While offering higher efficiency, tankless water heaters usually don't make sense
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There
are two primary types of water heaters: storage and tankless. In this
column I’ll try to explain the differences between these two approaches
and offer some guidance on choosing between them. (There are also
“hybrid” water heaters with features of both that I’ll cover in a future
blog.)
Storage water heaters
Most water heaters are
storage
models. These are insulated tanks holding 20 to 120 gallons with either
electric heating elements or gas burners. The storage tank stratifies
with hot water at the top and cold incoming water at the bottom, so that
as you draw off hot water (from the top), you get consistently hot
water until the hot water is nearly depleted. The “first-hour rating”
tells you how many gallons of hot water can be delivered in an hour.
Storage
water heaters constantly lose heat through the tank walls. Even though
the tank is insulated, the difference in temperature across that
insulated wall is large, so even with a lot of insulation the stand-by
heat loss is substantial. Gas-fired storage water heaters that have
standing pilot lights replenish some of that lost heat with the pilot,
but most of the pilot’s heat is lost up the flue.
Tankless water heaters provide constant hot water and energy savings
To address the issue of standby heat loss and running out of hot water,
tankless water heaters (also referred to as
demand
water heaters) were developed decades ago. These are sometimes
(especially in other countries) installed at the point of use, say in a
bathroom, but in this country they are usually installed centrally in
place of standard, storage water heaters.
A great feature of
tankless water heaters is that they never run out of hot water —
assuming the water heating capacity large enough to supply the needed
hot water demands. They also don’t have stand-by losses. Because hot
water isn’t stored in a tank, there is no heat loss when the water
heater isn’t operating (though there will be some losses through the
pipes during use).
A
2008 Consumer Reports article reported that gas-fired tankless water heaters used about 22% less energy than their storage-type counterparts. A
2010 study by the Center for Energy and Environment in Minnesota found that gas-fired tankless water heaters save an average of 36% over storage water heaters. So far, so good.
The size of heating elements
A key advantage of storage water
heaters is that the heating element(s) can be fairly small. Because a
significant volume of water is stored and because the tank remains
stratified as hot water is drawn off, a properly sized storage-type
water heater can provide a family’s hot water needs without requiring a
very large flow of gas or electricity to heat the water.
Most gas-fired storage-type water heaters have relatively small burners, typically 30,000 to 50,000 Btu/hour
(not much larger than the larger burner on a gas range). This means
that a half-inch-diameter gas line is usually adequate to supply the
water heater. It also means that the air intake (supply of combustion
air) can be fairly modest in size.
Gas-fired
tankless
water heaters, on the other hand, often have much larger burners. A
typical whole-house model, sized to allow two showers to be used at the
same time or for someone to shower while the clothes washer or
dishwasher is operating, will have a burner producing as much as 180,000
Btu/hour; the largest tankless water heaters have burners over 300,000
Btu/hour. Supplying the natural gas or propane to such a large burner
requires a larger gas-supply line (typically 3/4-inch) than needed for
storage water heaters — not an insignificant consideration.
Along
with the large gas line, these tankless water heaters require a lot of
combustion air. A small, 125,000 Btu/hour model operated at full
capacity requires about 30 cubic feet per minute (cfm) of air for
complete combustion, and a large, 180,000 Btu/hour model requires up to
45 cfm of air at full capacity. Such large airflow requirements can
limit the options for placement.
Bigger challenges with electric tankless water heaters
An
electric
tankless water heater large enough to serve a whole house requires a
huge current draw. A Seisco Model RA-28 that supplies 2.5 gallons per
minute at a 76°F temperature rise draws as much as 116 amps at 240
volts! Most homes have only 200-amp service, and the multiple breakers
and wiring required for such large current flows are expensive.
For
utility companies, the idea of a lot of customers switching to electric
tankless water heaters is downright scary, since hot water loads
typically fall during periods of peak morning and early-evening power
consumption. Utility companies are required to have capacity available
for whatever the demand is, and if a lot of electric tankless water
heaters were installed in a service district that would result in a
significant increase in those peaks.
Flow rates
Some tankless water heaters have a minimium flow
rate as high as 0.5 or 0.6 gallons per minute, meaning that at lower
flow rates they won't come on. This can be a problem with low-flow
plumbing fixtures, such as bathroom faucets.
Fortunately,
manufacturers are responding to this concern. The Rheem H95 condensing
tankless water heater pictured with this blog, for example, has a
minimum flow rate of 0.26 gpm, the lowest I've seen — though the minimum "activation rate" is somewhat higher at 0.4 gpm.
Higher cost for tankless water heaters
While tankless water
heaters save energy compared with storage water heaters, that doesn’t
mean they are cost-effective. Both the
Consumer Reports and
Minnesota study mentioned above reported that the significantly higher
cost of tankless water heaters resulted in payback periods longer than
the expected lifetimes of the water heaters.
Consumer Reports
found the cost of tankless models to range from $800 to $1,150 plus
about $1,200 for installation, compared with $300 to $480 for storage
water heaters and $300 for installation.
The Minnesota study reported a 20- to 40-year payback for the tankless water heaters.
With
certain usage patterns, though, the numbers could change. In a vacation
home that is only used for an occasional weekend, the standby losses
can be a huge percent of the total energy use for water heating, and a
tankless model might make more sense. Or, in a commercial building in
which a lavatory faucet is far away from the water heater and the hot
water demand is very low, a small point-of-use tankless water heater may
make sense — even an electric model.
Increased maintenance
On top of the questionable economics,
tankless water heaters have significantly greater maintenance
requirements than storage models.
Models designed for outdoor
installation (where supplying combustion air is not a problem) include
sophisticated freeze-protection systems. In places with hard water,
scale build-up is a significant problem. If the hardness is above 11
grains per hour, experts recommend installing a water softener,
according to
Consumer Reports, and special provisions may be needed during installation to allow periodic flushing the heat exchanger coils with a vinegar solution.
The bottom line
The bottom line is that tankless water
heaters simply don’t make sense for most whole-house applications. There
are exceptions, as noted above, but for the vast majority of
residential applications, storage water heaters make more sense.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. He also recently created the Resilient Design Institute.
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