Well, I'm thinking your heater core is plugged up just like you oil was. What I do is disconnect both heater hoses at the heater core, slip a couple of short lengths of hose on the heater core nipples (so they act as stand pipes)and fill the core with CLR and let it sit for 45 minutes then flush it out with a water hose both ways. This procedure really works, I have been doing it for years.