Tankless Water Heater Install Help

[MattyM]

I have mounted my EcoSmart 27kw in my garage near my breaker panel in order to save on the three runs of 8 ga wire I have to run to it. I started looking at what I need to connect it to the water lines from my current tank water heater which is in the attic this evening. While looking at it I started wondering if running the cold line down to my garage and then the hot line back up to the attic if it would significantly affect my water pressure. That led me to this forum, nice to meet yall. Im Matt in Louisiana.