Adding a whirlpool bath is a popular master suite renovation project, but homeowners first should do their homework to make sure they truly can relax in a "hot tub."

Standard 50-gallon heaters can fill these tubs with water, said Jeff Cohen of Montgomery Kitchen & Bath in Gaithersburg. The issue is keeping the water hot.

Jacuzzis with inline heaters keep the water warm via a pump that recirculates it around the tub perimeter, he said. But the system can encourage bacteria growth.

In jetted tubs, a compressor blows heated air into the bath. "It's a much cleaner version," Cohen said.

But jetted tubs do not have inline heaters, so a standard heater suffices for only 20-minute dips.

A luxurious, therapeutic soak of an hour requires more hot water than a standard water heater can supply, Cohen said. So soakers should aim for a tankless water heater system.

A tankless system's lag time depends on its distance from the spigot, so a separate tankless water heater near the master bath is your best bet for hot water on-demand.

Tankless heaters can cost twice as much as traditional ones but save on energy over time.