[Greenbuilding] vent waste heat recovery--in reverse?

[Alan Abrams]

   still perserverating about improving energy efficency in my home--a 250
   unit, 18 story condominium...
   with maybe 18 soil stack vents, times +/- 160' per stack...or more than one
   half mile of at least 4" id pipe...thermosiphoning air from the sewer,
   through tempered space, and out through the roof.  Assuming winter time
   ground temps (for a relatively shallow sewer) at 45-50d F, is there enough
   flow, and great enough delta, to try and recover this heat?  if so, how
   would one do it?  perhaps the outflowing air could assist a heat pump, which
   could in turn pre-warm water bound for the water heater.

   crazy?

   Alan Abrams AIBD
   Abrams Design Build LLC
   202-726-5894 o
   202-291-0626 f
   www.abramsdesignbuild.com