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


This might sound silly, but it sort of depends on what climate your unit was manufactured for. First let me tell you why you have an option to use O or B in the set up. The O/B terminal on the subbase of your thermostat controls the reversing valve in your heat pump systems outdoor unit. The reversing valve is the valve that switches the heat pump between heat mode and cool mode.

Manufactures 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 in the heat mode and units in warmer climates to fail in the cool mode. Kind of a customer curtesy.

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 as to 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 set up.

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

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

I personally don’t like taking the time to find the data, that’s why I like number 2. 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 below, it’s the shiny brass thing with 3 copper pipes sticking out the top.

Where Does The O Wire Go On My Thermostat?

I had to think about this question for some time and then I began to recall the times when I’d go on a service call and the customer had bought a thermostat at Home Depot. They tried their best to hook it up and just couldn’t make their heat pump work.

My first reaction is to look at the back of the thermostat and look for an O/B terminal. Lo and behold, there is no O. They didn’t buy a heat pump thermostat. It had to be a heat only stat or a heat/cool stat.

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, just hurry on 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 at Home Depot will be what works. Make sure you choose the 2/heat 1/cool heat pump.

Or if your equipment has more stages be sure to select the correct stat. When you remove your old thermostat you will notice a W1 and a W2. If W2 has a separate wire then you most likely have 3 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 home owners. Their number is 800-468-1502

For price I like the Honeywell T6 Pro, available with WiFi

For features and ease of use with a nice large touch screen I like the Honeywell 8000 Pro, also available with WiFi. I like the Redlink feature that allows you to hook up an outside wireless sensor that will give you temperature and humidity readouts on the screen. The outdoor sensor is sold separately.

What Does 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 thermostats setup means the opposite of the O setting. They could use 1 & 2 or x & y, it is simply an option of one or the other. O for cold climates and B for hot climates, that I wish I was in right now, it’s freezing here.

I always think about those very temperate climates that never get too cold or too 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 in manufacturing 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 are going to 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.

Most every manufacturer to my knowledge makes both and sends them to the appropriate climate.

Should OB Be Energized On Cool Or Heat?

This is your one and only test question. And it’s impossible to fail because I think repetition brings conviction. Answer: It depends on the installation instructions given with the equipment or available online by searching for “your model number, installation and startup instructions” or starting the the heat pump and checking to see if it’s in the mode you set the thermostat to, heat or cool.

If it’s heating when it’s set to cool, then switch the setting. Thanks for reading.

Since you’ve read this far, why not travel just a bit further and scope out a couple of my favorite heating and cooling products and tools.

Thank you for reading my article. I hope the information has led to your ability to make good decisions in either working on or discovering how to solve problems in your home HVAC equipment. These are affiliate links, so if you do decide to use any of them I do earn a commission. In all honesty they are products that I would use in my own home or the homes of my friends and family.

Fireplace remotes by Skytech Remote Control Products . My wife loves this and it probably took all of 30 minutes to install. Replace that wall switch or thermostat for your gas fireplace. Easy installation and instructions. In fact I wrote an article that covers the job pretty thoroughly I think.

Honeywell Thermostats. My favorite two Honeywell stats are the T6 Pro and the 8000 Pro series but their is a bunch to select from besides those two models. You can select WiFi that has the capability of operating your system from your smart phone no mater where you are. Honeywell tech support for homeowners is the best out there. Call them if you need at 800-468-1502.

Fieldpiece HS33 Expandable Manual Ranging Stick Multimeter for HVAC/R. I carry two of these in my service truck. Just like a spare tire, when I get out away from everything, it’s no time for a break down. The Fieldpiece HS33 gives me the ability to do any aspect of my trade. I can measure motor or element amps, check capacitors, and AC or DC voltages. Even different scales of OHMS for finding shorted windings or grounded conductors in motors or compressors.

Chad Peterson

Chad Peterson is near 40 year veteran of the HVAC industry. "I like to explain heating and air conditioning problems in a way the average home owner can understand. "

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