Water heaters are steam-driven tube bundle type heat exchangers. A temperature control valve typically modulates steam flow into the tubes to control water temperature. On the condensate side, a trap drains and discharges the condensed liquid into the return system. Steam air coils work much the same way with steam or hot water in the tubes. Fins on the outside of the tubes transfer most of the heat to the moving air.
Temperature control valves again throttle the steam or water flow to hold a desired air temperature.
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