steam used on hot water coils

JackEnnisMartin

Hello
I had an interesting job that proves the old adage you can use hot water on steam coils not the other way aroumd. The unit in question is a Herman Nelson heating unit that is equipped with a coil for hot water. The installers used steam because that is what the buildig is heated by.
The coil has a bleeder line off the top of the inlet side ofhte coil and it runs back to the return line of the system piping. In hot water this makes really good sense the coil cannot be air locked it is allowed to vent to the return side on start up and the coil is full of hot water no air bleeding required. However, this unit [ one of thirty five] is suppled by low pressure steam with a Trane Series A trap on the coil outlet. The call was the room was always too hot. I intially thought the control valve is passing or the damper are not working correctly. After some head shaking none of these causes for overheating made any sense. closing off of the steam valve still allowed the coil to overheat? Finally the cause came to light. The steam trap was not seating and the trap was allowing steam in the main return line to back into the coil. Under normal circumstances this would be self limiting -- you can onll blow so much air into a bottle and it stops flow - no pressure drop. The line on the hot water heating coil line that was intended to allow air to escape made a complete circiut for the steam to travel into the coil and go back into the connection made by the installer into the return pipe after the steam trap. n essence the steam would travel into the back of the coil travel along and find a relief opening provided by the air bleeder line. Once again the so called design engineer [ since retired]purchased whatever the sales person suggested and we have a problem time s thirty - five. The design engineers degree was in English - go figure.

All the best and have a great 2007
Jack Ennis Martin