Tankless water heaters, also known as demand water heaters, heat water only when you need it. There’s no storage tank like a conventional water heater, thus no standby energy loss as unused water cools in the tank. Tankless heating can reduce water-heating costs by as much as 50 percent. Because the drawbacks of conventional tank heaters — corrosion, mineral deposits and leaks — are avoided, tankless water heaters can have an efficient service life as long as 20 years.
Single, whole-house tankless units are increasingly popular and may meet the entire needs of your residence. Or, multiple tankless units may offer better coverage. Sizing a tankless system for your specific needs involves two main factors: flow rate and temperature rise.
- Flow rate is the gallons-per-minute of heated water a tankless heater supplies at a specific temperature. A unit’s flow rate should exceed your demands — the maximum expected hot water usage in your home at any given time — by 10 percent. Figure your demands by estimating the total gallons-per-minute of typical hot water outlets running simultaneously. For example, a faucet in the kitchen + a shower head in the bathroom + a washing machine.
- Temperature rise is the difference between the temperature of water as it enters your home and your desired hot water temperature. If the water inlet temperature is 58 degrees and the preferred output is 128 degrees, the unit will need a 70-degree temperature rise at its maximum gallons-per-minute flow rate. The higher the flow rate at any given time, the more difficulty your system will have achieving the required temperature rise.
One way to deal with a shortfall in the flow rate/temperature rise equation is to reduce demand by installing low-flow faucets and shower heads. Another way is to utilize one or more dedicated point-of-use tankless water heaters in high-demand locations such as a kitchen or laundry room, while having a whole-house unit service all other outlets.
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