My house has a gas based water heater(geyser) on the outside. The plumber has connected it such that without the use of electric pump, just through pressure alone, hot water reaches the 1st and 2nd floor. How does this work?
The water coming to your home is under pressure. Typically about 60 psi.
Imagine a bucket with a lid and in the lid you have two holes cut. In one hole, you place a garden hose that fits snugly. You turn the water on and the bucket begins to fill. When the water reaches the top of the bucket, it will start to come out of the second hole.
Your water heater works the same way. The tank has two holes. One for water in and one for water out. The water coming into the tank will push water out of the outlet and up through your house because of the pressure.