Do I Use O or B On My Heat Pump Thermostat?

What is the O/B setting on a thermostat? The O/B setting controls the reversing valve in a heat pump. The O setting in setup is the most common. When setting up a new thermostat, and you select O, the heat pump should cool while in the cool mode.

If the heat pump’s indoor coil heats in the cool mode, switch the setting from O to B. It’s really that simple.

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 O/B setting in a thermostat controls the reversing valve in a heat pump.

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 stat 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. This one (ad) at Home Depot will be what works. Make sure you choose the 2/heat 1/cool heat pump.

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

For the price, I like the (ad) Honeywell T6 Pro, available with wifi

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

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/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 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.

Should O/B Be Energized On Cool Or Heat?

This 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.

Chad Peterson

Chad Peterson is a veteran of the HVAC industry since 1977. "I like to explain heating and air conditioning problems in a way the average home owner can understand. "

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