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Low Voltage vs Line Voltage vs Millivolt Thermostats: A Detailed Comparison

Not all thermostats consume the same amount of energy. Based on how much voltage a thermostat consumes; there are three types of them; low voltage thermostats, line voltage thermostats and millivolt thermostats. What is the difference between these three?

A line voltage thermostat needs to be connected to a power supply of fewer than 30 volts, while a millivolt thermostat needs only 0.75 volts and a line voltage thermostat needs 120 to 240 volts. A millivolt thermostat consumes the least amount of energy while the line voltage type consumes the highest.

Given below is a simple table showing the main differences between these three types of thermostats.

Thermostat type Volts Wiring system Installation Usage 
Low voltage thermostats <30VThin wires 2,3 wires with no cooling 4,5 wires with cooling  More than 7 wiresMedium difficulty Furnaces, baseboard heaters, boilers, heat pumps, air conditioning, radiant heat 
Line voltage thermostats 120V-240VSingle pole: 1,2 wires  Double pole:4 wires Low difficulty Electric space heaters, furnace, baseboard heater, any other heating system 
Millivolt thermostats 0.75V2 wires Low difficulty Direct vent furnaces, top vent furnaces, gravity wall furnaces, mobile home furnaces. 

If you are interested in learning more about the differences between line voltage, millivolt, and low voltage thermostats, keep on reading.

What is a low voltage thermostat?

This type of thermostat is one of the most common ones as you will come across them in many homes. Low voltage thermostats are heating and cooling regulators that need less than 30 V to function. They use less energy than a line voltage thermostat which, in consequence, makes them more popular as well. 

You can use a low voltage thermostat to manage any type of system from an air conditioning system to a furnace, and boilers as well as heat pumps and radiant heat. 

In terms of installation, these thermostats could be quite difficult to set up for someone with no experience as they have a more complicated wiring system. 

These low voltage thermostats can use between two and three wires with no cooling as well as four to five wires with cooling and they get even to more than seven wires with heat pumps. Considering that all these wires are thin and they are harder to manage, the installation will not come as easy. 

Understand how low voltage wiring works

Low voltage wiring is common but less common compared to line voltage wiring and it is not defined in the same way worldwide. Even so, The National Electrical Code (NEC) considers being a low voltage wiring any wiring circuit that uses less than 50V. While this is the general range of voltage for such a system, you have to consider that some tools and appliances use only between 12V ans 24V. 

Since low voltage wiring consumes so little energy compared to line voltage wiring, people usually opt for such a system to save money on their power bills. However, using a low voltage system will most likely require also investing in a transformer in order to adjust the voltage of the system with the voltage of the product you need to use. So, this aspect increases the difficulty of installation and also it increases the original cost.  

What is a line voltage thermostat

Line voltage thermostats use on average 120V-240V so they consume more power than low voltage thermostats. But in spite of the fact that these thermostats consume more energy than the low voltage thermostats, they come with many other advantages that might make you consider investing in one of these products. 

You will find these thermostats easier to install compared to low voltage thermostats as they only have two wires for the single pole thermostats and four wires for the double pole thermostats. They are ideal thermostats for baseboard heaters or electric furnaces but also for any other type of system. 

Single pole thermostats 

The single pole thermostats with line voltage come with only two wires that are connected to the main circuit and they use a single current. To turn off these thermostats you will have to disconnect the entire unit. 

Double pole thermostats

Unlike single pole thermostats, the double pole thermostats with line voltage use four wires that connect the main circuit. These thermostats use different currents as well in order to operate correctly. An advantage of the double pole thermostats is that you will be able to turn them off without having to disconnect the unit. 

Understand how line voltage wiring works

A line voltage wiring system will tell you how many volts your circuit uses or your socket uses and therefore, what kind of appliances you can use with such a circuit. Line voltage circuits use 120V and they accommodate most appliances you need in your home, starting with your line voltage thermostat and ending with your vacuum cleaner.

These systems bring several benefits which is why most people choose to invest in them. The cost of your appliances will be lower as well as the cost of any fixtures you might need. You will not need to use a transformer in order to convert the voltage and the range of products you can choose from for such a system is very wide. When it comes to American products, line voltage is the standard for most of them, which is a great advantage to consider as well. 

What is a millivolt thermostat

Millivolt thermostats are known as thermostats that use the least voltage. These thermostats are not so common as they will not function with any low voltage system and they need special compatibilities to work to the highest of their benefits. 

A millivolt thermostat uses an average 750 millivolts (mV) or 0.75 volts. These thermostats will not be connected to the wiring system in your house and they will not use a transformer like low voltage thermostats. 

You will make the most out of such a thermostat with such a low voltage if you use it with a direct vent as well as a top vent wall furnace that is made to accommodate a small surface. You can also use them with the furnaces of gravity walls and mobile home furnaces. If you are going to use a millivolt thermostat, you will also need a millivolt system in your home to create the functionality you are looking for. 

How to choose the right thermostat for your home

Choosing the right thermostat for your home will depend on a variety of factors that you need to consider. You might be able to check the following aspects yourself but when in doubt, always ask a technician to offer you the best information!

Get familiar with the wiring system in your house

The most important aspect to consider is what wiring system you have in your home. A low voltage thermostat will not be compatible with a line voltage wiring system. Similarly, a millivolt thermostat will only work with a compatible millivolt system and not your regular low voltage wiring or line voltage wiring. 

You might be able to convert the voltage differences by using a transformer so your thermostat can match your home wiring system the best. But chances are that you will not need to invest in a transformer as long as you choose the best thermostat for your wiring circuit. 

When you purchase a thermostat, you will be able to see what type of wiring system it is compatible with by checking the product description and its specifications. If you are not sure about the wiring system you have in your home, don’t hesitate to ask a professional before you decide to invest in a thermostat. At the same time, the technician that evaluates your wiring system will also be able to guide you towards the most compatible types of thermostats too. 

Are you going to install the thermostat yourself?

Installation is another type of aspect you definitely have to keep in mind when you invest in a thermostat. If you are going to install the thermostat by yourself, you want one that comes with a user-friendly process of installation. 

For instance, millivolt thermostats, as well as line voltage thermostats, tend to be easier to install than low voltage thermostats. If you have your heart set on a low voltage thermostat that is not matching well with your wiring system, you will need a transformer to balance out the difference of voltage. Adding a transformer to the installation process will make it that much more difficult and therefore, you might not even be able to do it yourself unless you have a minimum experience in the matter. 

Consider the size of your home

The size of your home is just as important as the other factors when you choose a thermostat. Millivolt thermostats are made to function with a millivolt system which is common and even optimal for small homes such as a studio or a small apartment and house. If you have a medium-size home or a large villa, a millivolt thermostat will not be able to accommodate your needs. 

For medium and large buildings you need to look at low voltage thermostats and line voltage thermostats. These two types are used in equal amounts, depending mostly on the wiring system in the house. If you are not sure about this aspect, it is also recommended to talk to a specialist or even use the customer service of the manufacturer to see if the thermostat you want to invest in, will be strong enough to cover the surface of your home. 

Find a thermostat compatible with your furnace

There might be different types of thermostats that are compatible with your furnace or, on the contrary, just one type of thermostat that you can use. Many thermostats, as well as furnaces, come with a manual of instructions that specifies this type of compatibility so you will know what to invest in. But if that is not the case, you can follow the general rule that says if both your furnace and your thermostat are compatible with the same wiring system, chances are that they are compatible with each other as well. 

Therefore, a low voltage thermostat will be compatible with gas furnaces as well as electronic ignition types. These thermostats also work with electric forced air furnaces and single or multi-stage heat pumps as well as electric central air conditioning systems. As we previously mentioned, you might be able to use a low thermostat with other types of systems as well if you use a transformer and install it properly. 

Like voltage, thermostats are specific for electric furnaces and electric baseboard heaters but also other electric types of heaters. They will work with radiant heaters as well as localized heating systems. 

When it comes to the millivolt thermostat, the options are not as wide. These small thermostats will work best with a direct vent or top vent furnaces, gravity wall furnaces, or mobile home furnaces. Keep in mind that millivolt thermostats will not cover a wide surface either. You want to use them if you have a small home and you want to control the temperature in one room. 

Final thoughts

Choosing the type of thermostat you need in your house doesn’t need to be difficult as long as you have the right information. In a nutshell, you will help yourself a lot if you get familiar with the wiring system you have in your house and take into account your subjective needs as well. These two characteristics will reduce your options significantly and you will be able to choose the right thermostat for your place with less hassle. 

On the other hand, you want to pay attention to other features a thermostat might have that could increase your comfort. For instance, you can choose to invest in a smart thermostat or a wifi thermostat that is compatible with your house system and you will make the most out of it. By using the information in this guide, you will understand why you need to choose a low voltage thermostat or a line voltage thermostat and even a millivolt thermostat as long as it is compatible with your home!