Long popular in Europe, on-demand (or tankless) water heaters provide hot water seconds after turning on the faucet.
A compact on-demand unit can be tucked into a cabinet or closet. It costs more than a standard water heater but uses less energy, so it may pay for itself in a few years.
It heats 3-5 gallons per minute, plenty for a dishwasher or a shower, but probably not enough for both at the same time.
Installing one calls for running gas lines and supply lines and installing a vent. A gas unit is shown here; electric units are also available.
At least a day to run a gas line, install a vent, and connect plumbing for an on-demand water heater
Drill, screwdriver, groove-joint pliers, tools for running water supply and gas lines, tin snips
Working with supply and gas pipe
Go over your plans with an inspector to make sure the installation will be safe.
On-demand water heater, gas pipe and fittings, supply pipe and fittings, vent pipe and fittings