Tankless water heaters do exactly what the marketing says. They produce hot water on demand, indefinitely, and they do not stand there burning gas to keep 50 gallons hot while you are at work. In the right house they are clearly the better appliance.
The part that gets left out of the sales conversation is what hard water does to one, and what that means for the total cost. Since much of Central Illinois has hard water, that part matters here more than it does in a lot of markets.
The genuine advantages
- No standby loss. A tank reheats its contents around the clock. A tankless unit fires only when a tap opens.
- Hot water does not run out. Real benefit for a household with back-to-back showers or a large tub.
- Space. A wall-mounted unit frees the floor a 50-gallon tank was occupying.
- Longer service life. A tank heater typically runs 8–12 years. A maintained tankless unit commonly reaches 20.
- No 50-gallon flood. When a tank fails, it fails by emptying itself onto the floor. A tankless unit does not have 50 gallons to lose.
The hard-water problem
A tankless unit works by running water through a heat exchanger and heating it very rapidly. Hard water plus rapid heating is precisely the condition that drives minerals out of solution — so scale forms directly on the exchanger surface.
As scale accumulates, the unit works harder for the same output, flow rate falls, and eventually it throws an error code. Left long enough it damages the exchanger, which is the expensive component.
This is not a defect and it is not a reason to avoid tankless. It is a maintenance requirement. A tankless water heater in hard water needs an annual descaling flush — and most manufacturers make that flush a condition of the warranty.
What that means for the real cost
Compare it properly. A tankless installation is more expensive up front than a tank swap, because the gas line frequently needs upsizing and the venting is different. Add the annual flush, and — in a hard-water house — realistically add a water softener, which the manufacturer may effectively require anyway.
Against that, put the gas savings, the doubled service life, and the shower that never runs cold. In a household with high hot-water demand it comes out ahead. In a low-demand household replacing a working tank purely to save money, it frequently does not.
Two things that surprise people
The cold-water sandwich
Turn off a tap and turn it back on a few seconds later, and you get a brief slug of cool water before the burner catches back up. Modern units with a small buffer tank largely eliminate it, but on a basic unit it is a real behavior.
Minimum flow rate
A tankless heater needs a certain flow before the burner fires. A trickle at the bathroom sink may not be enough to trigger it — worth knowing before you are surprised by it.
The short version
Go tankless if you have high hot-water demand, you are already dealing with the water quality, and you plan to stay in the house long enough to collect the payback. Stay with a tank if demand is modest and the current unit is not the problem you are trying to solve.
Either way, have someone look at the actual gas line, venting, and water hardness before committing. That is where the real cost difference lives.