Can a pellet stove be installed into an unlined masonry chimney?
No. However, a pellet stove, which is a power vented, positive draft appliance may be installed into a clay tile-lined chimney cavity provided that it is a listed appliance, that the manufacturer’s installation instructions explicitly allow it and provided the chimney is lined with a clay tile and is in good, serviceable condition. See 780CMR6007.6 and NFPA211 section 6007.6.13 for regulations pertaining to natural draft (non-power vented) appliances.