The heater inlet hose is usually plumbed to the intake manifold, and the heater outlet hose returns to the circulating pump. There are usually two plumbed fittings on the intake manifold, one of which has the water temp sensor, and the other one is plugged unless you have a heater. If you have a heater, there's often a valve there to close down heater circulation (doesn't work very well, IMHO). Likewise the heater return port on the circulating pump is only plumbed if there's a heater. Some people (and some mfgrs) plumb the heater intake line directly to the raw water intake using a Y fitting, the point of which (AFAIK) is to allow more heat to be extracted by the heater core. In that case the heater return line is connected to the barb on the Y fitting, and the usual return port on the circulating pump is plugged.