HVAC: Programmable Thermostats
|| Print This Page ||
Thermostats provide a means of controlling HVAC operations to ensure occupant comfort. Programmable thermostats offer facility managers and occupants an automated way to reduce energy costs by turning down heating or cooling systems when a space is unoccupied (Figure 1). The energy savings that can be achieved depend on the local climate, how well a building is insulated and sealed, the utility rate structure, and the temperature settings used. Savings also depend on how effectively the thermostat is used—studies of residential applications show limited savings because the thermostat can be too hard to program and users often use the override option and then don't reset. Savings will also be modest if occupants are already diligent about setting back the thermostat. The same things may be happening in commercial facilities, but they are not as well documented.
What Are the Options?
Manufacturers of programmable thermostats offer a wide range of choices in type and capabilities.
Electromechanical thermostats. These devices use mechanical means such as movable tabs or sliding levers to set temperatures for daytime and nighttime use. Mechanical devices don't provide as many options as digital thermostats; they usually just use the same settings for each day of the week. They are most appropriate for use in facilities with regular schedules, and they should not be used with heat pumps because they don't have the capability to minimize auxiliary electric resistance heating.
Digital thermostats. Digital units offer varying degrees of programmability and can be used with any type of heating or cooling equipment. The simplest are only capable of maintaining a single program that must be the same for each day. Most offer the ability to set different times and temperatures for each day of the week, with four or more different temperatures in the course of each day. Other units are able to provide one program for weekdays and another for weekends, or a different schedule for Saturdays and Sundays and holidays. Most units also include a manual override capability. Some adjust furnace or air-conditioner run time based on outside temperatures. (For more on gas furnaces, see the Purchasing Advisor on Gas Furnaces.)
Occupancy-based thermostats. This type of thermostat doesn't base settings on the time of day, but instead keeps the space at a setback temperature until someone calls for heating or cooling. After that, the thermostat will maintain its setting for a predetermined period—typically between 30 minutes and 12 hours. These devices, which are simple to use but not very flexible, work best in areas that remain unoccupied for long periods.
Power supply. Batteries are the most common power supply for programmable thermostats, and they power both digital displays and the relays that control the HVAC equipment. One drawback is that the batteries need to be replaced on a regular basis. Most units provide an alert when it's time for a change. Hard-wired thermostats, on the other hand, draw power from the HVAC equipment via an additional wire. They are preferred in commercial settings where facility managers or contractors don't want to deal with batteries in the hundreds of thermostats a facility might have.
Programmable thermostats provide an automatic means of changing heating and cooling settings to reduce energy use when a space is unoccupied.
© E Source
Changing modes. Programmable thermostats can change between heating and cooling modes manually or automatically. Manual changeover works best when it happens only seasonally, as it does in residential or light commercial facilities. Larger commercial buildings use automatic systems that switch between heating and cooling based on predetermined setpoints. The temperature difference between the heating mode and the cooling mode is known as the deadband. Most automatic systems feature a minimum deadband of two degrees.
Independent fan and temperature operation. Most thermostats for commercial buildings can be set to program fan operation independently of temperature settings to ensure that indoor air quality is maintained.
Button type. The typical programmable thermostat uses buttons for control, but in recent years touchscreen products have become available.
Service reminders. Thermostats may come with a variety of service reminders. Most common is a notice to change air filters or air cleaners—either based on HVAC runtime or a set interval. Some thermostats also feature alerts on the thermostat screen for maintenance of humidifiers (change water pad) and ultraviolet (UV) lights (change lamps) in facilities that use UV for improving indoor air quality.
Networking. Many programmable thermostats can communicate with higher-level controls, such as a building automation system. This capability enables facility managers to program multiple thermostats from a single computer. Many systems can also connect to the Internet, giving users the ability to monitor and control the system from anywhere that Internet access is available. (For more information, see the Purchasing Advisor on Building Automation Systems.)
Humidity control. Some thermostats now offer the ability to sense and maintain humidity levels.
Security. Most thermostats offer security features that protect the thermostat from unauthorized use. Some offer two levels of security: The first requires the use of a personal identification number (PIN) to get access to all options and settings; the second allows users to change temperature setpoints without entering a PIN, but locks all other options and settings.
Demand response. Some utilities are using programmable, communicating thermostats to alter heating or cooling setpoints a few times per year when the electric load approaches the utility's ability to provide power. Under these conditions, the utility sends out a signal (typically using a paging or cellular network) that adjusts thermostat setpoints (higher during the cooling season, lower during the heating season) for a limited time. To encourage customers to participate in these load-management programs, utilities sometimes offer free programmable thermostats and usually offer a monthly bill credit.
How to Make the Best Choice
Make sure your thermostat and HVAC equipment are compatible. Different types of heating and cooling systems may require different types of programmable thermostats. For example: some systems require dual-stage control, heat pumps use thermostats that minimize the use of less-efficient auxiliary electric resistance heating, and baseboard electric heaters require line-voltage setback thermostats that can directly control 120-volt or 240-volt line-voltage circuits.
Analyze heating and cooling patterns. To determine what level of programming capability you need, look at occupancy patterns and heating and cooling loads to determine the number of different settings required. Develop an HVAC schedule, which will not only help in selecting a thermostat, but will also help in estimating potential energy savings and in programming the unit after it is installed.
Choose a battery-powered thermostat when replacing a mechanical unit. Battery-powered thermostats use the same wiring as a mechanical thermostat, so it's easy to remove one and put in the other.
Choose the right level of complexity. Programmable thermostats can be complicated to use properly, so choose a model that occupants and facility managers are comfortable with. Otherwise you run the risk that the unit won't be used—or will be misused—and no savings will result.
What’s on the Horizon?
The next frontier for thermostats is wireless units on mesh networks. In mesh networks, wireless nodes are linked together to form a self-organizing network that allows bidirectional communications and requires little power to operate. These units will be especially effective in large facilities that require frequent reconfiguration to accommodate new tenants or changing office architecture. A wireless system eliminates the cost of relocating and rewiring sensors and controls.
Copyright 2006 - Platts, a Division of The McGraw-Hill Companies, Inc.