A.Martin Holladay responds: Before you consider
installing an instantaneous gas water heater for energy
efficiency, consider whether your customers will be satisfied
with the flow rate of the model chosen. The most common models
of instantaneous water heaters have maximum flow rates in the
range of 2 to 3 gallons per minute. Three gpm is the bare
minimum to supply two simultaneous showers, and most American
families expect their water heater to provide up to 6 gpm of
hot water when necessary.
Assuming you’ve decided to install an instantaneous gas
water heater with at least 3 gpm of flow, a typical choice
would be the AquaStar 240FX (800/642-3199,
www.controlledenergy.com), which is
available for about $900 to $1,030, depending on whether you
need the outside vent
hood. The AquaStar 240FX has an efficiency factor (EF) of
0.84. An ordinary 40-gallon gas water heater with an EF of 0.56
costs about $270, so you would need to save around $700 on your
fuel bill before your energy savings would repay the added cost
of the instantaneous heater. An instantaneous water heater will
probably last 10 to 20 years, compared with 7 to 10 years for a
conventional gas water heater.
| | Purchase price of water
heater | Annual gas in therms
consumption | Annual natural gas bill (at 87¢
per ccf) | Annual propane bill (at $1.40 per
gallon) |
| Conventional gas water heater (EF
0.56) | $270 | 267 | $233 | $409 |
| Instantaneous gas water heater (EF
0.84) | $970 | 187 | $163 | $275 |
Investing in an instantaneous model makes the most sense for
those with high fuel costs. (See the table above, which is
based on hot water use of 64.3 gallons per day, or 23,470
gallons per year, by the “average U.S. household,”
as shown in the Energy Guide Labels.) If you have access to
natural gas, you won’t save much, since the payback
period (assuming that gas costs $0.87 per ccf) is 10 years. If
you’re burning propane, however, the payback period is
shorter: At $1.40 a gallon, it would be a little over 5 years.
In some areas of the country, including northern New England,
where I am currently paying $2 a gallon for propane, the
payback period is less than 4 years.
Of course many other factors can affect payback calculations,
including how much hot water is used (high-use families see a
quicker payback), differences in maintenance costs, the
likelihood that an instantaneous heater will not need to be
replaced as frequently as a conventional heater, and possible
differences in installation costs (an instantaneous heater may
require a larger gas supply line and a larger flue than a
conventional water heater).
Martin Holladay is editor of Energy Design Update. For subscription
information, call 800/638-8437.