After you enjoy a relaxing bath during the cooler moths of the year, leave the cork in and leave the water in the tube for a while.  The water's heat dissipate into your bathroom! 

Try it! after having a bath in colder months leave the used but still warm water in the tub and come back to the bahroom later.  Notice how warm the bathroom is for a long while thereafter?  The heat in the water (which you paid good money for and used good energy to heat!) radiates into the room.  If you pull the drain-plug right after you finish bathing, that heat will escape your house along with the water and heat up your local sewers instead! 

Get full efficiency out of that heated water by leaving it in the tub until it's at room temperature.  If you wash dishes by hand do the same with the used dish-water.

How does this work?  Water is a very efficient 'thermal mass' which means it stores heat, like a battery stores electricity, and releases it when the ambient air around it is cooler than itself.