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Conservation: Home Heating
The thermostat controls how your heating system operates. It senses temperature and turns the heaters according to your wishes. Well, sort of...

Under the best of circumstances most thermostats work poorly. They often sense temperature inaccurately, and they tend to be slow to respond to changes in the temperature they sense. Unfortunately, most thermostats don't even enjoy the best of circumstances. More times than not they are placed in locations like hallways from where they cannot accurately measure the temperature in places where occupants spend most of their time. Let's have a closer look.

There are two types of heating system thermostats, line-voltage thermostats and low-voltage thermostats. In general, line-voltage thermostats control baseboard heaters or the like in zonal systems while low-voltage units control furnaces or boilers in central heating systems.

Line-voltage thermostats
A line-voltage thermostat acts like a switch for the electricity that runs a baseboard heater or similar device. It is commonly located on a wall in the room being heated, but it may also sit on the heater itself. Heaters controlled by line-voltage thermostats are either on or off. They have no intermediate settings, so turning them on to a high temperature does not result in the room heating up faster.

The heaters they control usually run on 220 volt current, which means they have two hot lines connected to them. Line-voltage thermostats come in to varieties: single-pole (or two-wire) units that only switch one of the hot wires and double-pole (or four-wire) units that switch both hot wires. The only sure way to tell which kind you have is to remove it from the wall and count the wires connected to it. However, you can often tell that a thermostat is of the double-pole type because it has an "off" setting. Most single-pole units will only show "low" or a similar designation at one end of the settings. The advantage of a double-pole thermostat is that you can turn it completely off and the heat will not come on no matter how cold your room gets. But with a single-pole unit, the thermostat will turn the heat on even if set on the lowest setting.

The vast majority of line-voltage thermostats installed in the Seattle area employ a bimetallic element to sense and control the heaters. This design works on the principle that different metals expand and contract at different rates as the heat and cool. These thermostats are cheap to make, but they tend to be inaccurate and to allow a very broad deadband. The deadband is the temperature range between when the thermostat turns the heaters on and when it turns them off. An old bimetallic thermostat could easily allow for an eight degree deadband or greater. That means that, if you set the thermostat for 68 degrees, it wouldn't turn the heaters on until the temperature dropped to 64 degrees, and it wouldn't turn the heaters off until the temperature reached 72 degrees. If you have baseboard heaters and you experience large changes in temperature while keeping a constant thermostat setting, the thermostat is causing the phenomenon.

Low-voltage thermostats
If your home has a central heating system, chances are you have a low-voltage thermostat controlling it. It's probably place on the wall in hallway or the living room, but you might find it anywhere. If you have an older thermostat, the mechanism inside it can take many different forms. One common design contains a coil of metal attached to a mercury switch. If you have one of these, you will need to take the old thermostat to a hazardous waste disposal station when you replace it with a new unit.

Unless you have a newer electronic thermostat, you should consider replacing your old one with a new one. Electronic digital thermostats offer reliability, accuracy and closer temperature control. You can also install a programmable thermostat that will automatically change settings according to a schedule you set. Since proper control of your space heating can result in substantial energy savings with no loss of comfort for you or your family, replacing an old thermostat may be the best conservation upgrade you can make to your home.

For more information about themostats, visit the Consumer Energy Center at

For more information from Seattle City Light on home heating, please e-mail or call 206.684.3800.

Contact an Energy Advisor at
or call (206) 684-3800

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