Why Use a Tankless Water Heater?

The cost of heating water, for uses as different as taking a shower or washing dishes, is about 13-15% of the typical homeowner’s total energy costs.  It makes sense to find ways to reduce your energy use for heating water.  Tankless water heaters can do that.  This note will show you how.

How does a tankless water heater work?

As its name implies, a tankless water heater does not have a tank of ready-to-use hot water like the common storage water heater.  It consists of a box of about 20 pounds that sits on your wall, connects to your electric (or gas) service and your water supply – point (1) in the diagram.  In the normal state – that is, most of the time – the heating element of the unit is off, and there is no hot water available. When you turn on your hot water faucet, sensors in the unit (2) detect water flow.  The computer in the unit turns on the heat source (3) – either electricity or a gas burner.  Water absorbs heat from the heating element in the heat exchanger (4), and flows out of the unit (5).  The time from cold water in, to hot water out, is a matter of seconds.

Note that the water sitting in the hot-water pipe between the water heater and the hot-water faucet must be flushed out before hot water will be felt at the faucet.  For this situation, neither tankless nor tank water heaters offers an advantage.  There are solutions for this situation, but NRG SVRS does not consider them cost-effective.

What are the advantages of tankless?

The tankless water heater heats water only when it is requested, unlike the traditional storage water heater that must use energy to keep water hot in its tank ready for use, even when not requested.  This is called “standby loss” of energy and is the primary amount of energy saved with the tankless appliance.  In addition to this energy saving, the tankless water heater has the following benefits:

• Unlimited. While its heated water is not instantly hot at your faucet (as explained above,) a tankless heater supplies an unlimited amount of full-temperature water.  The tankless solution is perfect for large families in that the last shower will still be hot.  The traditional storage water heater keeps a certain quantity of water hot and ready for use, but once that water is used up, the heater must heat another nearly full tank of cold water in order to supply full-temperature water again.  In the meantime, luke-warm or cold water is supplied to the hot-water pipes. In fact, the temperature of the delivered water begins to decrease well before the hot water in the tank is exhausted because cold water begins mixing with the hot water the instant hot water is demanded.
• No tank to break. There is no pressurized tank to crack or break, lessening the chance of a catastrophic water leak.  There is less on-going maintenance with a tankless heater.  For example, there is no need to flush water from the tank quarterly to remove silt; no need to check pressure valve twice a year; nor checking the anode in electric heaters. There is no storage tank to break and leak water in your house.
• Small size. The tankless water heater is a small box – about the size of a briefcase or backpack – that attaches to a wall in your home.  It takes up very little space compared to the traditional storage water tank.  This will free up space for additional storage or some other use.
• Added value. Finally, if you are planning to sell your house, the more modern approach of the tankless water heater will add value to your home.  Most tankless water heaters have an expected lifetime of 20 years or more.

Are there disadvantages?

• Power usage. An electric tankless water heater requires a lot of electrical power, around 40 amps on each of the two phases.  Older houses may not be wired for this much power, so additional wiring and changes to an electric panel may be required.  On the other hand, a gas-fired tankless heater requires about the same gas supply and flue requirements as the traditional gas storage water heater it replaces.
• Limited supply. The amount of hot water produced by a tankless heater varies with the size of the unit.  Most units can supply sufficient hot water to run a dishwasher and a shower at the same time.  But putting too much demand on the unit (such as trying to run two showers and a dishwasher) will decrease the temperature of the water the can supply continuously.
• Savings? The amount of energy saved by a tankless heater is significant but may not make up the difference in initial cost of a tankless system compared to a storage system.  You should make the calculations for your specific situation.

How can I estimate the size of water heater I need?

The two models of tankless water heaters NRG SVRS installs can supply unlimited amounts of hot water at the rate of about 2.0 or 3.0 gallons per minute at 115 °F.  At 105 °F, they can supply about 15% more.  The table below shows how much different common water appliances use.

You need to size your tankless water heater to your expected water use. Demanding a flow greater than the unit's ability to supply hot water will result in unexpectedly cold water at the point of use. (However, note that some of the appliances in the table will not demand 100% hot water all the time.)

Conclusion

While tankless water heaters do save energy, they may not be a cost-effective solution for reducing your energy bills, and NRG SVRS does not recommend them as a strictly energy-saving strategy. It may make sense for those building new homes, those with large families, or those early adopters who want to save energy and are not terribly cost-conscious.

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Disclaimer: The information presented herein is provided for informational purposes only. Technical descriptions, specifications, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by NRG SVRS. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction is not permitted. ©,NRG SVRS, LLC.