Thermostat Compatibility: HVAC System (You Should Know!)

HVAC compatible thermostats

What is HVAC?

HVAC stands for Heating, Ventilation and Air Conditioning. It is a system that is used to offer comfort in both indoor and vehicular environments. The main aim of this technology is to give heat comfort and adequate indoor air conditions.

The HVAC system design is based on the concepts of thermodynamics, heat transfer and fluid mechanics. It is a vital component of living spaces which include: family homes, hotels, apartments, hospitals, ships, skyscrapers, and many more. In these living spaces, healthy and safe structural conditions are controlled with respect to humidity and temperature, using fresh air from outside.

How HVAC System Works?

The 3 main concepts of heating, ventilating and air conditioning are intimately connected. This is mainly with the need to give heat comfort and adequate indoor air conditions within a straightforward installation, maintenance, and operational costs. HVAC technology can be utilized in both home and work environments. They give sufficient ventilation and regulate the pressure conditions in living spaces.

In contemporary structures, the design, maintenance systems, and installation of these concepts (I.e., heating, ventilating and air conditioning) are joined into at least one HVAC system. When it comes to small structures, contractors usually do an estimation of the capacity and sort of operation required. Afterward, they design the system by choosing the suitable refrigerant and other necessary components.

When it comes to larger structures, building service designers or mechanical engineers study, design and make specifications for the HVAC system. After that, specialty mechanical contractors manufacture and commission the designed system.

HVAC System


How to Check Thermostat / HVAC Compatibility

It is essential to study the kind of heating and cooling system that is in your building. This will help you in finding the right thermostat model to suit the needs of your HVAC system.

A lot of thermostats work well with different HVAC systems. However, there's a possibility of acquiring one that does not work with your system, or if it does work, it may damage your HVAC system. You will then have to incur some expensive repair costs. As a result, there are essential measures you should take before you think of the particular specifications you would want with a thermostat.

​1. What kind of HVAC System do You have?

​​Knowing the kind of HVAC system you have will make your job much more comfortable, particularly if you do not have technical experience. There is a wide variety of different heating and cooling systems available in the market. Determining the system you have will enable you to decide wisely on the thermostat you need.

  • ​​High voltage systems (Direct line) usually require a 110-240 volts power source. In general, these include baseboards and most of the electric heating systems. They need added attention while searching for a suitable thermostat model. You need to also note that in some older buildings, high voltage systems are also utilized in powering the thermostat.
  • ​Low voltage systems are very common in the modern world. They only need a 24 volts supply of power. A lot of thermostats will be compatible with a low voltage system.
  • ​Another type that is available in the market is the 24 millivolt systems. Furnaces that are not powered by electricity are an example of the 24 millivolt systems.

​2. How Many Stages is Your System?

​​A heating and cooling system may either be a 'one-stage' or 'two-stage' type. You may have to look inside the current thermostat you have in your system. This will help you to determine the type of system and whether it is 'one-stage' or 'two-stage.'

By so doing, you will be able to get the information you require. However, this involves dealing with your power supply to the system, therefore, if you're not sure, get a technician to assist you.

  • ​One-stage is similar to an on and off switch. It means that the units you have can operate either at a full capacity or not at all. Older HVAC systems tend to have this stage.
  • ​Two-stage is also termed as multistage. It means that your system can heat and cool at both slow and fast speeds.

3. Process

When checking your system, you are able to know the number of stages it has. Looking inside the current thermostat, you will see some numbers and letters. If your system has multiple stages for the cooling search for cables that are connected to 'y1' and 'y2' terminals. A two-stage furnace has a similar connection; it has cables that are connected to the 'w1' and 'w2' terminals.

​Some Thermostat Models which are HVAC Compatible


1. A Honeywell Wi-Fi Smart Thermostat:
To be able to power this type of thermostat, there should be a c-wire in your heating and cooling system. This Wi-Fi Thermostat is incompatible with high-voltage systems that operate at 110 volts or more. In addition to that, if you have thick wires that are attached with wire nuts, this thermostat is not suitable.

2. A Nest Learning Thermostat: 
The Nest Learning Thermostat is usually compatible with only the 24V low voltage systems. This does not include mini volt systems or high-voltage systems. Most low voltage systems work with the Nest Learning Thermostat. There are some low voltage systems, however, that are incompatible such as:


  • ​High impedance systems: Quite many high impedance HVAC systems are not able to provide sufficient power for the Nest Learning Thermostat. In such a case, the Nest thermostat will require a common wire to work with the system.  
  • ​Proprietary systems: A proprietary system uses a serial communication protocol for communication between the thermostat and the HVAC system. The Nest Learning Thermostat cannot communicate using serial protocols. To counter this, some proprietary systems are wired with some standard HVAC cables.
  • ​Micro-controller based systems: HVAC systems that utilize micro-controllers rather than relays, are at times unable to power the Nest thermostat. In such a case, a common wire is connected to the thermostat.
  • ​Wired remote sensors: The Nest Learning Thermostat might work with systems that utilize remote sensor wiring. However, remote sensor wires cannot be connected to the thermostat. These wires are typically used to give the thermostat data on the weather conditions. The Nest Learning Thermostat uses the location of your building to collect this weather data via Wi-Fi instead.

​If you're uncertain about the kind of system you have, you can switch off the power and remove the covering of your current thermostat. You will then be able to see the type of wires that are installed. Use a compatibility tool to test whether you can connect a Nest Thermostat to your system.

This type of thermostat does not work with HVAC systems that have incompatible wires. The Nest Learning Thermostat is incompatible with some wiring types such as:

​1.  3-stage cooling: The second generation Nest thermostat only works with HVAC systems that have one-stage and two-stage cooling.

2.  Heat pumps that use Service light (L) wires: L wires are usually used in showing the system's status. While the Nest Learning Thermostat is compatible with HVAC systems that have L wires, these wires cannot be attached to this thermostat. The Nest Learning Thermostat has a Nest app that shows the system's status; therefore, there's no need for the L wires. 

3.  International HVAC systems: Quite many international brands have incompatible wiring. These systems need relay panels so that they may be installed with the standard thermostat wiring used in North America.

In addition to that, there's also some hardware that is incompatible with the Nest Learning Thermostat. Here are some examples:

  • ​Some types of gas valves: Certain types of gas valves on HVAC systems will buzz or vibrate when the Nest thermostat is connected. This can be countered by connecting a common wire from the system to the thermostat.
  • ​Zone relay panels/control panels: Some HVAC systems possess control panels that need a common cable to be able to work with the Nest Learning Thermostat. Examples of such panels that require a common wire include Bryant 548E036, Carrier HK42FZ011, Honeywell AQ25110B, White-Rogers 36C03-300, Nordyne 624631-A, and Waterfurnace ATV045A 110CIT.

3. A Lux WIN100 5-2 Day Programmable Thermostat: 
This Lux Programmable thermostat is a wonderful gadget to use at home or in any other living space. This thermostat is designed to work only with HVAC systems that re-start automatically. As a result, it is not compatible with systems that are unable to back up power automatically after a black-out. It works well with a portable air conditioning system and a space heater as well.  

4. Ecobee 3 and Ecobee 4
The Ecobee 3 and Ecobee 4 are advanced thermostat systems. They have the ability to control heat in multiple rooms inside your home. They have external sensors which read the temperature levels of a room and transfer that data back to the thermostat. The thermostat will then sense the variations in temperature levels of different rooms and make the necessary adjustments.



The Ecobee 3 and Ecobee 4 thermostat systems are usually termed as 3-stage HVAC systems. They can also separately monitor a second heating system, e.g., baseboards heating. These thermostats can work with electric, gas and oil furnaces.

​Wrapping Up

​There are many different types of thermostat models available in the market. Finding the right thermostat for your HVAC system requires some added attention. Start by learning about the kind of system you have in your building, then study the current thermostat that is installed. This way you will be able to get a HVAC compatible thermostat very easily.

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