Can Two Thermostats Control One Zone?

If you have a two-story house with only a one-zone HVAC system installed, you most probably have problems balancing the temperature in your home. You’ve asked around, trying to find a solution to the problem. It seems to you that more than one solution is possible, but the option to simply add another thermostat could be a good and also the least expensive option. However, you wonder whether two thermostats can effectively control one zone and, if it is possible, what will be the effect on your energy bill. 

In principle, it is possible for two thermostats to utilize the same HVAC system and control one zone and the centralized air conditioning. There might be some older thermostats that will not allow this, but generally speaking, it is possible. If the two thermostats are correctly installed and connected, you can also save on your electricity bill.  

In this article, I’ll discuss how two thermostats can control one zone. I’ll also briefly refer to some of the other possible solutions to your problem. 

Reasons why you need more than one thermostat

The main reason why owners of houses with more than one floor want to install a second thermostat is that the temperature on the two floors differs. This is a result of the existing thermostat only reading the temperature on the floor where it is installed.

The movement of warm and cool air causes different temperatures on the first and second floors. Having a thermostat on each floor will give temperature readings on the specific floor. 

Examples to understand the concept

Let’s look at examples of how two thermostats can make the temperature throughout your home more comfortable.  In the examples, I assume that there is a thermostat installed on each floor and both are correctly wired to control the centralized air conditioning. 

Example 1 – Winter 

The overall temperature in your home will be more comfortable in the winter if you set the thermostat on the first floor to the preferred temperature. Then set the temperature on the thermostat on the second floor to two degrees lower.  The warm air on the first floor will rise and make the upper floor comfortable.

If your house has more than two floors, you follow the same principle. If you have a thermostat on every floor, you set the preferred temperature on the thermostat on the first floor and decrease the temperature by two degrees on every floor.

Example 2 – Summer

For the summer heat, you do the same, except that you start with the preferred temperature on the top floor and then increase the set temperature on the first floor by two degrees. 

The rule of thumb for a two-story house is that you set each floor’s thermostat two degrees Fahrenheit apart from the other. 

How do two thermostats control one zone, and does it save energy?

Let’s look at an example of the way two thermostats operate in one HVAC zone. Usually, it gets warmer on the top floor than on the first floor of a two-story house in the summer. If you don’t have a zoned HVAC system installed in your home and your system only uses one thermostat, the thermostat cannot differentiate between the warmer temperature on the top floor and the cooler temperature on the first floor. The thermostat only read the temperature on the floor where it is installed.  

If the existing thermostat is on the first floor you can install a second thermostat against a wall on the top floor. Remember, for the two thermostats to control the same zone, they have to be wired in parallel. In this article, I’m not explaining how to wire the two thermostats but fortunately, there are excellent videos available online showing you step-by-step how to connect the thermostats and the HVAC system. For this discussion, I assume that the wiring has been correctly done. 

With the two thermostats installed, you can, for example, set your thermostat on the first floor at 74 degrees. The air conditioner will start to work without the blower fan when the temperature reaches 74 degrees. If you’ve set the thermostat on the top floor to trigger the blower fan at 76 degrees, the blower fan will only kick in when the temperature on the top floor rises above 76 degrees. When the blower fan is running, it will even out the difference in temperature.  

If the two thermostats are installed and set correctly you will also save electricity (and money) because the blower fan is only active when it is needed and is not, as with one thermostat, automatically running when the air conditioner kicks in. 

Benefits of two thermostats for one zone

Although the two-thermostat-one-zone solution is perhaps not the most effective way of balancing the temperature in your home, it offers certain benefits, such as the following:

  • A correctly installed two-thermostat system helps you to save substantially on your utility bills. 
  • You can control the temperature of the first and the top floor remotely. 
  • You can use the two-thermostat system successfully in winter and summer. 

Other options

If you cannot for some reason, like incompatible thermostats or an old HVAC system, use two thermostats in your one-zoned house, the other options you can consider include 

  • a completely separate HVAC system on every floor with unconnected thermostats, or 
  • a house divided into zones by using dampers in the ductwork to accommodate the different temperature settings on each thermostat. A zoned system can be designed in such a way that every room in the house is a different zone with its own thermostat.   Thermostats in zoned systems can either control the same heating and cooling system, or each thermostat can control its own system.

FAQs

Q1: What are the consequences if two thermostats are synced? 

A1: Multiple thermostats can be synced but once synced, they use the same temperature settings and maintenance methods. If two synced thermostats are installed in different rooms in your home, they will set the same temperature in both rooms.

Q2: Is there a difference between zoned and multistage systems? 

A1: There is a difference between the two systems. A multistage system is a one-zone system with more than one level of heating or cooling. A zoned system has more than one thermostat to control the heating and cooling in different parts of the house.

Conclusion

Although a zoned HVAC system in your home is the ideal setup because you can control the temperature in every room or other space in the house separately, it can be very expensive. Fortunately, the more affordable option of a two-thermostat-one–zone home can also help to balance the temperature in your home – especially if it is a double-story house.

Charles John

A novice DIYer who learns about home ventilation. I am a mechanical engineer and have a basic knowledge of HVAC systems but I learn continuously to make myself the best blogger in that space.

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