Tankless water heaters are fairly new to us, and we're just starting to find out the funny little tricks they play on us. The favorite one is water turning cold while in the shower. That like never happens with a standard vented tank water heater. The tankless is different though. For starters you plug it into electricity to run controls and a fan. With some you get an air intake, often plumbed to the outside and an exhaust, also plumbed to the outside. And don't even tell me you didn't do that one right. So for those wondering if the water turning cold while in the shower is fixable.............you may want to check the filter on the incoming air. It's a simple little thing to try and does need doing often. This was performed on a Navien CR-210 from 2009. Yep.........turning the unit off is a good idea. OMG, did that dirty filter come out of my tankless water heater? Come on, admit it. You've never, ever even looked at the dirty piece of wire screen before. This filter came from the top left of the tankless. And now hopefully you will get that warm fuzzy feeling about your tankless while in the shower again.