Feb 072014

Graph of presence of thermostats on central equipment, as explained in the article text

Source: U.S. Energy Information Administration, Residential Energy Consumption Survey 2009
Note: The ‘no central equipment’ group includes households whose main heating equipment is a portable heater, fireplace, or stove, or main cooling equipment is a room air conditioner.

Thermostats are used to control central heating equipment in 85% of households, and less than half of those thermostats are programmable. For households with central cooling equipment, 60% have a central thermostat and about half of those are programmable.

Because they automatically respond to temperature changes, thermostats play a key role in weather-related changes in energy demand. Programmable thermostats can be used to help households set temperatures back at night or when occupants are away. Of course, humans can also accomplish these tasks, and the mere presence of a programmable thermostat does not in itself save energy—most of them must be appropriately programmed. The ENERGY STAR® website provides guidelines for programmable thermostat usage.

Some new programmable thermostats have network connectivity features, to allow for remote control via mobile devices or the Internet. Such devices are often referred to as smart thermostats. Remote cycling capability, often installed by electric distribution utilities to control the outdoor compressor used in central air conditioners, is also a part of some demand response programs that aim to manage peak summer system loads.

Understanding the distinction between central and room-based space conditioning equipment and the different types of thermostats used to control central equipment can be useful for forecasting energy use as it relates to variations in weather. However, thermostat settings for heating and cooling can differ by both region and fuel. Many analyses of weather-related energy demand focus on a threshold of 65 degrees Fahrenheit for both heating and cooling analysis. Degree days are calculated as the difference between the average daily temperature and 65 degrees Fahrenheit, with positive differences accumulated as heating degree days and negative differences accumulated as cooling degree days. However, analysis of monthly utility bills and household consumption from the 2009 Residential Energy Consumption Survey revealed that a single temperature threshold (also called a base temperature) for analysis of both heating and cooling demand may not be entirely appropriate, as there is often a range of temperatures that would result in little or no heating or cooling demand, especially in certain parts of the country.

On average, households using natural gas may begin heating when average outdoor temperatures fall below 62 degrees. Houses using electric heat may not begin heating until outdoor temperatures fall below 59 degrees. Part of electricity’s lower threshold is explained by portable electric space heaters, which typically are not used until the outside temperature is much lower than 65. For cooling, thresholds for the use of cooling equipment are often higher than 65 degrees, with the South often not cooling until outdoor temperatures exceed 67 degrees.

Map of average base temperature, as explained in the article text

Source: Based on analysis of 2009 Residential Energy Consumption Survey data

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