I have an in floor radiant heat system using water that’s heated by a tankless water heater. However, there is also a PEX line running from the top of my main hot water heater/tank, which passes through a "Feed Water Pressure Regulator" with the arrow indicating the flow is toward the radiant system. This finally connects to the system just after the outlet from the tankless heater, after the circulator. Does anyone know what the purpose of this connection from the main household hot water tank would be?
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