I've been installing/servicing water boilers for several years . Came across a strange situation. After installing a replacement boiler, setting up the gas valve to the correct pressure there seems to be a strange smell coming from the area of the boiler itself. I've check the Co levels at all locations, zero on the readings.
I've used the recommended S.S. vent pipe, within normal lenghts. I thought it was the normal start up burn off, however with two weeks past it still produces a faint smell throughout the garage where it's located. I've double checked for enough fresh air supply, thats normal. The homeowner describes it as a "dead animal smell",,, yes it did check the flue for any furry critters. none there, and none to seen in the burn chamber area.

Any suggestion to the cause??

thanks ahead of time