Coil Won't Drain on Valve Closure

J.R. Madden

We are in COLD North East. Complete draining of this coil is imperative. WHen I first saw this unit there wasn't an air vent on the coil. We added the vent and PRESTO under daytime operation everything works OK! When we go into night setback it's a diffrent story.

Opening the wye strainer and manually draining about a pint of condensate puts back in operation. There are 2 AHU's on this line. There is never a problem with the one closest to the supply and condensate return. There is a long (50') run to this unit. When the control valve closes the steam in the run condenses and I'm sure creates a slug of condensate.Also the natural vacuum on the coil prevents the coil from draining. The bypass line was in place to allow the condensate in the supply to be pushed through the trap on start up. If I install a vacuum breaker as proposed on sketch, I would hope that the coil would drain. HOWEVER when the vacuum breaker opens would it allow steam to run through the coil backwards through the bypass line? Would it be better to install an end of main trap as well as the vacuum breaker? See attached sketch.

Steamhead

You need both a vacuum breaker

and a trap in the line marked "Bypass". The trap keeps steam from getting into the return. The trap from the coil should have its own connection to the return, separate from the bypass trap.



What pressure are you running? In general a float-and-thermostatic trap works better for this application than a bucket trap.