A water heater typically uses much less energy than a furnace, but it also costs less to replace, so replacing a water heater often turns out to be cost-effective.
Natural gas water heaters are now available that are as much as 74% efficient, compared to as low as 49% for old units. Electric water heaters are 100% efficient, but electricity costs more than gas, so they are actually more expensive to operate. Heat-pump water heaters are as much as 400% efficient and can often pay for themselves as quickly as gas, even though they cost more. The most efficient water heaters are solar, but not all houses get enough sun exposure to make solar water heating worthwhile.
We often find gas water heaters that are not drafting properly -- that is, their exhaust flows into the house instead of out the flue pipe -- or that are producing far too much carbon monoxide. Either of these issues by itself is not a problem, but when they are combined, there is grave danger of carbon monoxide poisoning! The immediate solution is to have the water heater repaired; however the long-term solution is to replace it with a sealed-combustion unit, or with an electric or solar one that does not produce carbon monoxide.
Commonly when someone's gas bill is high all year 'round, it is because they are using warm or hot water in the washing machine. Modern washing machines do not require hot water to get clothes clean. An Efficiency Kansas loan will not cover replacing a washing machine, but doing so at your own expense will usually pay for itself within a year or two.