There are three things we never discuss at the dinner table: religion, politics, and which wire to break with your float switch. A float switch fills with water whenever there is a drain backup, and it breaks power to one of the low-voltage wires. We clearly want the HVAC system to stop producing more condensate, so we want to break one of the circuits that makes it run (so, not something like G for the blower or O for the reversing valve). That leaves us with the choice to break R or Y.

People have some pretty strong opinions about this topic, and the truth is that what might be common sense in one part of the country could be completely wrong in another. There are specific reasons for breaking R or Y, and this tech tip will help you decide what to do.

R and Y Wires

R and Y are two of the low-voltage wires. These wires are usually red and yellow, respectively, though that isn’t necessarily the case for every system you come across. 

The R wire provides constant 24v power to the thermostat. In some thermostats, you may see two R terminals: R_H and R_C. Those just give you the option to determine whether there is constant 24v power in heating mode or cooling mode, and they’re functionally the same. The R wire connects the thermostat to the integrated circuit board, which connects to the transformer that powers everything. The R circuit also powers the defrost board in a heat pump.

The Y wire connects the thermostat to the contactor. When energized, the contacts close and complete the circuit for the compressor and condensing fan motor to turn on. The Y wire also has to run through the low-pressure and high-pressure switches; if one of those opens due to improper operating conditions, the compressor and condensing fan motor will shut off.

You can learn more about the low-voltage circuits in our tech tip and 3D video about understanding low-voltage wiring.

What happens if we break R?

Breaking R will simply cut power to the thermostat and defrost board. Nothing will be able to function until the circuit closes again, and the thermostat will be blank.

What happens if we break Y?

Breaking Y will prevent the compressor and condensing fan motor from running any longer. It does NOT affect the indoor components, including the blower or auxiliary electric heat.

Why Wire a Float Switch to Break R?

In our humid Florida climate, we train our installers and technicians to break R whenever they wire up a float switch. We mostly do that because an open R circuit doesn’t allow anything to run—but why wouldn’t we want something like the blower to run?  

Whenever we have a drain backup, there’s moisture on the coil and in the drain pan. Running the blower would allow that moisture to re-evaporate into the space. We don’t want any more humidity in the space.

Customers will also immediately get alarm bells in their heads when they see a blank thermostat, and they’ll know to call for help. We also try to empower our customers to solve drain backups on their own because it saves them a service call fee and allows us to save schedule space for more urgent or complicated calls. Our techs will also know to check the air handler first instead of beginning their troubleshooting at the outdoor unit.

Breaking R also makes sense if there’s an A/C unit or heat pump working with a high-efficiency (90%+) furnace. Those can have condensate issues as well, and breaking R will stop everything controlled by the thermostat, not just the A/C or heat pump.

Why Wire a Float Switch to Break Y?

We don’t personally wire float switches to break Y for the reason explained above; the blower will keep running even if the compressor is off. 

However, there are valid reasons to break Y.

If you need heating, breaking Y will still let the blower and heat run. It won’t cause the thermostat to be completely blank, so you won’t lose those functions. 

If the evaporator coil freezes in a climate where high humidity isn’t a problem, running the blower could help speed up the melting process without making the space too muggy. You wouldn’t be able to do that if you broke R, but you could if you broke Y. 

Plus, as much as we talk about the issues with high relative humidity, low relative humidity (<30%) could also present some health challenges. Dry environments present an increased risk of allergies and respiratory infections. Leaving the blower on can help some of the moisture in the drain pan reevaporate into the space. That can help a homeowner in a dry climate keep their home RH in the desired 30–60% range.

While our team fits squarely into the “Break R” camp, the decision to break R or Y really depends on your service practices and climate. 

What DOESN’T change regardless of climate is the fact that you need to wire float switches in series. If one trips, they all need to trip. Otherwise, the HVAC system could continue running if one of them trips, which could cause the pan to overflow and lead to water damage.