Hi Don, thanks!
Correct, the metal chimney cannot be used to vent your new power vent water heater
because all of the vent pipe components must be air tight. You can utilize unused metal chimney / use it as a chase for the new PVC vent pipe. However, there are a few things you should keep in mind:
1. Use the PVC vent pipe tables provided with the appliance to make sure you’re using correct diameters, number of elbows and length of the vent pipe
2. Water heater power vented system does not produce condensation but the condensation can form inside the CPVC, ABS or PVC vent pipe, especially in vertical runs.
The amount of condensation will depend on the environment; climate, metal chimney exterior exposure, proper seal at the top and bottom of the chase – at chimney entrance and roof termination, length of the vertical run. To prevent condensate from flowing back into the motor enclosure and contaminating it, make sure there is a proper condensate drainage kit installed.
3. Install 90° or 45° vent terminal + screen to prevent rain water and rodents / birds penetration.
Let me know if you have other questions