Naturally drafting residential chimneys develop draft because the warmer air inside the chimney is lighter than the air outside the chimney. The source of the heated air at the bottom of the chimney burns fuel and creates a column of gases that are lighter than the outside air. The taller the chimney and the hotter the contents - the stronger the draft.
Today, with the upgrading of furnaces to more efficient heating systems that vent their combustion gases through fan-assisted plastic vents, it is often left up to the fuel burning water heater to create enough heat by itself to make a sufficient draft that will vent the combustion gases through a chimney that was previously designed for much more heat - once provided from both the furnace and water heater. The water heater's over-sized chimney now needs a chimney liner.
The need for chimney liners is often ignored when the heating system has been upgraded. This oversight creates the potential for dangerous combustion gases to enter the living space.
Unlined chimneys and those deteriorated from acidic combustion gases should be upgraded, as well, to include stainless steel, aluminum or masonry liners.
Be sure that upgrades to your heating system include a review of the sufficiency of your gas fueled water heater's ability to properly vent its combustion gases.