How to prepare a chiller for winter? Glycol, drainage, and freeze protection

In cities with harsh winters such as Ankara, Erzurum, or Kayseri, we encounter the same fault every year: water leaks from the evaporator of a chiller started up in spring. The cause is most often that the water left in the circuit over winter froze and cracked the heat exchanger. This damage usually requires replacing the heat exchanger and means weeks without cooling at the start of the season.

The good news is that freeze damage is almost entirely preventable. Below we explain the three main winter preparation methods and the end-of-season checklist.

Why does a chiller freeze in winter?

Chiller freezing is when the water in the evaporator or outdoor piping turns to ice below 0 °C, expands, and cracks the pipe or the heat exchanger plates. When water freezes its volume increases by about 9 percent; this expansion produces a force far beyond the pressure a metal pipe can withstand.

The risk does not exist only when the unit is off. In a chiller running at low load in winter, local freezing can start inside the evaporator when the water flow drops or when the chilled water setpoint is set too low. Because of their narrow channels, plate heat exchangers are more sensitive to this than shell and tube heat exchangers.

Method 1: Adding glycol to the circuit

Glycol is an additive that lowers the freezing point of water and acts as antifreeze in the chiller circuit. It is the safest method for systems that must also run in winter or that are impractical to drain. Two types are common: ethylene glycol and propylene glycol.

At the same ratio, ethylene glycol gives better freeze protection and heat transfer. Propylene glycol is less toxic, so it is preferred in food, pharmaceutical, and facilities where there is a risk of contact with drinking water. In both cases, a product made for HVAC with corrosion inhibitors should be used; car antifreeze must not be put in a chiller circuit.

The values in the table are approximate; the exact value should be taken from the technical data sheet of the product used. As a general rule, choose the ratio that gives protection a few degrees below the lowest expected outdoor temperature of the region. We explain how to determine the ratio for process applications separately in our glycol ratio article.

Glycol is not free: as the ratio increases, the water's heat-carrying capacity drops, viscosity rises, and the pump uses more energy. Using a higher ratio than needed can reduce chiller capacity by a few percent in summer.

Glycol is not poured directly into the circuit. A pre-mixed solution with water is prepared, or it is pumped into the circuit in a controlled way with a filling pump; then the system is circulated for a while and the ratio is verified by taking samples from several points. If the total water volume of the circuit is unknown, it should be calculated first, otherwise the ratio is only a guess. Because later water top-ups dilute the glycol, this should be monitored especially in systems with an automatic fill line.

Method 2: Draining and drying the circuit

Draining means emptying the water circuit completely so that no freezable water remains, for chillers that will not run at all during winter. It looks economical because there is no glycol cost, but if not done properly it is the riskiest method.

The problem is that the water does not come out completely. Even a few liters left in the low points of the evaporator, in the plate heat exchanger channels, and in the pump housing can freeze and cause cracking. That is why, after draining, the circuit should be blown out with compressed air, and the drain and air vent valves should be left open all winter. Some manufacturers also recommend filling the heat exchanger with a small amount of glycol after draining.

In a drained circuit, the risk of corrosion increases because the inner surfaces are exposed to air. When refilling the circuit in spring, the water treatment chemicals should be dosed again and the filters cleaned.

Method 3: Protecting with heaters and circulation

This means keeping the water above its freezing point with electric freeze protection, an evaporator heater, a pipe heating cable, and pump circulation. Many chillers come from the factory with an evaporator heater and an antifreeze function that runs the pump at low temperature.

The weak point of this method is a power outage. When the power goes out, both the heater and the pump stop, and there is no protection on the coldest night. For this reason, rather than relying on the heater alone, it is recommended to use it together with glycol or to connect it to a generator supply. In addition, the insulation of outdoor piping and the condition of heating cables should be checked every season.

End-of-season checklist

Whichever method is chosen, the following checks should be done before entering winter:

1. Measuring concentration with a refractometer in a glycol circuit and topping up if needed
2. Checking the pH and inhibitor level of the glycol; degraded glycol turns acidic and causes corrosion
3. Verifying that water flow and the flow switch are working
4. Entering the antifreeze and low water temperature alarm settings in the control panel according to the fluid used
5. Testing the evaporator heater and pipe heating cables by current measurement
6. Checking the insulation, cladding, and valve boxes on outdoor piping
7. If draining, emptying all low points and blowing them out with air
8. Cleaning the condenser coil and recording end-of-season readings

Panel settings, electrical measurements, and refrigerant-side checks should be performed by a qualified technician. Facility staff can safely do only a visual check of the outdoor insulation and note the alarm history and temperature values.

What should be watched in a chiller that runs in winter?

In data centers, hospitals, and facilities with process cooling, the chiller runs all winter. In this case, condensing pressure drops very low at low outdoor temperatures; fan speed control, condenser pressure control, and compressor safety settings must be suitable for winter operation.

As outdoor air temperature drops in winter months, free cooling can also be considered. In properly designed systems, cold outdoor air takes the compressor partly or fully out of service and provides significant energy savings. Such systems also require a glycol circuit.

Spring start-up is also part of winter preparation. When refilling a drained circuit, air must be fully vented, tightness checked, and the unit monitored by a technician at first start. An unexpected trace of water at the evaporator outlet, an increased pressure drop, or low refrigerant pressure can be a sign of freeze damage that went unnoticed over winter.

Winter preparation with YOİ

At season close we do glycol measurement, drainage, heater testing, and panel settings in a single visit and record the measurements in a written report. For facilities with an annual maintenance contract, these checks are added to the schedule automatically. For details you can look at our chiller service page, and you can read which system suits you in our chiller selection guide.

Detailed guide: Chiller maintenance: intervals, checklist, and maintenance form