I have a hot water heater that was replaced. It had a 3" diameter exhaust, which according to the town has to be 4" now. The exhaust is in a condo, so the pipe runs through a concrete basement wall makes a 90 degree turn and goes into a common chimney. When I replace the metal exhaust pipe, is there any good way to seal the opening in the chimney. There is probably a 6" diameter terra cotta sleeve (not sure what it really is called) the hot water heater pipe goes into. There appears to be cement sealing the gap between the sleeve and the hot water heater pipe.. After I break it all away and put the new pipe in, is there something better to use other than cement?