How to Maintain Controls in Commercial Refrigeration Systems

Every commercial refrigeration system depends on a set of control devices working in harmony. These controls regulate refrigerant flow, manage pressure, and maintain the right temperatures across the system. When even one of them malfunctions, the result can be anything from wasted energy to spoiled inventory and compressor damage. Preventive maintenance of these controls is not optional – it is the backbone of reliable, efficient, and safe refrigeration operation.

Why refrigeration controls need regular attention 🔗

Control devices in a refrigeration system act as regulators. They decide how much refrigerant enters the evaporator, when it flows, and at what pressure. Over time, these components experience wear and tear from constant operation, temperature fluctuations, and exposure to moisture or contaminants. Without scheduled inspections and adjustments, small issues like a slightly misaligned valve seat or a drifting temperature sensor can snowball into major system failures.

According to MJ HVAC, emergency repairs on commercial HVAC and refrigeration systems can cost two to five times more than planned maintenance. Beyond cost, poorly maintained controls lead to higher energy consumption, inconsistent temperatures, and a shortened lifespan for expensive equipment like compressors and condensers.

Thermostatic expansion valves (TXVs) 🔗

The thermostatic expansion valve is one of the most critical control devices in any refrigeration system. It regulates the flow of liquid refrigerant entering the evaporator based on the temperature and pressure at the evaporator outlet – a measurement known as superheat. A TXV ensures that enough refrigerant enters the evaporator to absorb heat effectively, while preventing liquid refrigerant from reaching the compressor, which can cause severe mechanical damage.

How a TXV works 🔗

A TXV operates through the balance of three forces: bulb pressure, spring pressure, and evaporator pressure. A sensing bulb mounted at the evaporator outlet detects the suction temperature and transmits pressure to a diaphragm inside the valve. The spring provides a constant opposing force, and evaporator pressure acts against the diaphragm from below. When the evaporator needs more refrigerant (rising superheat), the bulb pressure increases and opens the valve wider. When cooling demand drops, the valve closes partially. As Danfoss explains, this feedback loop maintains stable superheat and protects the compressor from flooding or overheating.

Key maintenance tasks for TXVs 🔗

Superheat check: Under normal operation, TXVs are typically set for around 8-12°F (or approximately 10°C) of superheat. Technicians should measure superheat regularly by comparing the refrigerant temperature at the evaporator outlet with the saturation temperature corresponding to the suction pressure. A low-pressure gauge on the suction line and a temperature probe at the evaporator outlet are the essential tools for this check.

Sensing bulb inspection: The sensing bulb must be tightly mounted to the suction line and properly insulated. A loose or poorly insulated bulb reads ambient temperature instead of the actual refrigerant temperature, sending incorrect signals to the valve. This can cause the valve to overfeed (risking compressor liquid slugging) or underfeed (starving the evaporator).

Valve seat and strainer inspection: Over time, wear on the needle valve and valve seat, or contamination from dirt and debris, can prevent the valve from closing properly. According to ACHR News, cleaning or replacing the inlet strainer and internal parts is essential if the valve is restricted or physically damaged.

Ice and oil blockage: If moisture is present in the system, ice can form at the valve orifice when temperatures drop below 0°C, blocking refrigerant flow. Similarly, refrigerant oil can separate and accumulate around the orifice at low temperatures. Proper filter-driers upstream of the TXV are the first line of defence against these issues.

Float control valves 🔗

Float control valves regulate the level of liquid refrigerant in receivers, accumulators, or evaporators. They work on a simple buoyancy principle – a float rises or falls with the liquid level and opens or closes a valve accordingly, much like a float mechanism in a water tank.

Types of float valves 🔗

There are two main types. A high-side float valve is installed on the high-pressure side (typically in the receiver) and meters liquid refrigerant into the evaporator as it accumulates. A low-side float valve sits on the low-pressure side (inside or near the evaporator) and maintains a constant liquid level for optimal heat exchange.

Maintenance requirements 🔗

Freedom of movement: The float mechanism must move freely without sticking. A stuck float in the closed position starves the evaporator of refrigerant, cutting its cooling capacity and increasing compressor running time. A stuck-open float does the opposite – it allows excessive refrigerant into the evaporator, risking liquid carryover to the compressor.

Valve seat condition: The valve seat should be inspected for wear, erosion, or debris that could prevent proper sealing. Even minor damage to the seat can cause continuous leakage, leading to inefficient operation.

Replacement schedule: Float and valve kits should ideally be replaced at least once every six months, or more frequently if the valve does not respond properly to adjustments. In ammonia-based systems common in dairy and food processing, this is especially important because ammonia is aggressive on seals and moving parts.

Solenoid valves 🔗

Solenoid valves are electrically operated on/off valves that control refrigerant flow at various points in the system. When the coil receives an electrical signal, it generates a magnetic field that lifts the valve stem to open the flow path. When the signal is cut, a spring pushes the stem back down to close the valve. These valves are found in liquid lines, hot gas bypass circuits, defrost systems, and oil return lines.

Why solenoid valves matter 🔗

The most common application is the liquid line solenoid valve, installed just before the expansion valve. Its primary purpose is to prevent high-pressure liquid refrigerant from migrating into the evaporator when the compressor shuts down. Without this valve, the next compressor start-up could result in liquid slugging – a condition where incompressible liquid enters the compressor cylinders, potentially causing catastrophic mechanical failure. As Danfoss notes, solenoid valves are found in nearly every dry-expansion commercial refrigeration system above a certain capacity.

Preventive maintenance steps 🔗

Electrical connections: Check all wiring connections for tightness and signs of corrosion or overheating. Loose connections can cause intermittent operation, while corroded terminals increase resistance and reduce coil performance.

Coil condition: The magnetic coil is the most common failure point in a solenoid valve. A burned-out coil means the valve stays closed (for normally closed types) and stops refrigerant flow entirely. Listen for the characteristic click when the valve energises – silence usually indicates a coil failure, blown fuse, or mechanical binding.

Valve stem movement: The stem should move freely when energised. Dirt, debris, or worn seals can cause the stem to stick, preventing the valve from opening or closing completely. During maintenance, the O-ring between the valve body and coil should be replaced, and all internal sealing surfaces should be cleaned.

Seat inspection: A worn or damaged valve seat can cause refrigerant to leak through even when the valve is supposed to be closed. This undermines the pump-down cycle and defeats the purpose of having the solenoid in the system.

Back pressure regulating valves 🔗

A back pressure regulating valve (also called an evaporator pressure regulator or EPR valve) is installed in the suction line between the evaporator and the compressor. Its job is to maintain a minimum pressure – and therefore a minimum temperature – in the evaporator, regardless of the compressor suction pressure.

Where they are used 🔗

These valves are particularly important in multi-temperature systems, such as supermarket refrigeration installations where produce display cases (requiring around 2-4°C) and freezer cases (requiring -18°C or lower) share a common compressor rack. Without EPR valves, the lower-temperature circuits would pull down the suction pressure so far that the higher-temperature evaporators would freeze over.

Maintenance priorities 🔗

Pressure setting verification: The valve’s spring tension determines the minimum evaporator pressure it maintains. Over time, springs can lose tension or the adjustment can drift. If the valve allows the evaporator pressure to drop too low, frost or ice accumulates between the evaporator fins and eventually clogs the entire unit. Technicians should verify the setting using a calibrated pressure gauge on the suction line and adjust according to the manufacturer’s specifications.

Valve seat and piston inspection: Dirt, oil residue, or wear on the valve seat and piston can cause the valve to stick open, closed, or in an intermediate position. According to ACHR News, worn pistons and valve port plugs that show a dull, sandblasted appearance often indicate exposure to flashing liquid, which is a sign of deeper system problems that need attention.

Safe service procedures: Before servicing any pressure regulating valve, the adjustment stem should be backed off to release spring tension. If the valve is solenoid-operated, the coil must be de-energised and locked out. The valve section should be isolated and evacuated of refrigerant before disassembly.

Temperature controllers 🔗

Temperature controllers are the command centre of a refrigeration system. They monitor the temperature inside the refrigerated space using sensors (thermocouples, RTDs, or thermistors) and send signals to other components – solenoid valves, compressors, defrost heaters, and fans – to maintain the desired setpoint.

Types of temperature controllers 🔗

Electromechanical controllers use a pressure-sensing element (such as a bellows filled with temperature-sensitive fluid) connected to a switch. They are simple and reliable but offer limited precision. Electronic/digital controllers use electronic sensors and microprocessors to offer programmable setpoints, differential settings, alarm functions, and data logging. Modern systems increasingly use digital controllers integrated with building management systems for real-time monitoring and predictive maintenance.

Maintenance of temperature controllers 🔗

Sensor calibration: Temperature sensors drift over time. A sensor that reads 2°C higher than the actual temperature means the system runs colder than necessary, wasting energy. A sensor reading low means the system runs warm, risking food safety. Sensors should be checked against a calibrated reference thermometer at regular intervals, and recalibrated or replaced when readings fall outside acceptable tolerances.

Setpoint and differential verification: The controller’s setpoint (the target temperature) and differential (the range between cut-in and cut-out) should be verified to ensure they match the application requirements. A too-narrow differential causes excessive compressor cycling, while a too-wide differential allows unacceptable temperature swings.

Electrical connections and contacts: For electromechanical controllers, switch contacts can pit and corrode over time, causing unreliable operation. Connections should be checked for tightness, and contacts cleaned or replaced as needed. Digital controllers should have their firmware and programming checked periodically.

Alarm functions: If the controller has high-temperature or low-temperature alarm functions, these should be tested regularly to ensure they activate properly. An alarm that fails to trigger during a genuine temperature excursion defeats the purpose of having the safety feature.

Relief valves and water flow regulating valves 🔗

While not always categorised alongside the main control devices, relief valves and water flow regulating valves are part of the overall control system and need attention during preventive maintenance.

Relief valves 🔗

A relief valve, typically installed above the condenser, releases excess pressure to prevent equipment damage or rupture. After relieving, the valve should reseat automatically. However, even a small particle of dust on the valve seat can hold it slightly open, allowing refrigerant to leak continuously. In ammonia systems, this is both a safety hazard and an environmental concern. Relief valves should be inspected and tested at regular intervals as prescribed by equipment manufacturers and local safety codes.

Water flow regulating valves 🔗

In water-cooled condensers (such as shell-and-tube types), a water flow regulating valve adjusts cooling water flow based on condenser pressure. This keeps condensing pressure constant despite variations in water supply temperature or refrigerant load. If this valve is neglected, it may fail in the open position, wasting thousands of litres of water, or fail closed, causing dangerously high condensing pressures.

Building a preventive maintenance schedule 🔗

A structured maintenance schedule ensures no control device is overlooked. Here is a practical frequency guideline:

Monthly checks: Verify superheat settings on TXVs, check temperature controller readings against a reference thermometer, inspect solenoid valve operation (listen for the click), and monitor system pressures for any deviation from normal.

Quarterly checks: Inspect float valve mechanisms for free movement, clean or replace inlet strainers on expansion valves, check all electrical connections on solenoid valves and controllers, and verify EPR valve pressure settings.

Semi-annual checks: Replace float and valve kits if they do not respond to adjustments, test relief valves, inspect water flow regulating valves, perform a comprehensive calibration of all temperature sensors and controllers, and check for acid or moisture contamination in the system oil.

Annual checks: Perform a complete system review including pressure and safety control settings, oil analysis, leak testing on all valve connections, and a review of operating logs for any trends that indicate developing problems. As Heatcraft recommends, the annual check should also include verifying the operation and calibration of all timers, relays, and safety controls.

Common signs of control device failure 🔗

Knowing what to look for between scheduled maintenance visits can help catch problems early. Watch for these warning signs:

Fluctuating temperatures: If the refrigerated space temperature swings wider than normal, the temperature controller or expansion valve may be drifting out of calibration.

Frost buildup on evaporator coils: Excessive frost, especially if it blocks airflow through the fins, often points to a malfunctioning back pressure regulating valve or a faulty defrost timer/solenoid.

Compressor running continuously or short-cycling: Continuous running can indicate a TXV that is underfeeding the evaporator or a stuck-open solenoid valve. Short-cycling may point to a controller with too narrow a differential setting.

Unusual noises: Hissing at the expansion valve may indicate a partially blocked orifice. Absence of the click from a solenoid valve suggests electrical or mechanical failure. Knocking or hammering in the compressor can mean liquid refrigerant is getting through due to a failed control valve.

Higher than normal energy bills: When controls are out of adjustment, the system works harder to maintain temperatures. This shows up directly in electricity consumption.

The cost of skipping maintenance 🔗

Neglecting control device maintenance leads to a predictable chain of consequences. First, energy consumption rises as the system compensates for poorly regulated refrigerant flow and pressure. Then, components like compressors and condensers experience accelerated wear because they are operating outside their design parameters. Eventually, a complete breakdown occurs – often at the worst possible time. According to Forced Air Mechanical, the combined savings from reduced repair costs and lower energy consumption typically outweigh the cost of a preventive maintenance programme many times over.

For industries like dairy processing, food retail, and cold chain logistics, the stakes are even higher. A control failure that causes temperatures to rise above safe limits can lead to regulatory penalties, product recalls, and loss of consumer trust.

What do you think? Which control device in your refrigeration system do you find most challenging to maintain, and how often do you currently schedule inspections for your system’s valves and controllers?

References

1. https://mjhvac.com/blog/why-preventive-maintenance-is-critical-for-commercial-hvac-electrical-refrigeration-systems/
2. https://www.danfoss.com/en-us/service-and-support/case-stories/dcs/how-thermostatic-expansion-valves-work/
3. https://www.achrnews.com/articles/97596-servicing-thermostatic-expansion-valves
4. http://dairy-technology.blogspot.com/2014/11/preventive-maintenance-of-controls-of.html
5. https://www.danfoss.com/en-us/service-and-support/case-stories/dcs/solenoid-valves-for-refrigeration-systems/
6. https://www.achrnews.com/articles/85444-refrigeration-control-valve-maintenance
7. https://ultra-refrigeration.com/preventative-maintenance-for-commercial-refrigeration-systems/
8. https://www.heatcraftrpd.com/support/preventive-maintenance
9. https://automationforum.co/how-to-do-control-valve-preventive-maintenance/
10. https://www.forcedairmech.com/preventive-maintenance/why-preventive-maintenance