Heat exchanger freezing is a common winter problem in HVAC, refrigeration, industrial cooling, process heating and outdoor equipment. When water or another water-based fluid freezes inside a heat exchanger, ice can block the flow channels, reduce heat transfer and, in severe cases, cause permanent mechanical damage.
This is especially important for plate heat exchangers, brazed plate heat exchangers and water-to-water heat exchangers, because their compact flow passages can be sensitive to ice formation.
So, how do you prevent a heat exchanger from freezing in winter?
The correct solution depends on whether the system is operating continuously, operating intermittently, or completely shut down.
Why Does a Heat Exchanger Freeze?
A heat exchanger freezes when the fluid temperature falls below its freezing point while sufficient water remains inside the heat exchanger.
The risk becomes higher when:
- Outdoor temperatures fall below freezing
- Water flow becomes too low
- A pump stops unexpectedly
- The heat exchanger operates with very low outlet temperatures
- The system is shut down while water remains inside
- The cold side temperature is too low
- Glycol concentration is insufficient
- Temperature sensors or freeze protection controls fail
- Fouling causes uneven flow distribution
- Outdoor piping and heat exchanger connections are poorly insulated
For a plate heat exchanger, freezing does not necessarily require the entire fluid volume to reach 0°C.
Local areas of the heat transfer surface can become cold enough for ice to form. Once an ice layer develops, it can restrict the channel, increase pressure drop and further reduce flow. This can create a cycle in which freezing becomes progressively worse.
How to Prevent a Plate Heat Exchanger from Freezing
There is no single freeze protection method suitable for every application. In most systems, the safest approach is to combine several measures.
1. Use Glycol Instead of Pure Water
For systems exposed to freezing temperatures, a properly selected water-glycol mixture is one of the most common freeze protection methods.
The two most common options are:
- Ethylene glycol
- Propylene glycol
The required glycol concentration depends on the minimum operating temperature and the required freeze protection level.
Do not select glycol concentration simply by using a standard percentage such as 20%, 30% or 50%.
The design should consider:
- Minimum fluid temperature
- Required freeze protection temperature
- Heat exchanger operating temperature
- Flow rate
- Pressure drop
- Pump capacity
- Fluid viscosity
- Heat transfer performance
- Materials and corrosion protection
Increasing glycol concentration can improve freeze protection, but excessive glycol can also reduce heat transfer performance and increase viscosity and pumping requirements.
For example, Danfoss notes that glycol concentration must be determined according to the minimum freezing temperature while also considering the effect of glycol concentration on heat transfer.
2. Keep Sufficient Flow Through the Heat Exchanger
Low flow is one of the most important factors in heat exchanger freeze-up.
When water flow decreases, the fluid can remain in contact with a cold heat transfer surface for longer. Local temperatures can therefore fall below the safe operating range.
Freeze protection should consider:
- Minimum flow rate
- Pump failure
- Valve position
- Flow switch operation
- Partial-load operation
- Start-up and shutdown conditions
A heat exchanger should not be designed only for its normal operating condition. The freeze protection strategy should also consider abnormal conditions such as pump failure and low-flow operation.
3. Install Low-Temperature and Freeze Protection Controls
Temperature sensors can provide an early warning before freezing causes serious damage.
Depending on the application, a control system may monitor:
- Fluid inlet temperature
- Fluid outlet temperature
- Outdoor temperature
- Differential pressure
- Flow rate
- Heat exchanger wall or surface temperature
If the temperature approaches the freezing limit, the control system can increase flow, open a bypass, reduce cooling capacity, stop the refrigeration circuit or activate another protection sequence.
This is particularly important for heat exchangers used as evaporators or in refrigeration systems, where the water-side temperature can approach freezing conditions.
4. Protect the Heat Exchanger During Shutdown
A heat exchanger can be more vulnerable to freezing after the system stops than during normal operation.
If the equipment is installed outdoors or in an unheated area, water may remain trapped inside the heat exchanger and connected piping.
For seasonal or emergency shutdowns, possible protection methods include:
Drain the heat exchanger completely.
If the system cannot be drained, use an appropriate glycol solution.
For critical equipment, automatic freeze protection should be considered instead of relying only on manual operation.
The correct procedure depends on the heat exchanger design, installation and operating conditions.
5. Insulate the Heat Exchanger and Piping
Insulation does not create heat, but it reduces heat loss and slows down temperature changes.
Proper insulation should cover:
- Heat exchanger body
- Water piping
- Valves
- Flanges
- Connections
- Other exposed components
However, insulation alone should not be treated as complete freeze protection.
If the fluid temperature can remain below freezing for a long period, insulation may only delay freezing rather than prevent it.
For outdoor installations, insulation may need to be combined with heat tracing, glycol or automatic temperature control.
What Should You Do If a Heat Exchanger Has Already Frozen?
Do not immediately restart a frozen heat exchanger.
Ice can restrict the flow passages, and forcing high pressure or high flow through a blocked exchanger can create additional mechanical stress.
A practical inspection procedure is:
- Stop the system safely.
- Isolate the heat exchanger if necessary.
- Determine whether the fluid inside has frozen.
- Allow the equipment to thaw under controlled conditions.
- Check for leakage or mechanical damage.
- Check the inlet and outlet pressure.
- Confirm normal flow after thawing.
- Check the glycol concentration if glycol is used.
- Inspect temperature sensors and freeze protection controls.
- Restart the system gradually.
If the heat exchanger has experienced severe freezing, do not assume that it is undamaged simply because the system operates again.
A detailed inspection may be required.
Why Do Plate Heat Exchangers Freeze More Easily?
Plate heat exchangers have narrow channels and a large heat transfer area in a compact package.
This provides excellent heat transfer performance, but it also means that local ice formation can affect the flow passage.
The actual freezing risk depends on the complete operating condition, including:
- Fluid temperature
- Flow velocity
- Heat exchanger design
- Temperature difference
- Pressure drop
- Fluid properties
- Fouling condition
- Refrigerant evaporation temperature
- Control strategy
In refrigeration applications, the relationship between refrigerant temperature and water-side wall temperature is especially important.
The coldest point of the heat exchanger may not simply correspond to the average fluid temperature. Flow distribution and operating conditions can influence where freezing begins.
How to Choose a Heat Exchanger for Cold-Weather Applications
If a heat exchanger will operate in a cold environment, freeze protection should be considered during the equipment selection stage rather than after installation.
When requesting a heat exchanger, provide the manufacturer with:
- Heating or cooling capacity
- Fluid type
- Glycol type and concentration
- Inlet temperature
- Outlet temperature
- Design flow rate
- Allowable pressure drop
- Minimum ambient temperature
- Minimum operating temperature
- Operating pressure
- Maximum operating pressure
- Installation environment
- Continuous or intermittent operation
This information allows the heat exchanger manufacturer to select an appropriate heat transfer area, connection size, plate configuration and material.
For example, some modern brazed plate heat exchangers are specifically designed for applications where freezing risk must be controlled. Certain specialized designs can tolerate controlled ice formation under defined operating conditions rather than allowing ice to block critical flow passages.
Heat Exchanger Freeze Protection Checklist
Before winter operation, check the following:
Fluid
- Is the fluid suitable for the minimum operating temperature?
- Is the glycol concentration correct?
- Is the glycol condition acceptable?
Flow
- Is the minimum flow rate sufficient?
- Can the pump stop unexpectedly?
- Is there a low-flow alarm or protection?
Temperature
- Is the outlet temperature monitored?
- Is there a low-temperature alarm?
- Is freeze protection integrated into the control system?
Installation
- Is the heat exchanger exposed to outdoor temperatures?
- Are the heat exchanger and piping properly insulated?
- Is heat tracing required?
- Can the heat exchanger be completely drained?
Maintenance
- Is the heat exchanger clean?
- Is pressure drop within the expected range?
- Are temperature sensors working correctly?
- Are valves and pumps operating correctly?
Can Glycol Completely Prevent Heat Exchanger Freezing?
Glycol can significantly reduce the freezing risk when correctly selected and mixed, but it should not be treated as a universal solution.
The required concentration depends on the minimum temperature of the system.
A higher glycol concentration is not automatically better. Too much glycol can increase viscosity, pumping requirements and reduce heat transfer performance. Danfoss recommends determining glycol concentration from the required freeze protection while also considering system materials and corrosion protection.
The correct concentration should therefore be calculated from the actual operating conditions rather than selected by guesswork.
Final Recommendation
The most reliable way to prevent heat exchanger freezing is to address the problem at the system design stage.
For a typical water-based system exposed to winter conditions, the freeze protection strategy may include:
Correct glycol concentration + sufficient flow + temperature monitoring + proper insulation + reliable control + a safe shutdown/drainage procedure.
For refrigeration and low-temperature applications, the heat exchanger itself also needs to be selected according to the actual freezing conditions.
If you are experiencing plate heat exchanger freezing, brazed plate heat exchanger freeze-up, low-temperature operation, winter heat exchanger problems or repeated ice blockage, provide the operating conditions to the heat exchanger manufacturer before selecting a replacement unit.
The right heat exchanger should be selected according to the actual fluid, temperatures, flow rate, pressure drop and minimum ambient conditions—not simply by matching the connection size or heat transfer capacity.
A properly designed freeze protection system can prevent unexpected shutdowns, reduce the risk of heat exchanger damage and improve reliable operation throughout the winter.
Need Help Selecting a Heat Exchanger for Freeze Protection?
Send the following information to your heat exchanger supplier:
Fluid / glycol type → glycol concentration → inlet temperature → outlet temperature → flow rate → operating pressure → minimum ambient temperature → required heat transfer capacity.
With these parameters, a heat exchanger manufacturer can evaluate the freezing risk and recommend a suitable heat exchanger design for your application.