Automatic and Programmable Thermostats

 In our modern, high-tech society, we don't think much about some of the
electronic gadgets in our homes. Take, for example, the ever-present
thermostat--a staple of American households for decades. It usually
takes the shape of an unassuming box on the wall, but that modest device
controls the comfort of your family on the coldest day in January and
the hottest day in July.

What Is a Thermostat?

It is a temperature-sensitive switch that controls a space conditioning
unit or system, such as a furnace, air conditioner, or both. When the
indoor temperature drops below or rises above the thermostat setting,
the switch moves to the "on" position, and your furnace or air
conditioner runs to warm or cool the house air to the setting you
selected for your family's comfort. A thermostat, in its simplest form,
must be manually adjusted to change the indoor air temperature.

General Thermostat Operation

You can easily save energy in the winter by setting the thermostat to
68øF (20øC) when you're at home and awake, and lowering it when you're
asleep or away. This strategy is effective and inexpensive if you are
willing to adjust the thermostat by hand and wake up in a chilly house.
In the summer, you can follow the same strategy with central air
conditioning, too, by keeping your house warmer than normal when you are
away, and lowering the thermostat setting to 78øF (26øC) only when you
are at home and need cooling.

A common misconception associated with thermostats is that a furnace
works harder than normal to warm the space back to a comfortable
temperature after the thermostat has been set back, resulting in little
or no savings. This misconception has been dispelled by years of
research and numerous studies. The fuel required to reheat a building to
a comfortable temperature is roughly equal to the fuel saved as the
building drops to the lower temperature. You save fuel between the time
that the temperature stabilizes at the lower level and the next time
heat is needed. So, the longer your house remains at the lower
temperature, the more energy you save.

Another misconception is that the higher you raise a thermostat, the
more heat the furnace will put out, or that the house will warm up
faster if the thermostat is raised higher. Furnaces put out the same
amount of heat no matter how high the thermostat is set--the variable is
how long it must stay on to reach the set temperature.

In the winter, significant savings can be obtained by manually or
automatically reducing your thermostat's temperature setting for as
little as four hours per day. These savings can be attributed to a
building's heat loss in the winter, which depends greatly on the
difference between the inside and outside temperatures. For example, if
you set the temperature back on your thermostat for an entire night,
your energy savings will be substantial. By turning your thermostat back
10ø to 15ø for 8 hours, you can save about 5% to 15% a year on your
heating bill--a savings of as much as 1% for each degree if the setback
period is eight hours long. The percentage of savings from setback is
greater for buildings in milder climates than for those in more severe
climates. In the summer, you can achieve similar savings by keeping the
indoor temperature a bit higher when you're away than you do when you're
at home.

But there is a certain amount of inconvenience that results from
manually controlling the temperature on your thermostat. This includes
waking up in a cooler than normal house in the winter and possibly
forgetting to adjust the thermostat (during any season) when you leave
the house or go to bed.

Thermostats with Automatic Temperature Adjustment

To maximize your energy savings without sacrificing comfort, you can
install an automatic setback or programmable thermostat. They adjust the
temperature setting for you. While you might forget to turn down the
heat before you leave for work in the morning, a programmable thermostat
won't! By maintaining the highest or lowest required temperatures for
four or five hours a day instead of 24 hours, a programmable thermostat
can pay for itself in energy saved within four years.

Programmable thermostats have features with which you may be unfamiliar.
The newest generation of residential thermostat technologies is based on
microprocessors and thermistor sensors. Most of these programmable
thermostats perform one or more of the following energy control
functions:

They store and repeat multiple daily settings, which you can manually
override without affecting the rest of the daily or weekly program.

They store six or more temperature settings a day.

They adjust heating or air conditioning turn-on times as the outside
temperature changes.

Most programmable thermostats have liquid crystal temperature displays.
Some have back-up battery packs that eliminate the need to reprogram the
time or clock in case of a power failure. New programmable thermostats
can be programmed to accommodate life style and control heating and
cooling systems as needed.

Types of Automatic and Programmable Thermostats

There are five basic types of automatic and programmable thermostats:

electromechanical,
digital,
hybrid,
occupancy, and
light sensing.

Most range in price from $30 to $100, except for occupancy and light
sensing thermostats, which cost around $200.

Electromechanical (EM) thermostats, usually the easiest devices to
operate, typically have manual controls such as movable tabs to set a
rotary timer and sliding levers for night and day temperature settings.
These thermostats work with most conventional heating and cooling
systems, except heat pumps. EM controls have limited flexibility and can
store only the same settings for each day, although at least one
manufacturer has a model with separate settings for each day of the
week. EM thermostats are best suited for people with regular schedules.

Digital thermostats are identified by their LED or LCD digital readout
and data entry pads or buttons. They offer the widest range of features
and flexibility, and digital thermostats can be used with most heating
and cooling systems. They provide precise temperature control, and they
permit custom scheduling. Programming some models can be fairly
complicated; make sure you are comfortable with the functions and
operation of the thermostat you choose. Remember-- you won't save energy
if you don't set the controls or you set them incorrectly.

Hybrid systems combine the technology of digital controls with manual
slides and knobs to simplify use and maintain flexibility. Hybrid models
are available for most systems, including heat pumps.

Occupancy thermostats maintain the setback temperature until someone
presses a button to call for heating or cooling. They do not rely on the
time of day. The ensuing preset "comfort period" lasts from 30 minutes
to 12 hours, depending on how you've set the thermostat. Then, the
temperature returns to the setback level. These units offer the ultimate
in simplicity, but lack flexibility. Occupancy thermostats are best
suited for spaces that remain unoccupied for long periods of time.

Light sensing heat thermostats rely on the lighting level preset by the
owner to activate heating systems. When lighting is reduced, a photocell
inside the thermostat senses unoccupied conditions and allows space
temperatures to fall 10ø below the occupied temperature setting. When
lighting levels increase to normal, temperatures automatically adjust to
comfort conditions. These units do not require batteries or programming
and reset themselves after power failures. Light sensing thermostats are
designed primarily for stores and offices where occupancy determines
lighting requirements, and therefore heating requirements.

Choosing a Programmable Thermostat

Because programmable thermostats are a relatively new technology, you
should learn as much as you can before selecting a unit. When shopping
for a thermostat, bring information with you about your current unit,
including the brand and model number. Also, ask these questions before
buying a thermostat:

Does the unit's clock draw its power from the heating systems's
low-voltage electrical control circuit instead of a battery? If so,
is the clock disrupted when the furnace cycles on and off?
Battery-operated back-up thermostats are preferred by many
homeowners.

Is the thermostat compatible with the electrical wiring found in
your current unit?

Are you able to install it yourself, or should you hire an
electrician or a heating, ventilation, and air conditioning (HVAC)
contractor?

How precise is the thermostat?

Are the programming instructions easy to understand and remember?
Some thermostats have the instructions printed on the cover or
inside the housing box. Otherwise, will you have to consult the
instruction booklet every time you want to change the setback times?

Most automatic and programmable thermostats completely replace existing
units. These are preferred by many homeowners. However, some devices can
be placed over existing thermostats and are mechanically controlled to
permit automatic setbacks. These units are usually powered by batteries,
which eliminates the need for electrical wiring. They tend to be easy to
program, and because they run on batteries, the clocks do not lose time
during power outages.

Before you buy a programmable thermostat, chart your weekly habits
including wake up and departure times, return home times, and bedtimes,
and the temperatures that are comfortable during those times. This will
help you decide what type of thermostat will best serve your needs.

Other Considerations

The location of your thermostat can affect its performance and
efficiency. Read the manufacturer's installation instructions to prevent
"ghost readings" or unnecessary furnace or air conditioner cycling.
Place thermostats away from direct sunlight, drafts, doorways,
skylights, and windows. Also make sure your thermostat is conveniently
located for programming.

Some modern heating and cooling systems require special controls. Heat
pumps are the most common and usually require special setback
thermostats. These thermostats typically use special algorithms to
minimize the use of backup electric resistance heat systems. Electric
resistance systems, such as electric baseboard heating, also require
thermostats capable of directly controlling 120 volt or 240 volt
line-voltage circuits. Only a few companies manufacture line-voltage
setback thermostats.

A Note for Heat Pump Owners

When a heat pump is in its heating mode, setting back a conventional
heat pump thermostat can cause the unit to operate inefficiently,
thereby canceling out any savings achieved by lowering the temperature
setting. Maintaining a moderate setting is the most cost-effective
practice. Recently, however, some companies have begun selling specially
designed setback thermostats for heat pumps, which make setting back the
thermostat cost effective. In its cooling mode, the heat pump operates
like an air conditioner; therefore, manually turning up the thermostat
will save you money.

A Simpler Way to Control Your Environment

The best thermostat for you will depend on your life style and comfort
level in varying house temperatures. While automatic and programmable
thermostats save energy, a manual unit can be equally effective if you
diligently regulate its setting--and if you don't mind a chilly house on
winter mornings. If you decide to choose an automatic thermostat, you
can set it to raise the temperature before you wake up and spare you
some discomfort. It will also perform consistently and dependably to
keep your house at comfortable temperatures during the summer heat, as
well.

Source List

The Energy Efficiency and Renewable Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
(800) 363-3732
Fax: (703) 893-0400
E-mail: doe.erec@nciinc.com

EREC provides free general and technical information to the public on
the many topics and technologies pertaining to energy efficiency and
renewable energy.

Reading List

"Electronic Thermostats," Radio-Electronics, June 1992.

"Energy Saving Thermostats," Consumer Reports, October 1993." "Good News
on the 'Setback' Front," T. Wilson, Home Energy, Jan-Feb 1991. 2124
Kittredge Street, No. 95, Berkeley, CA 94704, (510) 524-5405.

"Home Environment," Home Mechanix, February 1992.

"Home Q&A," Home Mechanix, November 1995.

"The Latest in Home Thermostats," Consumers' Research Magazine, February
1990.

"New Electronic Thermostats Save Money," Consumers Digest, January 1989.

"Programmable Thermostats: How to Buy and Install One in Your Home,"
Family Handyman, January 1989.

"Smart Thermostats for Comfort and Conservation," March 1994, EPRI
Journal.

This document was produced for the U.S. Department of Energy (DOE) by
the National Renewable Energy Laboratory (NREL), a DOE national
laboratory. The document was produced by the Information Services
Program, under the DOE Office of Energy Efficiency and Renewable
Energy. The Energy Efficiency and Renewable Energy Clearinghouse (EREC)
is operated by NCI Information Systems, Inc., for NREL/DOE. The
statements contained herein are based on information known to EREC and
NREL at the time of printing. No recommendation or endorsement of any
product or service is implied if mentioned by EREC.

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