EREC Fact Sheet: 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
thermostata 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 68°F (20°C) 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 78°F (26°C) 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 setthe variable is how long it must stay on to reach the set
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
10° to 15° for 8 hours, you can save about 5% to 15% a year on your heating billa
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
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
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 10° 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.
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
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
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 settingand 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.
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.