Three Common Metering Devices Found on Typical Heat Pump Systems

Three Common Metering Devices Found on Typical Heat Pump Systems

When diagnosing, repairing, or installing a heat pump system, one component plays a critical role in efficiency, capacity control, and system reliability—the metering device.
Whether you’re a homeowner researching HVAC upgrades or a technician brushing up on fundamentals, understanding the three metering devices commonly found on typical heat pumps helps explain why some systems perform better than others.
Short answer 
👉 The three metering devices commonly found on typical heat pump systems are Electronic Expansion Valves (EEVs), Thermal Expansion Valves (TXVs), and Fixed Orifice (Piston or Capillary Tube) devices.
Below, we break down how each one works, where it’s used, and why it matters.

1. Electronic Expansion Valve (EEV)What Is an Electronic Expansion Valve?An Electronic Expansion Valve (EEV) is the most advanced type of metering device used in modern heat pump systems. It electronically controls refrigerant flow using sensors and a control board rather than relying on mechanical pressure or temperature alone.
EEVs are most commonly found in:

• High-efficiency heat pumps
  
• Variable-speed and inverter-driven systems
  
• Ductless mini-splits
  
• Cold-climate heat pumps
  

How an EEV WorksAn EEV uses:

• Pressure sensors
  
• Temperature sensors
  
• A microprocessor
  
• A stepper motor–driven valve
  

Together, these components:

• Continuously monitor system conditions
  
• Calculate the exact refrigerant flow needed
  
• Adjust instantly to changes in heating or cooling demand
  

This level of precision allows the heat pump to operate efficiently at any capacity level, not just full load.

Advantages of Electronic Expansion Valves

• Extremely precise refrigerant control
  
• Instant response to load changes
  
• Ideal for variable-speed compressors
  
• Higher SEER2 and HSPF2 ratings
  
• Improved comfort and humidity control
  
• Better performance in cold weather
  

Because of these advantages, EEVs are now the industry standard for premium heat pump systems.

Disadvantages of EEVs

• Higher system cost
  
• More complex controls
  
• Requires manufacturer-specific components
  
• Not easily retrofitted into older systems
  

2. Thermal Expansion Valve (TXV)What Is a Thermal Expansion Valve?A Thermal Expansion Valve (TXV) is a mechanical metering device that regulates refrigerant flow based on superheat—the temperature difference between refrigerant vapor and its saturation point.
TXVs are widely used in:

• Standard residential heat pumps
  
• Mid-efficiency systems
  
• Single-stage and two-stage equipment
  

For many years, TXVs were considered the gold standard in residential HVAC.

How a TXV WorksA TXV uses:

• A sensing bulb attached to the suction line
  
• Internal diaphragm and spring pressure
  

As suction line temperature rises or falls, the TXV:

• Opens to allow more refrigerant
  
• Closes to restrict refrigerant
  

This helps maintain a consistent superheat level, protecting the compressor from liquid floodback.

Advantages of TXVs

• More efficient than fixed orifice devices
  
• Good protection for the compressor
  
• Reliable and well-understood technology
  
• Lower cost than EEV systems
  

TXVs strike a balance between performance and affordability, which is why they’re still very common.

Limitations of TXVs

• Slower response to load changes
  
• Less accurate at very low capacity
  
• Not ideal for inverter-driven compressors
  
• Mechanical wear over time
  

As heat pump technology evolves, TXVs are increasingly limited to mid-range efficiency systems.

3. Fixed Orifice (Piston or Capillary Tube)What Is a Fixed Orifice Metering Device?A fixed orifice device—often called a piston or capillary tube—is the simplest type of metering device. It allows a fixed amount of refrigerant to flow regardless of operating conditions.
These devices are commonly found in:

• Older heat pump systems
  
• Entry-level or builder-grade equipment
  
• Some packaged units
  

How Fixed Orifice Devices WorkA fixed orifice:

• Has a set opening size
  
• Does not adjust to load changes
  
• Relies on system pressures to regulate flow
  

There are no sensors or moving parts that respond to temperature or demand.

Advantages of Fixed Orifice Devices

• Low cost
  
• Simple design
  
• Easy to service
  
• Fewer components to fail
  

Because of their simplicity, they’re still used in budget-focused systems.

Major Disadvantages of Fixed Orifice Devices

• Poor efficiency
  
• No adaptability to changing conditions
  
• Higher risk of compressor damage
  
• Reduced comfort and temperature stability
  

For this reason, fixed orifice devices are not used in high-efficiency heat pump systems.


5. Why Metering Devices Matter in Heat Pump PerformanceThe metering device directly affects:

• Energy efficiency
  
• Heating and cooling capacity
  
• Compressor lifespan
  
• Comfort and humidity control
  

A poorly matched or failing metering device can cause:

• High energy bills
  
• Frozen coils
  
• Short cycling
  
• Compressor failure
  

That’s why manufacturers design the metering device, compressor, and controls as a matched system.

6. Which Metering Device Is Best?There is no “one-size-fits-all” answer—but:

• EEVs are best for high-efficiency, variable-speed heat pumps
  
• TXVs are suitable for mid-range residential systems
  
• Fixed orifice devices are best avoided when efficiency is a priority
  

For homeowners considering upgrades, the type of metering device is often a hidden indicator of system quality.

Final TakeawayThe three metering devices commonly found on typical heat pump systems are:

1. Electronic Expansion Valves (EEVs) – used in high-efficiency variable-speed systems
   
2. Thermal Expansion Valves (TXVs) – common in standard and mid-efficiency systems
   
3. Fixed Orifice devices – found in older or entry-level equipment
   

Understanding these differences helps explain why some heat pumps deliver better comfort, lower energy bills, and longer lifespan than others.