My Norcold DE-351D gave up earlier this summer after 12 years of use on 26' Nordic Tug. I replaced it with another Norcold, model DE0051. New unit included top and bottom vents, plus a fan. The old unit had neither vents nor a fan. With no air circulation, I don't know how it cooled at all or lasted as long as it did. Installation was relatively easy once I removed the rear cabin-entry steps. I removed the bottom mounting pads, and the unit slipped right into the original opening with no modifications. Only hang-up was a factory installed 12V connector which Norcold tech could not identify or provide a mating connector (boat-side) for. They advised me to cut off connector and discard. I suspect connector was for a factory installed truck or RV unit. There is a slight interference between the top vent and the bottom of a cabinet drawer which I still need to address. The new unit is performing well to date. Ron Matuska Two Far Out NTK26-160