I have a tankless water heater. It was installed in 2004. For the past year ( yes, I've been battling the problem that long), my shower will start out hot and then suddenly go ice cold. I called the manufacturer, Noritz, and they had me change the flow sensor, motherboard, ignitors, etc., but nothing worked. I paid a "certified" Noritz plumber $850, but he couldn't find the problem. I recently called Noritz, they told me that I must have a connection between the hot and cold side in one of my single-handled faucets. I replaced the cartridges on all of those faucets. But the problem persists.
I don't want to waste anymore money on a plumber who can't solve the problem. Does anyone have any ideas?