When a water heater is connected to an engine’s closed cooling system, certain protocols, as well as the engine manufacturer’s instructions, must be followed. Primary among these is the one governing the location, with regard to elevation, of the water heater. If the heat exchanger within the water heater or any portion of the plumbing between the engine and the water heater is located above the engine’s expansion-tank cap, then a remote expansion tank must be plumbed into the system. This tank, with its own pressure cap, then becomes the primary fill point for the closed cooling system, rendering the original legacy fill cap dormant and unusable. In most cases, when a remote expansion tank is installed, it must use the pressure cap called for by the engine manufacturer; the original cap located on the engine must then be replaced with a higher-rated cap; check with your engine manufacturer for specific instructions. This will ensure that the cap with the highest elevation in the system, the one that can vent air, is the only one that opens and closes with temperature-induced pressure changes.