Tankless vent requirement

[rfcomm2k]

I wish to replace my existing oil fired baseboard heat and domestic hot water with a gas fired tankless system. I realize that I will need at least 2 separate units, one for the domestic hot water and the other for baseboard heat. I have done the studies and compared the costs of using one of these for baseboard heat and this is not an issue.
My problem is that a local plumber claims that the tankless heaters he sells (Rinnai) cannot vent into the existing chimney structure. Surely there must be SOME product out there that will vent this way, or else a fan product that will assist with the vent.
I am desparate to get rid of this filthy oil burner in favor of a gas unit, and really like the economy of a tankless.

[DUNBAR PLUMBER]

You will have a hard time finding a tankless that doesn't vent through it's own specifications that separate it from any other existing flue structure.

Too many variables that can/could affect the operation of the tankless.


I would seal off the chimney and find a route for the tankless heater and be done with it. Going away from MFG. specs only complicates the install.