An S-76 helicopter pilot diverted to a nearby airport when air conditioning fumes misidentified as smoke filled the cabin shortly after takeoff from a nearby heliport.

Narrative

Prior to takeoff I had turned on the cabin heater due to the low outside air temperature. Upon applying power and lifting off the elevated heliport to the north; the Flight Medic in the cabin area stated she smelled something 'funny.' As I turned westbound she said; 'I've got smoke in the back.' I asked her to confirm that she had said smoke and she replied; 'Yes and it is pretty thick.' I was already in contact with ZZZ Tower as we were exiting the Class B airspace. I asked the Tower for permission to land at the FBO ramp area on the north side of runways XXL and R. I stated; 'I have smoke in the cabin.' The Tower responded that I was cleared to land abeam the ramp on the taxiway. She also asked if I needed assistance. I stated; 'Not at this time.' She asked me if I wanted the emergency equipment to roll and I replied again; 'Not at this time.'I complied with the immediate action items on the aircraft checklist which included opening the pilot's door window; turning off the environmental control unit (heater system); and closing the ECU vents. As soon as I did these items; the flight medic said the smoke cleared. Opening the pilot's vent window hastened the dissipation of the smoke and fumes. Upon landing and taxiing to the ramp; we opened the cabin and cockpit doors to further ventilate the cabin and cockpit area.I determined that the 'smoke' was actually the residual water and cleaning solution which had been used earlier that day after a morning flight when the engines were rinsed due to flight in a salt water environment. The fluid had not evaporated and was trapped in the bleed air lines that are part of the heater system. When the heater was activated; the water and cleaning fluid vaporized; causing a fogging of the cabin that resembled smoke in the darkness of the rear cabin. At no time was there any actual smoke. I recognized the smell for what it was but was determined not to make any further diagnosis in the air.Once the smell and vapor cloud had cleared; I was confident that the problem had been with the heater system and that there was no danger of continued flight. We then departed and returned to our home base. My diagnosis was confirmed by maintenance personnel and the daytime duty pilot when they performed a ground check of the aircraft later that morning. They saw only some residual vapor and smelled a slight chemical odor from the heater vents. The Mechanic confirmed my evaluation of the chain of events and recommended running the aircraft engines after any engine rinse to ensure all residual fluid was eliminated from the bleed air lines to the heater.