Do I Select O or B In My Heat Pump Thermostat Settings?

Last Updated on May 27, 2025 by Chad Peterson

Here, you will learn which to select, O or B, in your thermostat’s settings. The most common setting in my area is O. If you are unsure, just pick one. Read on.

If you’re staring at your digital thermostat wondering whether to choose O or B, you’re not alone. Many newer models display this option during setup, like the screen shown above. The choice determines how your heat pump’s reversing valve behaves — and picking the wrong one can leave you heating when you want cooling.

Just pick O for example and turn on the heat pump to cool (or heat). If it does the opposite, Select B in the setup. You just reversed the reversing valve in your heat pump.

What is the O/B setting on a heat pump thermostat? The O or B setting controls the reversing valve in a heat pump. The reversing valve inside the outdoor heat pump unit switches the heat pump between heat and cool mode. The O setting in the setup is the most common setting that I use. When you select O, the heat pump should cool while in the cool mode.

You just learned the function of the O/B terminal on your thermostat. But I have more to offer if you read on.

Why Is There An Option For O or B

This might sound silly, but it depends on what climate your unit was manufactured for. Let me explain. When you choose O or B controls, you are selecting what mode the reversing valve will be in (heat or cool). The reversing valve is the valve that switches the heat pump between heat mode and cool mode.

Manufacturers make two basic heat pump designs when referring to reversing valve controls. One is for cool climates, and the other for warmer climates. They want the heat pumps in cool climates to fail (reversing valves get stuck) in the heat mode and units in warmer climates to fail in the cool mode. It’s a customer courtesy.

How miserable would it be to live in Arizona and have your heat pump fail in the heat mode in the summer? You could have cooling, except it won’t switch over. So, it depends on your climate whether you use O or B. Most of the time, anyway. I have worked on a few units in my cold climate in Central Oregon that use B in the stat setup.

Two quick tips on how to know if my heat pump is O or B

1. Read the installation setup for your particular model number.
2. In the thermostat setup, select O and start the unit. If it’s heating when set for cooling, switch it to B.

I don’t like taking the time to find the data; that’s why I like number two. I always check the operation after installing and setting up a thermostat. That way, I know the correct choice was made.

The image below is inside a typical air-to-air heat pump. The reversing valve, sometimes called a changeover valve, is marked in the picture below; it’s the shiny brass thing with three copper pipes sticking out the top.

Where Does The Orange Wire Go On A Honeywell Thermostat?

The orange thermostat wire is commonly used on the O/B terminal on the thermostat subbase.

I’ve had this experience more than once when I’d go on a service call, and the customer had bought a thermostat at Home Depot or another big box store. They tried to hook it up and couldn’t make their heat pump work.

The first thing I’d look at was the back of the thermostat, and there would be no O/B terminal. They didn’t buy a heat pump thermostat. It was a heat-only or a heat/cool thermostat.

The other check you can make to know if you have a heat pump thermostat is to the SYSTEM button on the front of the stat and switch it from HEAT to COOL to OFF to AUTO to EM HT. Oh No, No emergency heat setting. Heat pump thermostats have to have an Emergency Heat setting.

If this was you, hurry back to Home Depot and get a thermostat that says it operates a heat pump. But beware, some of their heat pump stats don’t have all the bells and whistles that a good heat pump stat has. If you click on the button below you’ll be whisked away to Amazon and get a good look at Honeywell’s VisionPro thermostat.

Monitor and control the heating and cooling in your home from your phone.

For features and ease of use with a nice large touch screen, I like the Honeywell TH8321WF1001 Touchscreen Thermostat Wi-Fi Vision Pro 8000 with Stages up to 3 Heat / 2 Cool. The Wi-Fi connection gives you outdoor temperature and humidity on the screen.

If your equipment has more stages, select the correct stat. When you remove your old thermostat, you will notice a W1 and a W2. If W2 has a separate wire, you most likely have three stages of heat with the heat pump included.

In case you’re interested in my favorite go-to stat, I have two that I use the most. I’m partial to Honeywell. They are a tested name, and when I call customer support, it’s generally not long until I get an answer. They have tech support for techs and homeowners. Their number is 800-468-1502

Setup is a straightforward step-by-step process. Just answer the questions as you power up the thermostat—super easy programming of your schedule for setback temperatures.

What Does O or B On My Thermostat Mean?

And why is there an O and a B on my heat pump thermostat? Short answer. The B selection in the thermostat setup means the opposite of the O setting. They could use 1 & 2 or x & y, simply an option of one or the other. O is for cold climates, and B is for hot climates, which I wish I were in now. It’s freezing here in Central Oregon.

I always think about those temperate climates that never get too cold or hot. Maybe they get a mixture of O & B units. Who knows?

Is A Trane Heat Pump Setting Supposed To Be On O or B?

It depends on what territory Trane manufactures the heat pump to be shipped to. Generally speaking, if the heat pumps are going to climates like Arizona and Texas, in your thermostat settings, you should select B. For climates where we get a lot of freezing winter weather, you will select O, but not always.

Start by selecting what makes sense for your region, and if it cools when you start the unit and you had it in the heat mode, reverse it to the other option. To my knowledge, most manufacturers make both and send them to the appropriate climate. But not always.

Should You Select O or B Be In Thermostat Settings?

O or B is your only test question. And it’s impossible to fail.

Answer: It depends on the installation instructions given with the equipment. Instructions are available online by searching for “your model number, installation, and startup instructions” or starting the heat pump and checking to see if it’s in the mode you set the thermostat to, heat or cool.

Switch the setting if it’s heating when it’s set to cool. Thanks for reading.