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It wouldn't be correct to say that thermostats are completely universal. While there are a number of thermostats that can fit a diverse range of HVAC systems, there are still some specifications that need to be kept in mind before buying one. Thermostats usually depend on power systems, type and size of HVAC systems, and the range of functionality you require.

Are Thermostats Universal?

To answer this question in one word: no, thermostats are not exactly universal. The type of thermostat you choose to buy will depend on the type of HVAC system you have at home and when it was installed. 

Thermostats differ by the voltage level they operate on, the wiring that they have and how many HVAC components they can control. There are thermostats on the market that are specific to only a particular type of device, while others control multiple devices and multi-stage heating/cooling.


The next thing to consider is the functionality that you’re expecting from your thermostat. There are simple thermostats that will just do the bare minimum in changing the temperature for you. Then, there are the more advanced types that let you program your week ahead of time.

Smart thermostats usually allow for remote control, meaning you can connect to your HVAC system from your phone. Finally, the number of devices you want to control with your thermostat will also play a role. 

At SmartHomePerfected we always advise readers not to undertake any work they are not competent in performing and if in any doubt, it is best to seek the assistance of a professional.

Finding out which thermostat works best for you

Choosing the Power System

When it comes to choosing the right thermostat for your household and HVAC system, you should first consider the power system you will go for. You have to pick one of three: mV, 24V, and High Voltage.

Keep reading to find out everything you need to know about these three power systems commonly seen in thermostats.

mV Power System

The mV (or millivolt) system is not very common with thermostats. It is most commonly used for gas fireplaces and low power heating systems. You can also find it in floor heaters and wall heaters that don’t require electricity.

Millivolt thermostats are designed specifically for these types of heaters and cannot be used with anything else. They also require two wires.

24V Power System

The 24V system is the most common power system among all thermostats. If you shop around, you will find that most modern thermostats use the 24V system. This type of system requires 3-5 wires.

The way it works is that a transformer carries the power to the thermostat. The thermostat then initiates contact between the wires and the 24-volt power, thus initiating cooling or heating.

High Voltage/Line Voltage Power System

The final type of power system is the Line Voltage/High Voltage type. This system uses 110V or 240V depending on where you are located. It is most commonly used for older HVAC systems, as well as some boilers, air conditioners, and furnaces.

Most modern smart thermostats are specifically designed for low voltage, so they would not be compatible with this system.

Though there are a few exceptions, The 24V system is usually a safe bet for most homes. If your HVAC system is really old, you might want to go for a High Voltage system. If you’re looking to regulate gas heating in your walls or floors, you can go for an mV system. 

As you can see, there is no one-size-fits-all when it comes to thermostats! While most people might go for the 24V system, you will have more options down the line to choose from. 

Size of HVAC system

Safe in the knowledge that thermostats are not universal, the stat you choose will greatly depend on the size of your HVAC system. More components would mean a more advanced thermostat and probably more temperature sensors, too. 


Your HVAC either consists of one device or multiple devices. Let’s say you only have one heater and that’s all you need to control with your new thermostat. For this case, there are simple baseboard thermostats, which can either be plugged into an outlet, or connected directly to the heater. 

In older houses, where there is only heating from a single boiler, traditional line voltage thermostats with a dial or single-button can be seen.  

If your HVAC system consists of both heating and cooling it will have multiple devices, such as a boiler and a split AC system. For these systems, 3-5 wire thermostats are more common. Most of the time, such systems will have a thermostat that works on 24 volts.


Finally, a more complex, multi-stage heating and cooling system requires a little bit more attention. Your thermostat will need to manage multiple heating/cooling devices while monitoring multiple sensors.

These thermostats are usually programmable, digital and have four or five wires. 

As you can see, the size of your HVAC system plays a big role in the type of thermostat you are going to get. Pay attention to that! 

Type of controls on your thermostat

The type of control of your thermostat matters, but it is up to your preference. Unlike the previous criteria, this one is not determined by your HVAC system, but rather by the features you are looking for in a thermostat. To get a better idea of the type of thermostat you can buy, let’s go step by step:

Mechanical thermostats

Mechanical thermostats are usually used for older HVAC systems that don’t need much in terms of capabilities. Let’s say you don’t have many devices to service. If that’s the case, you can go for an old-school mechanical thermostat.

These are usually the Line Voltage type and require 2-3 wires. You will find that most mechanical thermostats still have dials, as the models were developed a long time ago.

Programmable thermostats

Programmable thermostats gained popularity recently. They are much more modern and come with LCD displays. The whole point of a programmable thermostat is that you can schedule your days in advance and not touch the thermostat for weeks.

For example, let’s say you like to wake up to a warm house in winter. Schedule it in your thermostat and it will turn up the heat before you wake up. 

Smart thermostats

Finally, smart thermostats are the most modern thermostats on the market right now. They have LCD displays and touchscreens. You can connect your smart thermostat to your virtual assistant (Amazon Alexa or Google Assistant for example) and use voice commands to change the temperature.

If that’s not enough, smart thermostats can be controlled remotely using a phone app. Smart thermostats can also learn your schedule and automatically adjust to it in the future. As a result, they can save energy!

What to know about your home before buying a new thermostat


In this article, we’ve talked a lot about wiring and for good reason. Some thermostats require two wires, while others need 4-5. Before you settle on a thermostat, you should know what kind of wiring system you have at home. Do you have 2, 3, 4, or 5 wires? Find out more about thermostat wiring here.

HVAC Devices

Then, consider the number of devices you have in your HVAC system. That should greatly affect your final decision. Just like you wouldn’t buy a small wardrobe to fit piles of clothes, you should buy a fitting thermostat for your HVAC system. 

Do you have a C-wire?

The C-wire is an additional wire that is used to power thermostats. You don’t always need one, though! If you ever do and you don’t have one, you can find a C-wire adapter.  However, if your system does not have a C-wire, it is better to get a thermostat that does not require it.

Are Thermostats Universal? Conclusion

Now you should have a better understanding of the kind of thermostat your house needs. Consider things like the size of your HVAC system and the capabilities you are looking for in a thermostat. Remember that the more modern models usually provide more capabilities and give you better control over your HVAC.



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