Rheem RTG20213A MIC-6 Manifold Controller System Features:
• Connect up to 6 tankless water heaters together in a Manifold
• This kit mounts inside a single tankless unit and connects to up to 5 additional units for a total of 6. 
• For the RTG-95, RTG-84, and RTG-64 Series models only
• This kit will not work on any model manufactured before Jan 2010.
• Model: RTG20213-A
• Rheem offers many venting accessories to make your tankless water heater installation easy.