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The Science of Heating: How Thermostats Work

Old Thermostat

If keeping comfortable in your home is important to you, then your thermostat is probably one of the most important appliances you own. It's also probably the one thing you think about the least, other than to adjust the programming when Daylight Savings Time comes along. But the growing availability of "smart" thermostats may have you thinking of upgrading. These devices allow you to control the settings from your smart phone and can actually learn to adjust the temperature automatically based on your habits and preferences, making it easier than ever to save money while managing your home climate.

Even if you're not rushing out to buy one of the shiny new toys, upgrading your thermostat may be a necessary household fix. But the display of thermostats available at the hardware store may leave your head spinning if you don't have a basic understanding of how thermostats work. In this article, we'll introduce the basic mechanics of different kinds of thermostats, and discuss the heating systems each works best for. We'll also provide some ways to manage your thermostat to save money on heating and cooling all year long.

Digital Thermostats Vs Mechanical Thermostats

Most new thermostats today are digital and require a power source to operate both the thermometer and the LCD screen that displays the current temperature and settings. They have no moving parts, and measure the temperature with sensors made of semi-conductor metals. By measuring electrical resistance in the sensor, microcontrollers are capable of measuring temperature very precisely, and trigger relays that turn the HVAC units connected to it on or off within 1 degree or less of your set point.

The power needed by a digital thermometer can come from a battery in the device, or by connecting it directly to an electric circuit (with a battery as backup in case of a power outage). In the later option, a professional electrician must install the electrical wiring if it is not already in place. Digital thermostats may or may not be programmable.

Open digital thermostat

Electromechanical thermostats are constructed around bimetallic coiled strips, pieces of two different metals laminated together and wrapped into a coil. As the temperature shifts, the metals expand and contract at different rates, causing the coil to move between contact points, triggering the heater to turn off or on within 3 to 6 degrees of the set point.

Bimetal coil animation

Older thermostats of this type often have mercury in the internal switches. Since this is a hazardous material, they have been banned in many states and require special disposal.

The power for a mechanical thermostat is minimal, and comes directly from the heating device itself (via a thermopile or transformer installed on the equipment) and therefore will continue to operate even in the event of a power failure (unless it is an electrically-fueled heater).

Open thermostat

Programmable Thermostats Vs Manual Control Thermostats

Programmable thermostats have clocks that allow you to program them for automatic temperature adjustments at set times. Since this can lead to substantial energy savings programmable thermostats are increasingly popular. The US Department of Energy states that you can save as much as 10% by turning your thermostat back 10°F for the eight hours a day you are not at home. Comparable savings can be made by raising the temperature on the air conditioner in the summer.

Programmable thermostat

Today, most programmable thermostats are digital and offer increasingly sophisticated programming options, like the ability to program different weekday and weekend operating schedules. They're also growing "smarter" - instead of telling the heater to turn on at 6 AM, you can tell the thermostat that you want it to be 70°F at 6 AM, and it will plan ahead to turn the heater on early enough to reach the desired temperature at the desired time.

Non-digital clock thermostats are also available, with simpler programming options that are set by manually adjusting bars or pins. These are often simpler for people to figure out how to use.

A manual control thermostat offers no scheduling options at all, just a means to set the desired temperature, switch between heating and cooling, and to turn the fan off or on. Both digital and electromechanical thermometers come in manual control models.

Low-Voltage Vs Line-Voltage

Most home thermostats today operate on low-voltage control circuits. These thermostats operate on 24 volt AC current, using a device called a "step-down transformer" mounted on the furnace to adapt your household's 120 volt line voltage.

Line voltage thermostats, or high voltage thermostats, are used primarily to operate electric resistance heating systems, such as electric baseboard heaters, and aren't especially common. These run directly off your home's 120 volt or 240 volt current.

Line voltage thermostat

If you don't know for sure which kind of thermostat you have, you can easily check the voltage by removing the cover of the thermostat and looking at the type and number of wires connected. Two to five thin copper wires indicate a 24-volt thermostat. Line-volt thermostats have thicker wires (12 or 14 gauge) like you might see connected to an outlet or light switch. The voltage should also be noted on the inside of the cover, or on the furnace or boiler itself.

A third, mostly obsolete alternative are millivolt thermostats. These are very uncommon and considered obsolete for their inefficiency. You'll find them used primarily in direct or top vent wall furnaces. They have a special thermocouple device called a powerpile or thermopile generator to produce 750 millivolts direct current (0.75 volts). These systems won't work with standard low-voltage thermostats.

The Right Thermostat for Your Heating System

As you can see, thermostats come with a great deal of variety. But before you start picking and choosing, it's essential to know what's required for your heating systems. Heating systems and thermostats aren't mix and match. Make sure a new thermostat is compatible with your heating system before you buy it.

Here are some important details to keep in mind:

  • Electric heaters must have a line-voltage thermostat. This is because they are actually burning electricity as fuel, and require a higher electric input to operate.
  • Programmable thermostats are not generally recommended for heat pumps. This is because lowering the temperature reduces its efficiency level and causes the back-up resistance heat system to kick in, and that cancels out the heat savings. Maintaining a moderate setting in heat mode is a more cost effective strategy. However, some manufacturers are developing special thermostats for heat pumps that use special algorithms to minimize use of electric resistance systems. If a programmable thermostat is necessary, be sure to select one designed specifically for heat pumps.
  • Some thermostats (particularly those with enhanced digital controls and displays) require a common wire - a low voltage power wire - in order to provide constant electricity from the heating and cooling equipment to the thermostat. If your current thermostat is not already wired with a common wire, then you will need to have an electrician install one.
  • Many newer HVAC systems offer multistage heating and cooling. Single-stage systems are either on or off - the heater or AC runs until the set temperature is reached, and then turns off. A multi-stage system has the capability of maintaining the set temperature at a reduced power level when conditions are moderate, and then shifting to full power when required by extreme conditions. Multistage systems require thermostats designed to accommodate this functionality.

Thermostat Management Tips & Tricks

Green Check SquareTurning the thermostat higher doesn't heat your space up faster. Heat is produced at a constant rate. When it takes 20 minutes for your house to reach 72°F, if you set the thermostat to 74°F it's still going to take 20 minutes to reach 72°F. In fact, if you forget to turn the thermostat back down to 72°F you'll probably just lose energy making your home warmer than you actually want it.
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It doesn't cost more to re-heat your house than it does to leave the heater running when you're gone. Because your home is always losing heat to the outside, if you leave the heat running when you're not there, all you're doing is providing a continual heat source. In fact, the cooler your house gets, the more slowly it will lose heat. The same is true in reverse, too, when it comes to hot weather and cooling your home.

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Steam heat and radiant floor heat have slower heat-up times that make setting the thermostat back problematic. Look for newer digital thermostats that track system performance and automatically calculate the ideal time to turn the system on to achieve the right level of heat when you want it.

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Many thermostats include remote temperature sensors that let you monitor outdoor conditions, or can be used indoors to calculate an average temperature reading for your home from two separate locations. This is a great way to achieve more balanced heat if your home has hot or cold spots.

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Thermostats should always be installed on interior walls, away from direct sunlight, drafts, doorways, skylights and windows. The airflow around it should not be blocked by furniture in front of it or below it, or it may provide inaccurate temperature readings.

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Advanced thermostats are capable of controlling multiple household systems, including: heaters, coolers, humidifiers, dehumidifiers and ventilation systems.

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Thermostats should be checked, cleaned and calibrated regularly. If you have an HVAC contractor in for maintenance on your heating or cooling systems, make sure thermostat upkeep is part of his routine. Or follow these thermostat maintenance tips to do it yourself, and ensure more accurate temperature control in your home.



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