Nest thermostat heat pump O B wire questions in Greater Vancouver

Wiring a Nest thermostat to a heat pump trips up a lot of Greater Vancouver homeowners, and the O/B terminal is almost always the culprit. This one wire tells the system how to switch between heating and cooling, and getting it wrong means the heat pump can run backwards from what you set. Here is what the O/B wire does and how to get it right.

What the O/B Wire Actually Controls

On a heat pump, a reversing valve decides whether the outdoor unit pulls heat into your home or pushes it out. The O/B wire is the signal that energizes that valve. The catch is that manufacturers do not agree on a default: some energize the valve in cooling (O), and others energize it in heating (B). Nest handles both, but it needs to be told which logic your equipment uses.

During Nest setup you will be asked whether your heat pump activates O/B for heating or cooling. If you pick the wrong option, the symptom is unmistakable: you call for heat and get cool air, or you call for cool and the home warms up. The thermostat is doing exactly what it was told, it was just told the wrong thing.

• O energizes the reversing valve in cooling mode (common on many North American units)
• B energizes the reversing valve in heating mode
• A single wire usually lands on the Nest O/B terminal, not two separate wires
• If air comes out opposite to your setting, the O/B logic is reversed in the app

Getting It Right Safely

You can safely confirm your wiring by looking at how the old thermostat was labelled before you removed it, and by checking the Nest app's heat pump questions during setup. If the previous thermostat had a wire on O or O/B, note which letter it used. Changing the O/B behaviour in the Nest app itself is a software setting and is safe to adjust.

What you should not do is open the outdoor unit, the air handler control board, or start moving low-voltage wires onto unfamiliar terminals to chase the problem. Heat pumps carry refrigerant and the control board ties into compressor and reversing-valve circuits that a licensed technician should handle. In our damp Lower Mainland winters a heat pump running in reverse can also frost up quickly, so it is worth getting the configuration verified.

• Safe: adjusting the O/B heating-or-cooling choice inside the Nest app
• Safe: photographing the old thermostat's wiring before removal for reference
• Leave alone: outdoor unit panels, the reversing valve, and refrigerant lines
• Call a pro if the air keeps coming out backwards after the app setting is corrected

Frequently Asked Questions

My Nest heat pump blows cool air when I set it to heat. What is wrong?

This is the classic sign that the O/B reversing-valve setting is backwards in the Nest app. Open the heat pump settings and switch whether O/B activates for heating or cooling, then test again. If the air is still reversed, the issue is at the equipment and a technician should check it.

Does my Nest need a C wire for a heat pump?

Heat pumps draw more from the thermostat than a simple furnace, so a dedicated C (common) wire is strongly recommended for reliable Nest operation. Without it you may see the Nest randomly lose power or restart. A technician can add a C wire if your run does not already have one.

Can I just guess the O/B setting and try both?

You can safely toggle the O/B choice in the app and observe the result, since it is only a software setting. Just give the system a few minutes between changes so the heat pump is not short-cycling while you test.

Are heat pumps eligible for any rebates in BC?

Qualifying heat pump installations in British Columbia may be eligible for FortisBC and CleanBC rebates, and financing can also be available. The exact amount depends on the equipment and your home, so it is worth confirming eligibility before you upgrade.

HeatLand Heating & Cooling is a licensed and insured, TSBC-certified HVAC company serving homeowners across Greater Vancouver. Our technicians diagnose the real fault, explain your options in plain language, and complete the work safely and to code.

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