I don't think you have enough information to make an informed decision. If there is no draft stopping and ambient air can follow the conduit up the chimney space I would not expect much temperature rise but if this is only open at the top your mileage will vary, based on how much makeup air could displace the hot air that exits.
It may not get hot enough to support combustion but you have to start derating at 86f/30c.
At a combined temp (ambient plus I2R heating) of 90c/194f your THHN insulation may start failing.
