How a thermostat works
How does a thermostat work?
How does a thermostat control the temperature of my home heating or air conditioning? What's the difference between a regular thermostat and a programmable thermostat?
Answer from Green Energy Efficient Homes
Most central heating and cooling systems are connected to a thermostat on a low-voltage DC current. The furnace or air conditioner waits for a current on this DC line in order to operate. So when there's current on the thermostat line the furnace or AC runs, and heats or cools the house, and when there is no current, the furnace or AC stops adding heat to or extracting heat from the system.
The thermostat works by sensing the temperature where it is located, and sending current to the heating or cooling system only when the temperature would require heat or cooling.
A simple-minded dial thermostat
Photo by mulmatsherm, via FLICKR
In its simplest form, a thermostat can consist of a bimetalic coil: a flat strip that has been coiled into a spiral, where the strip consists of two different types of metal. Because the two types of metal expand at different rates as the temperature rises, the coil will expand or contract as the temperature changes. The inner end of the coil is attached in place; the outer end is used to make an electrical contact when the coil has contracted to the point where heat is required, or has expanded to the point where cooling is required. As your house warms up or cools down as a result of the heater or AC being turned on, the coil will expand or contract until the electrical contact is broken, and the heat or AC stops running.
A programmable thermostat gives you the ability to set different temperatures for different times of the day and night, so that you can keep the house at a comfortable temperature when you're up and about, and keep it at an energy-saving temperature when you're asleep or typically out of the house (for example, at work). For heating, the energy-saving temperature should be lower than the comfort temperature, and for cooling, the energy-saving temperature should be warmer. The lower you can set the energy-saving heating temperature, and the higher you can set the energy-saving cooling temperature, the more energy you will save.
Switching from a simple thermostat to a programmable thermostat can save you 5%, 10%, or even 20% on your heating and cooling bills, depending on how far apart you set the comfort and economy settings. In our house we keep the heat at a cool 66F when we're up and about and in the house - cooler than the 70F or so most people like - but we do get some savings from that 4F difference. The economy setting meanwhile is a frigid 56F - the temperature overnight and when we're away at school or work. The only issue with setting your economy temperature much lower (in winter - higher in summer) than the comfort temperature is that you need to give it extra time, at the start of a comfort phase, to reach the comfort temperature. So if you normally wake up at 7am and get home from work at 5pm, for example, set the comfort temperature to kick in at 6:30am and 4:30pm.
Some people will tell you that setting the temperature lower in summer or higher in winter for part of the day is a waste of energy, because the air conditioner or furnace has to work that much harder to cool the place back down or warm it back up. That's a fallacy. It's true that it takes more energy to cool a house from 80F to 72F in summer than to keep it steady at 72F. But it's also true that it takes no energy at all to let the temperature rise from 72F to 80F while the air conditioner is off, and overall you'll save more by letting your house move to its economy temperature, winter or summer, when you don't need the comfort temperature.
Return from How a thermostat works to the Home energy efficiency questions page or to the Green Energy Efficient Homes site home page.
© Green Energy Efficient Homes Inc. 2011 Privacy policy | Disclaimers
