How to save money on
water heater
use
Go Tankless!
Old-style tank water heaters are wasteful
because of standby losses -- heat leaches
from the tank to the surrounding air, and then the
heater has to heat up the water again. The
modern replacement is an tankless unit,
which heats the water instantly when you turn on
the faucet. This eliminates standby losses. You can
choose either gas or electric, just like with an
old tank heater. A typical tankless unit starts at
around $400
for either flavor, though they cost a bit more
to install. They last up to twice as long as old
tank models makes up for the higher installation
cost.
Here are some typical costs for various water
heaters, but remember that everyone's situation
is different, so do not assume these numbers
are valid for your particular situation.
You especially should
not quote this table with something like, "Mr.
Electricity says XXX water heater costs $YY to
run."
Electric
|
Gas
|
|
|
|
|
|
Tank
|
Tankless
|
Tank
|
Tankless
|
|
$40.48/mo.
|
$36.96/mo.
|
$30.00/mo.
|
$21.08/mo.
|
|
$650
installed
|
$1000 - 2000
installed
|
$865
installed
|
$1500 - $2500
installed
|
|
lasts 10-13
years
|
lasts
20 years
|
lasts 10-13
years
|
lasts
20 years
|
Source:
EPA's Residential Water Heaters Analysis
(PDF,
350k, April 2008).
RMI's figures are similar, except they say a Gas
Tank saves 36% (not 26%),
and they give a much lower installed cost for all
models. (PDF,
2004)
Here's how the cost breaks down over 20
years:
|
|
Gas Tank
|
Gas
Tankless
|
|
Installation cost
|
$1,730
|
$2,000
|
|
Operating cost
|
$7,200
|
$5,059
|
|
Total cost over 20
years
|
$8,930
|
$7,059
|
As with most things in life, your results may
vary, for many reasons:
- The numbers for
tanks above are for 50-gallon
tanks.
If you use a 30- or 40-gallon tank, it
will cost you less to run it, so the savings
with a tankless model will be less.
- The temperature
where you keep your tank matters. The
colder the ambient air temperature, the more
heat your tank will lose, and the more it will
cost to run.
- The larger your
household (i.e., the more hot water you use),
the less standby losses you're going
to have from your tank, because you'll be using
a lot of the water before the heat can be lost
to the air. The less the stanndby losses, the
less it costs to operate, and the less you'll
save with a tankless model.
- If having an
endless supply of hot water with a tankless
model encourages you to take ridiculously long
showers, then the tankless unit could
wind up costing you more.
- The costs above use
national
average rates
for gas and electric. So if, for
example, gas is really expensive in your area,
electric could actually be cheaper.
The EPA has a calculator
that can better approximate the cost for your
situation, since it lets you enter both your
daily use in gallons and your local energy cost.
And do make sure to use your actual local rates,
since the default rates in the calculator are
woefully outdated. If you don't know your actual
rate, use the national averages in 2008 of
$0.11/kWh and $1.42/therm.
|
Factors
affecting savings when installing a
tankless water heater
|
|
|
Bigger savings from going
tankless
|
Smaller savings from going
tankless
|
|
Family size
|
Small to average
|
Large
|
|
Ambient air temperature
|
Cold
|
Warm
|
|
Length of showers after
installation
|
Same length showers
|
Longer showers
|
There's a lot of misinformation floating
around about tankless water heaters, so keep a few
things in mind. First, every method of heating
a given amount of water uses the same amount of
energy. That's true whether you're using gas or
electric, and whether you're using a big tank or a
new tankless system. This is basic physics. If you
want to heat some water, it's going to require some
specific amount of energy to do it, no more, no
less.
So even though heating a given amount of water
uses the same amount of energy no matter how you do
it, here's why your costs can be
different depending on your heating method.
- The cost of fuel differs. If you use
the same amount of gas energy and electric
energy, the electric will cost more, simply
because the price of electricity is generally
higher. (Your local rates could be different, of
course.)
- You're heating the water more than
once. Both a tank and a tankless use about
the same amount of energy to heat, say, 50
gallons of water initially. But with a tank
system, some of the heat from the tank is lost
to the outside air, so the tank has to heat up
the water again. The tankless system
doesn't have to do that. So a tank uses more
energy because some of the energy that went to
heat the water the first time is wasted.
- Standby losses differ. While you'll
use the same amount of gas or electric to heat
the water initially, the gas tank will lose more
heat than the electric tank because the gas tank
requires a vent, from which heat can escape. So
more gas is required to keep the water hot.
However, gas tanks still generally cost less to
operate than electric tanks,simply because the
price of gas is cheaper, even though you're
using more gas.
- You use more water than before. When
some people get a tankless system they love the
fact that the hot water doesn't run out so they
take ridiculously long showers. At that point
it's no longer a fair apples-to-oranges
comparison, because of course it's going to take
more energy if you wind up heating more
water. And in that case, the extra cost
isn't the fault of the tankless system, it's the
fault of the person who's decided to take much
longer showers.
Other advantages of
tankless water heaters
Tankless water heaters have other advantages
over tank heaters:
- You can't run out
of hot water by using all the hot water in the
tank, because there is no tank.
- The unit should
last about 20 years vs. about 10 for a tank
heater. That will help the tankless
heater pay for itself. (In theory a tank will
last longer if you drain it once a year or have
a self-cleaning model, but there are doubts
about whether
self-cleaning models really work.)
- Leaks are rare with
tankless systems, while all tanks
will leak eventually. The damage caused by a
leaking tank could cost you a bundle.
- Tankless systems
save space. Your old huge tank heater
is replaced by a small box which mounts on the
wall. Check out these before
& after photos.
How to buy a tankless
water heater
You'll choose a tankless model based on the
flow rate (how much how water it can deliver
per minute) and the temperature rise (how
much it heats the water). You
don't look at those separately, because they're
related. For example, one unit might heat the water
54° at 1.5 gallons per minute (gpm), but only
27° at 3.0 gallons per minute.
So let's first look at typical gpm requirements.
Here's the typical flow rate for water use in your
house:
- 2.0 - 4.0 gpm - Bathtub
- 1.5 - 3.0 gpm - Shower
- 1.0 - 3.0 gpm - Dishwasher
- 1.0 - 2.0 gpm - Sink
- 0.5 - 1.0 gpm - Toilet
So for most uses, a 2.5 gpm unit would be fine,
as long as the temperature rise is also good.
If you want to use more than one thing at a time,
you need more gpm, so you'll need a bigger model.
Conversely, if you have an efficient showerhead and
run the water slowly when you shower, you might do
fine with a 1.5 gpm model, which will cost less
than a bigger model.
The temperature rise you need depends on the
climate where you live. If you live in
the north where the incoming water is cold, you'll
need a bigger rise to get your water up to shower
temperature than you would in the south, where the
incoming water is warmer. You can get a thermometer
from the grocery store to measure your cold water
temperature. You'll need to get it up to about
104°F (40°C) for showers or dishwashing.
And keep in mind that the temperature will drop a
few degrees as the water travels from the heater to
the faucet.
Here are some links to check check out electric
models and gas
models at Amazon.
I also found a Tankless
Water Heater Guide that tells you pretty much
everything you'd want to know about selecting and
installing a tankless water heater.
Gas vs. Electric (both
tank & tankless)
I cover operating and installation
costs above.
In this section let me give you some items
in favor of choosing an electric model:
- Electric heaters are cheaper,
because they're less complicated
- They're easier to install -- no
gas pipes required, no venting required
- They're safer (no fuel to leak or
explode, no pilot light to go out leaking gas
into home, no combustion byproducts)
Electric tanks are actually more efficient
than gas tanks, because gas tanks constantly
lose heat through the venting flue (about
6°F per hour
[source]).
Electric tanks typically lose 8-30% of the heat
they generate while gas tanks lose a lot more,
about 40-50%. But even though electric is more
efficient, it generally costs more to operate
since electricity costs more than gas. On the
other hand, an electric model is cheaper to
purchase and install.
Water use
figures
|
Hot water used
(gallons)
|
|
Washing Machine (typical model;
hot/hot)
|
40
|
|
Washing Machine (efficiency model;
hot/hot)
|
13-28
|
|
Shower
|
20
|
|
Bath
|
20
|
|
Dishwasher
|
12
|
|
Cooking
|
5
|
|
Washing Dishes
|
4
|
Most figures from the
Dept.
of
Energy.
Typical hot water
temperatures
|
Water
Temperatures
|
|
104°F
(40°C)
|
For showers and dishwashing. You can
get a thermostatic
shower valve to keep the water
temperature constant even when people
flush toilets or turn on sinks in other
parts of the house.
|
|
123°F
(50.6°C)
|
Bacteria fram Legionnaires Disease can
grow in water up to 122°F,
so many people set their heater tanks to
at least 123°F. Of course, if you
have a tankless model (no water standing
around in a tank), then you don't have to
worry about bacteria multiplying.
|
|
130°F
(54.4°C)
|
Water this hot will give you
third-degree burns in just 30 seconds.
(Tap
Water
Burn)
|
|
140°F
(60°C)
|
Hotels, quite ridiculously, often
deliver hot water at this dangerous,
super-scalding level.
(AllStays.com)
|
Tips for saving with
an old-style tank heater
If you're not upgrading to a tankless system,
here are some tips for conserving with your
old-style tank heater.
- Turn down the
thermostat. When it's set on scalding
hot you have to mix in cold water in your shower
to lower the temperature, and why make your
heater boil the water if you don't need it that
hot? And remember that 130°F water is hot
enough to scald you. Note though that many
people prefer to set their tanks to 123°F
or higher to prevent Legionnaires Disease.
(Though I personally think the best way to avoid
LD is to just get an instant tankless system, so
you don't have festering water sitting around in
a tank.).
- Use a timer on
older electric heaters. A $40 timer
can automatically turn off an electric heater
when you go to work, then back on right before
you come home, off after you go to bed, and on
again right before you get up. These don't save
as much money as you'd expect, though. That's
because a typical electric water heater only
runs about three
hours a day anyway, and modern
energy-efficient water heaters run only
1.3
hours or so. Standby losses (how much heat
the tank loses by just sitting there) aren't
that great, especially for modern heaters. (In
fact, if your heater was made after 1998, it's
possibly not worth using a timer at all.) And
even with a timer you'll still have standby
losses as soon as you leave for work and after
the tank shuts off for the night. A timer for an
old (pre-1998) heater will save about 25kWh/mo.
for a family of two using 40 gallons a day with
the heater off four to six hours a day, but only
14kWh/mo. for a family of four using 80 gallons
a day. (Florida
Extension
Service)
(See our separate page on how
to install a water heater timer.)
- Wrap your heater in
a special tank blanket. Home
improvement stores sell a special water heater
blanket that you can put around your heater to
help insulate it. This reduces energy use by
10-15% -- for a family of two, that's about
21kWh/mo., or $20/yr. at 8¢/kWh. The
savings are only slightly less for a family of
four. (Florida
Extension
Service)
- Fix leaky
faucets. Every
30 drops per minute from a hot water faucet
costs you around 18kWh/mo. with an electric
water heater, according to this
article.
- Insulate the room
where the heater is. If it's in a
garage and it's cold outside, keep the garage
door closed!
- Take advantage of
utility promotions. Some utility
companies like Dakota
Electric will install a special water heater
which heats primarily at night for a lower kWh
rate. (In Dakota Electric's case, only 3¢
kWh.)
- Install a drain
recovery system. A drain recovery
system uses the heat from the water flowing down
the drain to preheat water entering the heater.
A system like the one by GFX
saves $180 to $340 a year when used with an
electric water heater (according to the
manufacturer), and prices start at $334. These
are only really suitable for homes with
basements or bathrooms on the second floor,
because access to a vertical drainpipe is
needed.
- Turn it off when
you're out of town. For electric
heaters without a switch, you can turn it off at
the breaker box. For gas heaters, follow the
instructions printed on the heater. (Usually you
can just turn the thermostat, NOT the gas knob,
to OFF -- unless you feel you're competent to
re-light the heater when you return.)
Energy required to heat
one gallon, and one tank
Energy required to heat a tank of
water
- A Btu, or British thermal unit, is the
amount of energy needed to raise one pound of
water from 60°F to 61°F at sea level.
(Wikipedia)
- A gallon of water weights 8.33
lbs.
- If the incoming water is 60°F and we
want to raise it to 123°F, that's a
63°F rise.
- Heating a gallon of water thus requires 8.33
x 63 = 525 Btu's, at 100% efficiency.
Cost to heat water in a gas tank
- A typical gas tank water heater is only
59%
efficient. So it takes 525 ÷ 59% = 890
Btu's to heat a gallon of water in a gas
tank.
- One therm is 100,000 btu's. So one Btu is
0.00001 therms.
(Pacific
NW Natl.
Lab.)
- 890 Btu's is 0.0089 therms.
- So we've got 0.0089 therms to heat a gallon
of water, or 0.0089 x 40 = 0.356 therms to heat
a 40-gallon tank.
- At $1.42/therm,
it costs 0.356 x $1.42 = $0.51 to heat a
40-gallon tank.
- Another source comes up with a similar
figure: 0.40 therms for the tank (based on 0.11
therms to heat 11 gallons of water.
(Multi-housing
Laundry
Association)
- MHLA also says it takes 3.3 therms to keep
11 gallons hot for one month.
Cost to heat water in an electric
tank
- A typical electric water heater is 90.4
to 95% efficient. Let's call that
92.7% on average.
- So it takes 525 ÷ 92.7% = 566 Btu's to
heat a gallon of water in an electric tank.
- One kWh is 3413
Btu's, so one Btu is 0.000293
kWh.
- 566 Btu's x 0.000293 kWh/Btu = 0.166
kWh.
- So we've got 0.166 kWh to heat a gallon of
water, or 0.166 x 40 = 6.63 kWh to heat a
40-gallon tank.
- At $0.11/kWh, it costs 6.63 x $0.11 = $0.73
to heat a 40-gallon tank.
Solar water
heaters
While solar
electric takes a while to
recoup its installation cost, solar water
heating works a lot better, and is
easier to install and maintain.
(More on solar water heating
from the Dept
of Energy,
SolarRoofs.com,
and the Centre for Enivronmental
Initiatives.)
On a separate page I have questions
I've received and answered about how
saving on water heating costs.
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