Available on PPI’s website, PPI Recommendation H Direct Connection of Plastic Piping Materials to Tankless Water Heaters for Domestic (i.e. residential) Applications provides clear guidance on the question of direct connection of piping materials to tankless water heaters.

"MAB-3 Model Specifications Guide for PE 4710 Buried Potable Water Service, Distribution and Transmission Pipes and Fittings" covers system design parameters, quality control, pressure classes and more.