Storage Water Heaters
Conventional storage water heaters remain the most
popular type of water heating system for the home.
Here you'll find basic information about how storage
water heaters work; what criteria to use when selecting
the right model; and some installation, maintenance,
and safety tips.
How They Work
A single-family storage water heater offers a ready
reservoir—from 20 to 80 gallons—of hot
water. It operates by releasing hot water from the
top of the tank when you turn on the hot water tap.
To replace that hot water, cold water enters the bottom
of the tank, ensuring that the tank is always full.
Set your thermostat as low as is comfortable in the
winter and as high as is comfortable in the summer.
Clean or replace filters on furnaces once a month
or as needed.
Clean warm-air registers, baseboard heaters, and radiators
as needed; make sure they're not blocked by furniture,
carpeting, or drapes.
Bleed trapped air from hot-water radiators once or
twice a season; if in doubt about how to perform this
task, call a professional.
Place heat-resistant radiator reflectors between exterior
walls and the radiators.
Turn off kitchen, bath, and other exhaust fans within
20 minutes after you are done cooking or bathing;
when replacing exhaust fans, consider installing high-efficiency,
During the heating season, keep the draperies and
shades on your southfacing windows open during the
day to allow the sunlight to enter your home and closed
at night to reduce the chill you may feel from cold
During the cooling season, keep the window coverings
closed during the day to prevent solar gain.
Long-Term Savings Tip: Select energy-efficient products
when you buy new heating and cooling equipment. Your
contractor should be able to give you energy fact
sheets for different types, models, and designs to
help you compare energy usage. For furnaces, look
for high Annual Fuel Utilization Efficiency (AFUE)
ratings. The national minimum is 78% AFUE, but there
are ENERGY STAR models on the market that exceed 90%
Long-Term Savings Tip: For air conditioners, look
for a high Seasonal Energy Efficiency Ratio (SEER).
The current minimum is 13 SEER for central air conditioners.
ENERGY STAR models are 13 SEER or more. The American
Council for an Energy-Efficient Economy provides tips
for buying energy-efficient furnaces, boilers, AC
units, and heat pumps
Ventilation and Air Conditioning
How much energy can I save by using fans instead of
my air conditioner?
The basic notion is that moving air (from ceiling,
whole-house, or portable fans) makes you feel cooler,
so you can turn up the air conditioner thermostat
or turn it off altogether. Whole-house fans are a
potential substitute for air conditioning, since they
move large amounts of air through the house and require
open windows. Savings from using a whole-house fan
can be large (it uses 20% or less of the energy of
a central air conditioner on a per-hour basis, although
they usually need to be used for fewer hours). Also
a whole-house fan provides good comfort levels when
it's not too humid or too hot outside (night time).
Studies by the Florida Solar Energy Center (FSEC)
show that ceiling fans can save energy *if the occupants
turn up the A/C thermostat*. Unfortunately, most people
don't adjust the thermostat. Often people leave them
on even when no one is home, which can result in negative
Research from FSEC indicates that whole-house fan
savings are quite variable, ranging from about 10%
to 65%. This range is due to the effect of climate;
a milder climate will see savings toward the upper
end of that range. FSEC found that if the air conditioning
thermostat is set 2o F higher when using ceiling fans,
the savings will be 14%. (With a higher thermostat
setting, savings are higher.) If the thermostat setting
is not changed, electricity consumption will actually
increase by 15%. FSEC's survey of actual behavior
showed no measurable savings from cooling fans (i.e.
in energy terms, the ceiling fan and the A/C were
a wash). There is at least one efficient ceiling fan
that uses less energy and has an occupancy sensor
(on some models).
Your Air Conditioner
An air conditioner's filters, coils, and fins require
regular maintenance for the unit to function effectively
and efficiently throughout its years of service. Neglecting
necessary maintenance ensures a steady decline in
air conditioning performance while energy use steadily
Air Conditioner Filters
The most important maintenance task that will ensure
the efficiency of your air conditioner is to routinely
replace or clean its filters. Clogged, dirty filters
block normal air flow and reduce a system's efficiency
significantly. With normal air flow obstructed, air
that bypasses the filter may carry dirt directly into
the evaporator coil and impair the coil's heat-absorbing
capacity. Keeping the filter clean can lower your
air conditioner's energy consumption by 5%–15%.
For central air conditioners, filters are generally
located somewhere along the return duct's length.
Common filter locations are in walls, ceilings, furnaces,
or in the air conditioner itself. Room air conditioners
have a filter mounted in the grill that faces into
Some types of filters are reusable; others must be
replaced. They are available in a variety of types
and efficiencies. Clean or replace your air conditioning
system's filter or filters every month or two during
the cooling season. Filters may need more frequent
attention if the air conditioner is in constant use,
is subjected to dusty conditions, or you have fur-bearing
pets in the house.
Air Conditioner Coils
The air conditioner's evaporator coil and condenser
coil collect dirt over their months and years of service.
A clean filter prevents the evaporator coil from soiling
quickly. In time, however, the evaporator coil will
still collect dirt. This dirt reduces air flow and
insulates the coil, reducing its ability to absorb
heat. To avoid this problem, check your evaporator
coil every year and clean it as necessary.
Outdoor condenser coils can also become very dirty
if the outdoor environment is dusty or if there is
foliage nearby. You can easily see the condenser coil
and notice if dirt is collecting on its fins.
You should minimize dirt and debris near the condenser
unit. Your dryer vents, falling leaves, and lawn mower
are all potential sources of dirt and debris. Cleaning
the area around the coil, removing any debris, and
trimming foliage back at least 2 feet (0.6 meters)
allow for adequate air flow around the condenser.
The aluminum fins on evaporator and condenser coils
are easily bent and can block air flow through the
coil. Air conditioning wholesalers sell a tool called
a "fin comb" that will comb these fins back
into nearly original condition.
Occasionally pass a stiff wire through the unit's
drain channels. Clogged drain channels prevent a unit
from reducing humidity, and the resulting excess moisture
may discolor walls or carpet.
Selecting a New Water Heater
You have a lot to consider when selecting a new water
heater for your home. You should choose a water heating
system that will not only provide enough hot water
but also that will do so energy efficiently, saving
you money. This includes considering the different
types of water heaters available and determining the
right size and fuel source for your home.
Types of Water Heaters
It's a good idea to know the different types of water
heaters available before you purchase one:
· Conventional storage
Offer a ready reservoir (storage tank) of hot water
· Demand (tankless or instantaneous) water
Heat water directly without the use of a storage tank
(becoming more popular)
· Heat pump water heaters
Move heat from one place to another instead of generating
heat directly for providing hot water
· Solar water heaters
Use the sun's heat to provide hot water
· Tankless coil and
indirect water heaters
Use a home's heating system (boiler) to heat water.
When selecting the best type and model of water heater
for your home, you also need to consider the following:
· Fuel type, availability
The fuel type or energy source you use for water heating
will not only affect the water heater's annual operation
costs but also its size and energy efficiency.
To provide your household with enough hot water and
to maximize efficiency, you need a properly sized
· Energy efficiency
To maximize your energy and cost savings, you want
to know how energy efficient a water heater is before
you purchase it.
Before you purchase a water heater, it's also a good
idea to estimate its annual operating costs and compare
those costs with other less or more energy-efficient
· Krigger, J.; Dorsi, C. (2004). Residential
Energy: Cost Savings and Comfort for Existing Buildings.
Helena, MT: Saturn Resource Management.
· Water Heating (PDF 836 KB). (August 2001).
DOE/GO-102001-0785. U.S. Department of Energy.
· Home Energy Briefs: #5 Water Heating (PDF
161 KB). (2004). Rocky Mountain Institute. 6 pp.
· Wiehagen, J.; Sikora, J. L. (March 2003).
Performance Comparison of Residential Hot Water Systems
(PDF 1.01 MB). NREL Report No. SR-550-32922. Golden,
CO: National Renewable Energy Laboratory. 61 pp.
Demand (Tankless or Instantaneous) Water Heater
Demand (tankless or instantaneous) water heaters are
rated by the maximum temperature rise possible at
a given flow rate. Therefore, to size a demand water
heater, you need to determine the flow rate and the
temperature rise you'll need for its application (whole
house or a remote application, such as just a bathroom)
in your home.
First, list the number of hot water devices you expect
to use at any one time. Then, add up their flow rates
(gallons per minute). This is the desired flow rate
you'll want for the demand water heater. For example,
let's say you expect to simultaneously run a hot water
faucet with a flow rate of 0.75 gallons (2.84 liters)
per minute and a shower head with a flow rate of 2.5
gallons (9.46 liters) per minute. The flow rate through
the demand water heater would need to be at least
3.25 gallons (12.3 liters) per minute. To reduce flow
rates, install low-flow water fixtures.
To determine temperature rise, subtract the incoming
water temperature from the desired output temperature.
Unless you know otherwise, assume that the incoming
water temperature is 50ºF (10ºC). For most
uses, you'll want your water heated to 120ºF
(49ºC). In this example, you'd need a demand
water heater that produces a temperature rise of 70ºF
(39ºC) for most uses. For dishwashers without
internal heaters and other such applications, you
might want your water heated at 140ºF (60ºC).
In that case, you'll need a temperature rise of 90ºF.
Most demand water heaters are rated for a variety
of inlet temperatures. Typically, a 70ºF (39ºC)
water temperature rise is possible at a flow rate
of 5 gallons per minute through gas-fired demand water
heaters and 2 gallons per minute through electric
ones. Faster flow rates or cooler inlet temperatures
can sometimes reduce the water temperature at the
most distant faucet. Some types of tankless water
heaters are thermostatically controlled; they can
vary their output temperature according to the water
flow rate and inlet temperature.