For example, I grew up in a 1960s vintage ranch house heated by an oil-fired steel fire tube boiler. The boiler was equipped with a tankless coil for domestic water heating. Upstairs was a single thermostat that controlled the entire 1,500-square-foot house.
When I replaced that boiler in 1989, the fire tube baffles were completely burned away. This helped explain the net stack temperature of about 550°F. Although this boiler kept our house comfortable for over 30 years, its "aged"