My oil-fired hot water boiler is 40 years old and I'm looking at replacing it soon. I will be looking at putting in an indirect hot water heater at the same time. I'll be getting quotes from HVAC contractors, but before the whole process begins, I want to educate myself about how the systems work, available features, and specs and reviews of particular brands of boilers and h/w heaters. One source I've looked at is the consumer guide website for the American Council for an Energy-Efficient Economy.
What other resources would you recommend I use to educate myself?