I've been doing a lot of reading, and I kept seeing some people recommending 18" of metal supply line clearance from water heaters, some saying it doens't matter, and some saying contact the manufacturer. I just did the latter, and thought I'd post their comments and solicit the forum.

I called Paloma (aka Rheem) and talked to their technical support person. He said there is no need for 18" of metal pipe clearance from the tankless water heaters - I could run pex all the way up to and out of it (minus the drains, check valves, pressure relief etc - they would be copper).

I'm curious why the 18" recommendation from folks - these units themselves have negligable clearances, and the temperature is set at a specific temp (ie. 120 degrees, and most pex is rated at 180 deg or higher). I can understand clearance from the vent itself, but my water supplies are on the bottom, and the vent on the top. Or is this a plumbing code that must be followed?

Forgive my curiousity, I'm an engineer and I just have to understand the WHY?