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
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 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 10° to 15° 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
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
Types of Automatic and Programmable Thermostats
There are five basic types of automatic and programmable thermostats:
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
|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
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 following table
shows an example of how to chart your weekly habits.
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
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.