Understanding Heat Pump Refrigerant Flow Challenges
The Thermostatic Expansion Valve (TXV) remains one of the most reliable metering devices in HVAC systems, but heat pump applications present unique challenges. Unlike standard air conditioning systems, heat pumps must accommodate refrigerant flow in both directions during heating and cooling cycles.
This is where specialized “Bi-Flow” TXVs become crucial to system performance. While some systems use standard TXVs with separate check valve bypasses or even dual TXV configurations, bi-flow TXVs offer an elegant solution by handling refrigerant flow in both directions with a single component.
In this article, we’ll explore how these specialized valves work, focusing on the Danfoss TGE Bi-Flow TXV, and why understanding their operation is essential for any HVAC professional working with heat pump systems.
Note: Understanding TXV operation and Heat Pump Reversing Valves is important to obtain the key takeaways from this article.
How Bi-Flow TXVs Solve the Reversing Problem
Referencing the above image, we will focus on the function of the Danfoss TGE Bi-Flow TXV. This drawing from the valve’s Data Sheet highlights the operation of the system in Cooling Mode.
Note: As mentioned, there are different ways to achieve heat pump operation with TXVs (this is also outlined in the TGE Data Sheet). Our example will focus on the use of a single Bi-Flow TXV with no check valves.
Cooling Mode Operation Explained
Cooling mode operation is similar to any other Air Conditioning or Refrigeration System. Through the Reversing Valve, the Compressor’s Discharge Gas is allowed to flow into the Outdoor Coil to reject heat and Condense. Liquid is then fed through the Bi-Flow TXV in its Conventional Flow Direction (more on this later). The liquid refrigerant absorbs heat and Evaporates in the Indoor Coil before returning to the Compressor.
Note: The TXV has its Sensing Bulb and External Equalization Tube installed in the Compressor Suction Line, instead of on the “Evaporator Outlet” like it would be in a plain AC System. This will allow proper TXV Control during the Heating Cycle as well.
Heating Mode Operation Explained
In Heating Mode, the piston in the Reversing Valve moves to allow system flow to reverse. This directs hot Discharge Gas to the Indoor Coil for heating, and the Condensed refrigerant now feeds the Bi-Flow TXV in the Reverse Flow Direction. The refrigerant is then able to feed the Outdoor Coil, and absorb heat from the outdoors while Evaporating.
Note: The above image from the TGE Data Sheet shows a setback of a Bi-Flow TXV. The setback of this set-up for a Heat Pump is that the TGE has a slight capacity reduction (how much heat transfer it can support) in the Reverse Flow Direction. In this example, we are “Bias towards Cooling”, as we have more capacity in the Cooling Mode. This is made up for in this design by fewer total components and gained system simplicity.
The Danfoss TGE Bi-Flow TXV Design
In the Danfoss TGE Manual (below), the design of the valve internals and pin is explained to give this TXV the characteristic to support refrigerant flow in both directions.
With the valve’s External Equalization Port (and Sensing Bulb) installed in the Compressor Suction Line (instead of one of the coil’s outlets), this allows the valve to reference “Evaporator” Outlet Pressure accurately, regardless of which mode it operates in or the current outdoor/indoor conditions.
Performance Considerations: Capacity in Reverse Flow
One important consideration when working with bi-flow TXVs is their performance in reverse flow mode. As shown in the Danfoss TGE documentation, there’s typically a slight capacity reduction when the valve operates in the reverse flow direction. System designers account for this when selecting components, often biasing the system toward cooling performance where maximum capacity is most critical.
This trade-off is generally acceptable because the simplified system design (fewer components, less potential leak points) outweighs the small capacity reduction. Additionally, modern heat pump systems often include supplementary heating for extreme cold conditions when maximum heating capacity would be needed.
Common Troubleshooting Issues
When working with heat pump systems using bi-flow TXVs, be aware of these common issues:
- Improper sensing bulb mounting: The sensing bulb must be securely attached to the suction line with good thermal contact
- External equalization line restrictions: Any kinks or blockages will cause improper valve operation
- Valve sizing issues: An undersized valve can restrict flow and reduce system capacity
- Refrigerant charge problems: Proper charge is critical for optimal valve operation in both directions
Related: Gary & Jamie Kitchen discuss discusses common misdiagnoses and solutions with TXVs & check valves
Key Takeaways
When working with heat pump systems using bi-flow TXVs, remember these key points:
- Bi-flow TXVs allow refrigerant to flow in both directions without additional check valves
- External equalization and sensing bulb placement are critical for proper operation
- Some capacity reduction in reverse flow is normal and accounted for in system design
- TXV selection should match the specific heat pump application requirements
- The simplified system design typically outweighs the minor capacity reduction in reverse flow
As the industry continues to evolve toward more electronic expansion valves (EEVs) and inverter-driven compressors, the principles of bi-directional flow control remain important. For technicians working on conventional heat pump systems, understanding bi-flow TXV operation is a valuable skill that leads to better diagnostics and more efficient system performance.