Heat Pump Reversing Valves and Their Control Designation
Key Takeaways
- O vs B Terminals: Most heat pumps default to heating mode (O terminal energized for cooling), while brands like Rheem, Ruud, and Bosch use B terminal (energized for heating)
- Light Commercial Systems: Carrier and York RTUs use conventional W1/W2 wiring instead of traditional heat pump controls, bypassing economizer controls
- Manufacturer Variations: Always verify the specific control designation for your equipment – never assume based on brand alone
- RTFM Rule: Reading the manual is critical for proper thermostat setup and avoiding costly wiring mistakes
Understanding O/B Terminal Designations
As Gary mentioned in a recent podcast, reversing valves are the heart of heat pump operation, directing refrigerant flow based on whether you need heating or cooling. These valves have a default position when de-energized, and understanding this default is crucial for proper system control.
Most manufacturers today default their reversing valves to heating mode, meaning the O terminal must be energized to switch to cooling. However, several manufacturers still default to cooling mode, requiring the B terminal to be energized for heating operation.
Manufacturers Using B Terminal (Less Common)
Here’s the list of manufacturers that energize the B terminal for heating:
- Rheem
- Ruud
- Weathermaker
- Ameristar
- Bosch Air Source (Note: Their water-source heat pumps use O)
Understanding these manufacturer differences is essential when troubleshooting heat pump reversing valve issues or setting up new thermostats.
Light Commercial System Considerations
Light commercial heat pump systems add another layer of complexity to control wiring. While many manufacturers stick with traditional heat pump control schemes, some notable exceptions use conventional wiring similar to gas furnace applications.
Carrier and York RTU Control Logic
Carrier and York rooftop units (RTUs) employ a unique control strategy that differs significantly from residential heat pumps:
W1 Terminal: Energizes all compressors for heating mode W2 Terminal: Activates auxiliary heat strips
This unconventional approach serves several important purposes:
- Economizer Bypass: In cooling mode (Y1 call), the control signal first passes through the economizer control before potentially energizing the stage one compressor contactor. Using W1 for heating bypasses this economizer logic entirely, preventing unwanted outside air infiltration during heating cycles.
- Maximum Heating Capacity: W1 brings on all available compressors simultaneously since we’re not concerned with latent heat removal during heating mode – only maximum BTU output.
- Simplified Valve Control: The logic board determines the appropriate reversing valve position based on the W1 signal, eliminating potential control conflicts.
A Note from the Field
Back in the day, I worked on packaged water-cooled heat pumps in ceiling spaces where they used mercury thermostats for control. These heat pumps failed in heating mode, so a call for W1 would run the heat pump in heating.
To run in cooling mode, they took an interesting approach: A call for Y1 would energize the reversing valve, and a call for Y2 would pull in the contactor for the compressor.
The building had many heat pumps throughout many floors that were wired this way. – Gary McCreadie
This unconventional wiring scheme highlights why understanding manufacturer-specific control logic is so critical in our field.
The Golden Rule: RTFM
As with any HVAC system, the most important step anyone can take is to RTFM: Read The Fantastic Manual.
This simple practice ensures:
- Proper thermostat configuration
- Correct control wiring
- Optimal system performance
- Avoided callbacks
Whether you’re installing a new system or troubleshooting an existing one, the manual contains manufacturer-specific details that no general guide can provide. This is especially crucial when dealing with bi-flow TXVs in heat pumps and other specialized components.
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When properly sizing heat pumps for replacement, understanding control logic is just one piece of the puzzle – you’ll also need to consider accurate heat load calculations to ensure optimal system performance and efficiency.
