how to convert brick chimney to metal pipe?

[xpinfo123]

This is a 50-year old, single-story house. It has a brick chimney that is in poor condition(chimney is broken in the attic and moves if I push on it, chimney needs repointing, etc). The chimney has a round 6" terra-cotta liner. The chimney is concrete block from the basement up to the attic floor and then brick from the attic floor to the roof. The chimney is used to vent the natural gas water heater and natural gas furnace.

I would like to remove the chimney(brick portion) from the attic floor to the top of the chimney and then replace this with a metal pipe. I want to do this because: 1) the chimney is broken and may not be safe,

2) roofing/flashing around a brick chimney is a lot of work and it seems that a metal chimney would be much easier to maintain.

How do I do this? I've looked at "Type B" vent pipe. Is this the correct product? If so, then it seems straightforward except for connecting the "Type B" pipe to my 6 inch terra cotta liner. How do I do that? Is there a special adapter? What is it called? Will I also need some kind of high-temperature silicone?

thanks in advance

[Art]

How old is your gas furnace and water heater. Modern efficient ones don't need a chimney. They can vent thru a wall with a short pipe, sometimes a double pipe for intack and exhaust. Before spending too much money on the chimney, maybe it would be cheaper to replace the water heater and furnace.

[xpinfo123]

I'm not sure about the age of the appliances. They are probably 15-20 years old. But I am fairly sure that they are not the high-efficiency models that can vent via PVC through a wall. I did consider replacing them, but I thought that modifying the chimney would be much cheaper(perhaps $100, DIY) than replacing the furnace($2000-ish, contracted) and water heater($400, DIY).

[Trekking Tom]

By replacing your heating system with a High Eff. one you will spent more but your heating bills will also go down.

I would use a stainless steel flexible liner for the chimney up to the level or the attic floor, type b gas vent after that. Repair the roof where the chimney was. ( mortar in where the masonary chimney ends and the type b starts)

Tom