Power-vent appliances (furnace or gas hot water heater) use a motor and fan to vent exhaust gas through a plastic pipe to the outside of the house.
With power-vent appliances, you do not need a chimney. It may be less expensive to get new power-vent gas appliances than to rebuild the chimney.
However, the power-vent hot water heaters run $900 vs normal (non-power-vent) hot water heaters about $250.
I think all new high-effeciency furnaces use power-vent technology.
If your furnace is over 12 years old, you may be better off replacing it.
By the way, you never said what was wrong with the chimney? Type of chimney? Or any other specifics that may help.
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