Hydronic heating systems utilize a heat source transfer fluid, such as boiler antifreeze, to deliver it to where it's needed throughout the RV. The transfer fluid is piped through a copper coil to heat water and through piping to heat radiators and other radiant heating components around the RV.
They can employ several heat sources such as diesel in large diesel pusher motorhomes, gasoline, propane and electric heating elements. They are also able to transfer heat between the engine cooling system and the hydronic heating system. This delivers free heat while driving or the ability to use the hydronic system to pre-heat the engine in cold weather. Many hydronic systems utilize multiple heat sources such as diesel, engine coolant and electric heating elements.
The downside of hydronic heating is its cost, both its initial cost as well as ongoing maintenance and parts. They must be serviced annually.