Buyers' guide: Programmable thermostats

The Old House Web

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.

Thermostat basics

manual thermostatA thermostat 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.

Myths and facts

You can easily save energy in the winter by setting the thermostat to 68degrees Fahrenheit (20 degrees Celsius) when you're at home and awake, andlowering it when you're asleep or away. This strategy is effective andinexpensive if you are willing to adjust the thermostat by hand and wake up in achilly house.

In the summer, you can follow the same strategy with central airconditioning, too, by keeping your house warmer than normal when you are away,and lowering the thermostat setting to 78 degrees only when you are at home andneed cooling.

Myth # 1: 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.

Fact: 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.

Myth # 2: 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.

Fact: 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 orautomatically reducing your thermostat's temperature setting for as little asfour hours per day. These savings can be attributed to a building's heat loss inthe winter, which depends greatly on the difference between the inside andoutside temperatures. For example, if you set the temperature back on yourthermostat for an entire night, your energy savings will be substantial.

By turning your thermostat back 10 degrees to 15 degrees for 8 hours, you cansave about 5% to 15% a year on your heating bill--a savings of as much as 1% foreach degree if the setback period is eight hours long.

The percentage of savings from setback is greater for buildings in milderclimates than for those in more severe climates. In the summer, you can achievesimilar savings by keeping the indoor temperature a bit higher when you're awaythan you do when you're at home.

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

Thermostats with automatic temperature adjustment

automatic thermostat

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

Programmable thermostats have features with which you may be unfamiliar. Thenewest generation of residential thermostat technologies is based onmicroprocessors and thermistor sensors. Most of these programmable thermostatsperform 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. Somehave back-up battery packs that eliminate the need to reprogram the time orclock in case of a power failure. New programmable thermostats can be programmedto accommodate life style and control heating and cooling systems as needed.

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.

Types of automatic and programmable thermostats

There are five basic types of automatic and programmable thermostats:

  • electromechanical
  • digital
  • hybrid
  • occupancy
  • light sensing

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

Electromechanical (EM) thermostats, usually the easiest devices tooperate, typically have manual controls such as movable tabs to set a rotarytimer and sliding levers for night and day temperature settings. Thesethermostats work with most conventional heating and cooling systems, except heatpumps. EM controls have limited flexibility and can store only the same settingsfor each day, although at least one manufacturer has a model with separatesettings for each day of the week. EM thermostats are best suited for peoplewith regular schedules.

digital thermostat

Digital thermostats are identified by their LED or LCDdigital readout and data entry pads or buttons. They offer the widest range offeatures and flexibility, and digital thermostats can be used with most heatingand cooling systems. They provide precise temperature control, and they permitcustom scheduling.

Programming some digital models can be fairly complicated. Make sureyou are comfortable with the functions and operation of the thermostat youchoose. You won't save energy if you don't set the controls or you set themincorrectly.

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

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

Light sensing heat thermostats rely on the lighting level preset bythe owner to activate heating systems. When lighting is reduced, a photocellinside the thermostat senses unoccupied conditions and allows space temperaturesto fall 10 degrees below the occupied temperature setting. When lighting levelsincrease to normal, temperatures automatically adjust to comfort conditions.These units do not require batteries or programming and reset themselves afterpower failures. Light sensing thermostats are designed primarily for stores andoffices where occupancy determines lighting requirements, and therefore heatingrequirements.

Choosing a programmable thermostat

Used correctly, a programmable thermostat can pay for itself in energy savedwithin four years. Because programmable thermostats are a relatively newtechnology, you should learn as much as you can before selecting a unit. Whenshopping for a thermostat, bring information with you about your current unit,including the brand and model number.

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 existingunits. These are preferred by many homeowners. However, some devices can beplaced over existing thermostats and are mechanically controlled to permitautomatic setbacks. These units are usually powered by batteries, whicheliminates the need for electrical wiring. They tend to be easy to program, andbecause they run on batteries, the clocks do not lose time during power outages.

Before you buy a programmable thermostat, chart your weekly habits includingwake up and departure times, return home times, and bedtimes, and thetemperatures that are comfortable during those times. This will help you decidewhat 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 pumpsare the most common and usually require special setback thermostats. Thesethermostats typically use special algorithms to minimize the use of backupelectric resistance heat systems.

Electric resistance systems, such as electric baseboard heating, also requirethermostats capable of directly controlling 120 volt or 240 volt line-voltagecircuits. Only a few companies manufacture line-voltage setback thermostats.

A simpler way to control your environment

The best thermostat for you will depend on your life style and comfort levelin varying house temperatures. While automatic and programmable thermostats saveenergy, a manual unit can be equally effective if you diligently regulate itssetting--and if you don't mind a chilly house on winter mornings.

If you decideto choose an automatic thermostat, you can set it to raise the temperaturebefore you wake up and spare you some discomfort. It will also performconsistently and dependably to keep your house at comfortable temperaturesduring the summer heat, as well.


The information in this article was provided by the National Renewable Energy Laboratory (NREL), a Department of Energy national laboratory.

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