On-demand water heaters heat water only when there is
a demand for it - when water flows through it. This is
accomplished with a small pilot light and a flow sensor. You
turn on the tap, water flows through the unit, and the burner
flame comes on and heat the water as it rushes through the
coils. The flame is modulated by the water flow. Water
flow stops, and the burner stops. You
can see that this has two advantages. You only heat the
water you use, saving propane, and you have an endless supply
- you aren't limited to the 6 gallons of a standard tank unit. The
key is that the unit must be sized right, so that it's GPM
capacity is greater than the demand flow of the shower or faucet.