How It Works
When you open a hot water tap, the flow of water triggers the heater to turn on. In a gas tankless heater, the powerful burners will turn on rapidly to heat the water as it passes through the unit. Most units take about three seconds to ignite and an additional three seconds to get up to temperature. The time it takes to get hot water to a tap is directly related to the piping diameter and distance to that fixture. Most units require a minimum flow rate to trigger the burner, approximately .5 to .75 GPM.
a tankless water heater only operates when the demand for hot water occurs, consequently one could realize a savings of anywhere from 11% to 69% off your hot water heating bill, depending on ther size and type (electric or gas) of your old water heater. Most homes use hot water for a cumulative total of about one hour per day, yet they are heating 40 to 50 gallons of water hot 24 hours a day. A system which eliminates the storage and heats water only as you use it can dramatically reduce your energy consumption. Most tankless water heaters do not have a standing pilot which will further contribute to energy savings.
A tankless water heater uses a different type of vent material than a standard water heater. Most of the modern tankless units are Category lll gas appliances and therefore require a tightly sealed stainless steel flue piping which allows for horizontal venting within the distance guidelines established by the manufacturer. The horizontal venting capability of these units allows for a variety of location options.
The tankless heater may take longer to get hot water to a tap than a tank water heater, therefore water consumption is a factor to consider prior to purchase.
The tankless water heater has a limited flow rate which is most noticeable when you exceed it, such as when using fixtures with unrestricted flow (bath tub fillers). It is important to be aware of these limitations or plan your installation with a unit that can overcome those limitations.
Most tankless water heaters have an energy factor (EF) of about .84 for natural gas. Some condensing units may have and EF of .90 or more. Tank type heaters generally have an EF of about .63 when new and decrease with mineral buildup inside the tank over time. The tankless water heater will usually maintain its efficiency throughout its lifetime.
Endless Hot Water
Since a tankless water heater does not have a finite amount of storage and it heats the water as the demand occurs, one can not run out of hot water as long as the maximum flow rates are not exceeded. For example, a Rinnai R-98 will deliver approximately 240 gallons of hot water per hour. It is important however to size the unit for the intended demand.
The modern tankless water heater can be located in a very small space allowing the homeowner to gain valuable space for other uses. Additionally, most units can be located on the home's exterior within certain limitations dictated by zoning ordinances.