Causes of Frost Buildup on Data Center Air Conditioning Compressors and Solutions

Precision AC Compressor Frosting: Causes & Fixes

The compressor is one of the most critical components in a constant temperature/humidity (precision) air conditioner. If the compressor fails, the entire cooling system stops working.

Among common compressor issues — unusual noises, frost buildup, not starting — compressor frosting is one we see frequently. Below, we explain why it happens and how to fix it.

1. Why Does Frost Appear on the Compressor Return Line?

Frost on the compressor return line (suction line) means the return gas temperature is too low. But what causes that?

In a precision AC, if you change the volume or pressure of the same amount of refrigerant, the temperature changes accordingly:

• More heat absorption → higher pressure, temperature, and volume
• Less heat absorption → lower pressure, temperature, and volume

So, low return gas temperature usually comes with low suction pressure and higher refrigerant mass per volume.

The root cause: Refrigerant flowing through the evaporator cannot absorb enough heat to reach its designed pressure and temperature. As a result, the return gas has low temperature, low pressure, and low volume.

Two possible scenarios:

1. Normal refrigerant flow from the expansion valve, but the evaporator cannot absorb enough heat — the refrigerant has nowhere to expand properly.
2. Normal evaporator heat absorption, but too much refrigerant flow from the expansion valve — in plain terms: too much refrigerant (overcharged). Yes, an overcharge can also cause low suction pressure.

2. Low Refrigerant Charge (Undercharge) Causing Return Line Frost

How an undercharge leads to frosting:

When refrigerant flow is too low, expansion starts immediately after the expansion valve — often causing frost at the distributor tubes. With too little refrigerant, the evaporator isn’t fully utilized. Only part of the evaporator gets cold, and local temperatures drop sharply, causing frost.

Once frost forms on part of the evaporator, that area becomes insulated and stops absorbing heat effectively. The refrigerant then expands elsewhere in the evaporator, causing more frost. Eventually, the whole evaporator ices up, forming a thick insulating layer. The expansion point then moves downstream to the compressor return line, causing frost there.

Also: Low refrigerant charge lowers evaporating pressure and temperature, which also leads to evaporator frost, then insulation, then return line frost.

Note: In most undercharge cases, you’ll see evaporator frost before the compressor return line frosts.

How to Fix It (for units with a hot gas bypass valve)

• Open the rear cover of the hot gas bypass valve.
• Use an 8mm hex wrench to turn the adjustment nut clockwise.
• Adjust slowly — about half a turn at a time.
• Let the system run and observe the frost pattern before adjusting further.
• Once the system stabilizes and frost disappears, retighten the cover.

For units under 15kW (no hot gas bypass valve)

If frosting is severe, you can slightly increase the cut-in pressure of the condenser fan pressure switch:

• Locate the pressure switch.
• Remove the small retaining clip.
• Use a Phillips screwdriver to turn clockwise.
• Adjust in small steps (half a turn), check the result, then adjust again if needed.

3. Cylinder Head Frosting (Severe cases: crankcase frosting)

Cylinder head or crankcase frost means the compressor is drawing in wet vapor or liquid refrigerant. Common causes:

A. Thermal expansion valve (TXV) issues

• TXV opening too large
• Bulb installed incorrectly or loose — causing false high temperature readings and overfeeding refrigerant

How a TXV works:
The TXV adjusts refrigerant flow based on superheat at the evaporator outlet. If the sensing bulb measures a temperature that’s too high (due to poor contact or wrong placement), the valve opens wider than it should. This sends excess liquid refrigerant into the evaporator, which may not fully evaporate — leading to liquid or wet vapor entering the compressor.

Proper superheat setting:

• Too high → long superheat section, reduced cooling capacity
• Too low → risk of liquid slugging or cylinder head frosting
• Recommended evaporator outlet superheat: 3°C to 8°C (5–15°F)

B. Liquid refrigerant sitting in the evaporator before startup

• Leaking liquid line solenoid valve
• Expansion valve not closing fully when the system is off

How it happens:
A temperature controller and solenoid valve manage compressor cycling. If the solenoid valve leaks or the TXV doesn’t seal tightly when closed, liquid refrigerant collects in the evaporator during off cycles. When the compressor starts, that liquid is pulled in, causing cylinder head frost.

C. Suction valve opened too far or too quickly at startup

When starting the compressor, opening the suction shutoff valve too wide or too fast can flood the compressor with wet refrigerant.

When to Call a Professional

Compressors are precision components. If you’re unsure about any of these adjustments — or if the problem persists — contact a qualified technician (such as the team at Jienengmao). It saves you time, prevents damage, and gets your precision AC back up and running faster.