place, will have to wait for rain to stop to dig into it

I would test each water source to see which ones trigger it. Could it be cold enough that the self heater is firing or are you getting hot water?

Keep us posted on what you found!

After checking all water lines to verify no crossed lines decided to increase water pressure from 35 lbs to 45.
The issue seems to have disappeared, so not sure if pressure was the issue or if one of the taps was not closing properly and allowing a bit of flow from hot side but all seems well at the moment.

I’ve also noticed the water heater will fire up when I flush the toilet. I was wondering if somehow it had been connected to the hot water, but I haven’t crawled in there to check it.
I need to trace the line, or hold it down long enough to see if it gets hot.