66 Safety relief valve Water backed furnace Hot water return Fully automated force draft burner Hinged access doors Exhaust air Tubes Hot water supply Fig 6.12 Commercial hot water boiler (image from the Creative Commons Attribution-Share Alike 3.0 Unported license, original author Dunnd74, English Wikipedia) Fig 6.13 Diagrammatic section of the hot water boiler from Fig 6.12
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