I have a customer with a strange problem. the Heatilator Nat. gas fireplace that was installed in his home in 1996 has had a strange smell when it is fired since he moved in. He has had about ten different companies out to try and resolve the problom but none have helped him. I am a HVAC business owner and I used to install and service fireplaces with a previous company. We have cleaned every nook and cranny of this unit, checked for CO, tried dampering freshair inlet on draft hood, Did a 10 hour burn and still the smell persists. the homeowner removed all drywall from around the unit to see if anything had been left there from new construction and still nothing. I am a little lost on what my next step should be. I thought maybe installing a power venter on it might help by creating a bit more of a negative pressure in the unit but have never seen this done to a fireplace and wonder if it is safe and or a plausable solution. The unit is a three sided heatilator model # GC920E-NAT. it has a standard b-vent flue and a 4" combustion air line. any help would be appreciated.