The household thermostat has evolved substantially from the round dial, simple models of the past. This article will endeavor to explain some of the features and benefits found in modern digital thermostats. On ProThermostats.com - we sell over 200 different thermostat models from over 6 different manufacturers. While we provide some robust tools to help select a thermostat that is right for you - this article will explain some common bells and whistles found on thermostats in greater detail.
The main job of the thermostat is to keep the indoor environment comfortable by turning on a heating or cooling source, depending on the sensed living space temperature. Essentially a heat-activated switch, a thermostat has a temperature sensor that causes the switch to open or close, completing or interrupting an electrical circuit that runs the house's heating or cooling system. Most residential models do this with a low-voltage circuit. The thermostat usually does more than just turning the heat on or off. Depending on the type of heating or cooling system in the home, the thermostat may control the system fan, which circulates air through the home ductwork during heating and cooling. Some thermostats can even control a whole house humidifier, dehumidifier, or ventilation system.
In all cases, the first thing to consider when shopping for thermostats is system compatibility. Not all thermostats work with all heating & cooling systems. This riddle of compatibility is where many homeowners get stuck when attempting to replace their old thermostat.
NOTE: If you have electric baseboard heat, or if you think you may use a high voltage thermostat - STOP HERE. Consult an electrician. The voltages in these systems can kill. If you have a system like this, you should contact a professional for thermostat replacement.
If you are reading this, let's assume that you are replacing a standard, low voltage thermostat. The easiest way to identify the system type and power requirements for your new thermostat is to physically remove your existing thermostat (or faceplate on the existing thermostat), and identify which terminals are currently in use and what wires terminate on the current thermostat.
To learn more about thermostat wiring, see our two wiring articles:
Understanding Thermostat Features
When shopping for thermostats, you will notice and abundance of terms relating to features. You might not understand what these features mean.
Programmable vs. Non-Programmable Thermostats
A non-programmable thermostat offers simple temperature control. The heating or cooling setpoint is held forever, and does not change until you change it. On the other hand - a programmable thermostat (aka "setback" thermostat) allows you to adjust the times you turn on the heating or air-conditioning according to a pre-set schedule with pre-set temperature setpoints. As a result, you don't operate the equipment as much when you are asleep or when the house is not occupied. By turning your thermostat back 10°-15° for 8 hours, you can save about 5%-15% a year on your heating bill-a savings of as much as 1% for each degree if the setback period is eight hours long.
Typically, there are 4 program periods per day - Wake, Leave, Return, Sleep. It is a common practice to "set back" the thermostat during the "leave" and "sleep" program periods.
What is a 7 day program vs. 5+1+1 vs. 5+2 vs. 24 hour?
- 7-Day Program - A 7-Day Programmable thermostat has a different program option for every day of the week. This flexible thermostat is perfect for people who have varying schedules and are also concerned about saving money on energy bills. Most newer thermostats have a "quick copy" feature that makes it easy to copy a program schedule from one day to the next.
- 5+1+1 Program - The 5+1+1 program style contains three separate programs. This program style is best suited to people who have the same schedule 5 days in a row (Mon-Fri), and then have different schedules for the next two days (Saturday is different from Sunday). If you require the same schedule 6 days in a row, the simplest method is to use the 5+1+1 style and program the 5 day and 1 day to the same schedule. This is useful for a business that is open 6 days a week or for a person who works Saturdays.
- 5+2 Program - The 5+2 program style contains two separate programs. This program style is best suited to people who work Monday through Friday, but are home Saturday and Sunday.
- 24 Program - The 24 hour program contains 1 programs - which is exactly the same every day.
One extremely popular new feature is "auto-changeover" Traditionally, it was necessary to manually switch the thermostat from heating mode to cooling mode. Thermostats with the auto-changeover feature will automatically switch from heating to cooling mode and back without any manual intervention. This is especially useful in climates where it can be very cool at night - but gets hot later in the day. An auto-changeover thermostat will have a "deadband" between the heating and cooling setpoints to prevent the heating cooling system from constantly cycling back and forth.
Hold Features on Programmable Thermostats
A programmable thermostat will automatically control heating and cooling to different setpoints, usually four different times during the day. In some instances it is desirable to override the preset program, and have the thermostat hold a setpoint for an extended period of time. Let's say you are home sick from work - and you don't want the thermostat to "set back" or let's say you leave on vacation and you want the thermostat to set back for 3 days, or 2 weeks. Most programmable thermostats will have some sort of hold functionality. There are a few different "flavors" of the hold feature that we will define.
- Temporary Hold - a temporary hold feature will allow you to override the pre-programmed setpoint to something different until the next program period kicks in. Let's say you have chronically cold guests over for dinner and you need a few extra degrees of heat to make them comfortable. You still want the thermostat to set back when you go to bed. Some thermostats offer a variation on this theme that provides temporary override that can be set in 15 minute increments so you don't have to wait for the next program period to start saving money.
- Permanent Hold - a permanent hold feature allows you to override the pre-programmed setpoint permanently. Once the thermostat is put in a hold state - it will stay this way forever until you change it. Essentially, this allows you to change your programmable thermostat to a non-programmable thermostat.
- Vacation Hold - a vacation hold feature allows you to change the programmed setpoint for an extended period of time, usually measured in days. For example, you could set back your thermostat for 4 days and then restart your program so you return to a warm house. The Honeywell VisionPro thermostats provide the most robust vacation hold functionality - allowing a hold period of up to 365 days.
Automatic Reminders: Filter, Humidifier Pad & UV Lamp Bulb
Your HVAC system certainly includes a filter (if you have a forced air system) and may also contain other IAQ (Indoor Air Quality) equipment. You may have a whole house humidifier with water panels that need to be replaced, and you may also have an Ultraviolet Air Purification System that has bulbs which must be periodically replaced. Your thermostat is in the unique position to keep track of how long your heating or cooling systems are running. Because of this - the thermostat can provide an alert when it is time to change your furnace/AC filter, humidifier water panel, or UV Bulb. More sophisticated thermostats will keep track of system fan runtime to decide when it's time to change the filter. In many cases, this is adjustable. It makes more sense to track fan hours rather than calendar days for filters - while the opposite is true for humidifier water panels.
Below is an example of adjustable change reminder options on Honeywell VisionPro thermostats:
Programmable Fan / Circulation Fan
A standard thermostat will turn on the system fan whenever there is a call for heating or cooling. The fan is necessary to bring the air from the living space into your mechanical room (via return ductwork) to be heated or cooled. Is then takes the conditioned air and distributes is through the home. There is usually another fan position on most thermostats which is "ON", which runs the fan 100% of the time whether or not heating or cooling is occurring. A new feature known as a "programmable" or "circulation" fan mode will cycle the system fan randomly even when there is no call for heating or cooling. With this fan mode in place - you do not have run the fan all the time (higher electric bills) but you are able to pull more air through your filter, and markedly improves the temperature control, humidity and filtration system in a house. It allows "averaging" of air temperature throughout the house and eliminates stagnant and uncomfortable air that a thermostat can't detect.
Note - "programmable fan" may also indicate that you can have a different fan mode for each of the four program periods on your programmable thermostat. In the case of the White-Rodgers blue thermostats, or the Honeywell VisionPro thermostats - the fan mode could be programmed like this:
- Wake Period - fan runs continuously
- Leave Period - fan runs on auto mode (only when heating or cooling)
- Return Period - fan runs in circulation mode (fan will run randomly if no call for heat/cool)
- Sleep Period - fan runs continuously
Bottom line - if you have a higher end whole house air filtration system connected to your HVAC - you should probably look at a thermostat with flexible fan settings like programmable fan.
Keypad Lockout / Setpoint Limiting
Keypad Lockout - It is often desirable to prevent others from tampering with thermostat setpoints or programs. Property managers and landlords that are responsible for paying utilities are especially interested in keeping tenants from wasting energy, but homeowners may also need to keep children or guests from making changes to their thermostat. Many thermostats offer some mechanism to "lock out' the thermostat keypad, either totally or partially. It's a good idea to find out how this is done and what this really means when considering a thermostat. A quick glance at the owner's manual or installer guide will tell you if this feature meets your requirements.
Setpoint Limiting is a feature (different from keypad lockout) that prevents unwanted changes to thermostat settings. This limiting is normally set up in the thermostat software - and will set a high and low setpoint limit. For example - you would be able to set the highest possible heating setpoint at 72 degrees (or any other temperature of your choosing).