Check the nameplate on the old unit, and note its capacity.  It’s safe to purchase a new one of the same size, unless you have recently installed or plan to buy an appliance that consumes a lot of hot water, such as a dishwasher.  A 30- to 50-gallon unit is usually large enough for a home.

Units designed to heat water quickly, called fast recovery units, are more expensive to buy and operate, but they handle peak demand times better.  Standard units heat more slowly but are more economical.

Though water heaters sometimes last 25 years or more, they usually give out sooner – the victims of rust and sediment.  When yours fails, there’s no need to call in a plumber.  Though it may seem like a complicated job, installing a gas water heater involves only two or three pipe hookups, and installing an electric heater requires that you connect some wires.  Removing the old unit is often the most difficult part of the job.

Before you take it out, make sure your old water heater can’t be fixed.  If the tank itself leaks – not the pipes – the lining has rusted and the heater must be replaced.  If your heater is not producing enough hot water, it may suffer from a buildup of rust and sediment, which insulates the water from the burner and forces the heater to work more often to satisfy demand.

WHAT THE PROS KNOW: Draining the heater may flush out enough sediment to make it efficient.  If it still produces too little hot water, replace the unit.