What Is the Difference Between the ”Coolant” and the ”Water” in a Hydronic Heating System for My Boat?

Question:

I watched the Hurricane hydronic heater  video on your YouTube channel. You mention instant hot water at every faucet, but you also mention "coolant." I think "water" is cirulated? What is the "coolant"?  Are there two piping sytems? A return hot water and return coolant? - Mel

Answer:

For the hydronic heating systems - a piping system is run though the boat and connected to cabin fan heat exchangers for heating, this heating loop is filled with coolant (or glycol). The hurricane system has an on demand hot water system that works by using a heat exchanger to pass hot coolant past a metal plate with your domestic water on the other side which is then plumbed into your hot water system, in place of a hot water tank.