Disadvantages tankless gas water heater

Tankless water heaters consume more energy per gallon of hot water than slower-heating units, so tankless save money only when consumption is reduced.
Evidence of this is larger gas and electric service required for tankless installation.

The promotion of 'unlimited hot water' is inconsistent with saving money, energy or water.
Tankless require 1/2-2/3 gallon per minute water flow before burner activates, so if water conservation is household objective, tankless may be bad choice.

Tankless burner will shut off with low water pressure, so if several faucets are running same time, hot water will disappear unless tankless is sized for maximum consumption, which means larger burner and more gas.

If you are in MUD district with low pressure or on a water well, you should check household pressure.

Tankless-gas draw more air to meet combustion demands. This air enters house through cracks and works against HVAC.

Tankless installation is usually located near gas meter, which might be further from faucets and tubs, so expect more energy consumption while waiting for hot water to arrive.

Tankless located in cold area has standby cost during winter as internal heater runs to prevent freezing.

Tankless are extremely sensitive to water quality and require correct filter and softener.

Tankless parts are complicated: parts are not interchangeable between brands, problems are tricky to diagnose, and repairs are generally not do-it-yourself. 
Check parts sheet and read manual and ask questions before buying a tankless.
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