This came to us from Con Edison:
Re: Oil to Gas Conversions

If you previously heated with oil and are now planning to convert to natural gas we are taking this opportunity to remind you that gas acts as a scrubbing or cleaning agent on the oil residue left in your chimney.

Hence, for your system to operate safely and efficiently, we (Con Edison) recommends that you have your licensed gas heating contractor or a chimney cleaning specialist take the following safety measures:

1) The chimney flue should be thoroughly cleaned from top to bottom when new gas heating unit is installed.
2) Six months after the gas heating equipment has been installed an inspection of the chimney should be made to check for the flue soot accumulation and if required, arrangements should be made for its removal.
3) A second thorough cleaning of the chimney should be performed one (1) year after the installation of the heating equipment.

If you retained your oil boiler and installed a new gas burner, the boiler should have been thoroughly cleaned prior to the gas burner installation, and a second cleaning of the boiler should be performed one (1) year later to remove accumulated residue.