Sensi Heat Pump Blowing Cold Air in Heat Mode? Fix O/B
- Reversing valve set to B when the system uses O (or vice versa)
- O/B wire not landed in the O/B terminal
- System configured as Conventional instead of Heat Pump
Problem Description
Your heat pump blows cold air when the Sensi is set to Heat, or blows warm air when set to Cool, meaning the system is running backwards. On a heat pump this is almost always the reversing valve setting, the O/B configuration in the Sensi, or the O/B wire, telling the reversing valve to switch to the wrong mode.
Why This Happens in Real Homes
A heat pump that blows cold in Heat mode, or warm in Cool, is running backwards, and on a heat pump that points straight at the reversing valve and its O/B control. The reversing valve switches the system between heating and cooling, and it is energized in just one mode: most heat pumps use O (energized in Cool), while some older systems use B (energized in Heat). If the Sensi's reversing-valve setting is on the wrong one, the valve flips at the wrong time and every mode comes out reversed. The fix is in the app's heat-pump wire-picker settings, where you set the reversing valve to O or B to match your equipment, plus confirming the O/B wire is actually landed in the O/B terminal and the system type is set to Heat Pump, not Conventional. Most systems are O, so try that first and test both modes. If it is configured correctly for your system and still runs backwards, the reversing valve itself may be stuck, which is a job for an HVAC technician.
Symptoms
- Heat pump blows cold air in Heat mode
- Blows warm air in Cool mode
- System runs the opposite of the setting
- Auxiliary heat works but the heat pump does not heat
- Only started after installing the Sensi
- Reversing valve does not switch modes
- Cooling is fine but heating blows cool
- Mode seems swapped in both directions
Recognize these? Here's what usually causes it.
Common Causes
- Reversing valve set to B when the system uses O (or vice versa)
- O/B wire not landed in the O/B terminal
- System configured as Conventional instead of Heat Pump
- Wire picker in the app set for the wrong reversing valve
- O/B wire loose or in the wrong spot
- Heat pump wiring miscopied from the old thermostat
- Reversing valve stuck (equipment fault)
- Aux heat masking a non-working heat pump
Most fixes happen in the first 3 steps.
Turn off power before rewiring at the base. Do not run a heat pump backwards for long; if it will not correct with the O/B setting, have it inspected, as a stuck reversing valve needs professional service.
Tools & Requirements
These tools will help you complete this fix.
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Step-by-Step Solution
Confirm You Have a Heat Pump
This backwards behavior is specific to heat pumps, which use a reversing valve to switch between heating and cooling. If your old thermostat had an O, B, or O/B wire, you have a heat pump. A gas or electric furnace with AC does not have this setting; for those, use the not-heating or not-cooling guides instead.
Understand O vs B
The reversing valve is controlled by the O/B wire, and it energizes in one mode. Most heat pumps use O, meaning the valve is energized in Cool. Some (older Rheem/Ruud, for example) use B, energized in Heat. If the Sensi is set to the wrong one, the valve switches at the wrong time and the system runs backwards.
Check the O/B Wire at the Base
Turn off power at the furnace/air handler switch, pull the Sensi off its base, and confirm the wire from the reversing valve is landed in the O/B terminal (not left out or in W). Note whether your paperwork or old thermostat labeled it O or B.
Set the Reversing Valve in the App
In the Sensi app, open the equipment/wire picker settings for the heat pump and set the reversing valve to O or B to match your system. If you are unsure, most systems are O; try O first, then test, and switch to B only if it still runs backwards.
Test Both Modes
Set the Sensi to Cool a few degrees below room temp and confirm cold air, then to Heat a few degrees above and confirm warm air (the heat pump takes a minute to switch and warm up). If both are now correct, the O/B setting was the issue.
Recheck the System Type
If the app is set to Conventional rather than Heat Pump, the O/B logic is ignored entirely and the system misbehaves. Make sure the system type is Heat Pump in the wire picker so the reversing valve control works.
Rule Out Aux Heat Masking the Problem
If auxiliary (backup) heat is running, the vents may feel warm in Heat even though the heat pump itself is running backwards. Temporarily note whether warmth comes from the heat pump or the aux strips, so you are testing the heat pump, not the backup.
Call a Pro for a Stuck Valve
If the O/B setting and wiring are correct for your system and it still runs backwards in both modes, the reversing valve itself may be stuck or failing, which is an HVAC repair rather than a thermostat setting.
Quick Solutions
Still having issues? This is usually the deeper cause below.
If this comes back after following these steps, check whether a recent app or firmware update reset a default setting — the fix works, but the setting gets reverted silently.
When in doubt, set the reversing valve to O first, since most modern heat pumps use O. Switch to B only if the system still runs backwards after testing.
This issue almost always looks more complex than it is — the majority of cases trace back to a single setting, a stale credential, or a default that shipped wrong.
- Reversing valve set to B
- O/B wire not landed in the O/B terminal
- System configured as Conventional instead of Heat Pump
- Wire picker in the app set for the wrong
- O/B wire loose or in the wrong spot
Before you go — try one of these (they fix most cases).
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Official Manufacturer Manual
Emerson Sensi provides official product documentation through their online manual rather than downloadable PDF. Access setup guides, troubleshooting steps, and product specifications for your Emerson Sensi Thermostat.
Source: sensi.emerson.com
Need More Help? Emerson Sensi Support
Note: The contact information below connects you directly to Emerson Sensi's official customer support team, not Trunetto. They can help with warranty claims, device replacements, and advanced technical issues.
