Cold store defrost heater (defrost heater) is an electric heating element mounted directly on the evaporator, whose job is to melt the ice that forms on the coil surface on an automatic cycle, keeping the cold store's cooling performance stable. It works in concert with a defrost timer and a defrost thermostat, dissipating 200 W to 600 W (0.2 kW to 0.6 kW) depending on the design rating, and cutting out when the coil reaches +10°C to +13°C (50°F to 55°F).
From installations on industrial cold storage projects, Tân Long's engineers find that a failed or incorrectly specified defrost heater is the direct cause of a completely iced-up evaporator, an overloaded compressor and a cold store temperature out of control. This article sets out the construction, working principle, types, specifications, testing procedure and selection criteria for defrost heaters across every kind of cold store.
Definition and role in the refrigeration system
Cold store defrost heater is the key heat-generating component in an automatic defrost system, working on the principle of resistive heating. When current passes through a conductor of high resistivity, electrical energy is converted entirely into heat, melting the frost and ice on the evaporator surface.
Its role directly determines the performance of the whole industrial refrigeration system. When the evaporator is covered in ice, the heat transfer coefficient falls sharply, forcing the refrigeration compressor to work at a higher load to hold the target temperature. That raises power consumption by 15% to 30% against standard operating conditions. Cold store defrost heater solves the problem outright by breaking down the ice at regular intervals, typically 2 to 6 times a day depending on store temperature and the actual operating load.
Defrost heaters, door heaters and hot gas defrost compared
In cold storageinstallation practice there are three common methods of defrosting and heating. Each has its own function, and they usually complement one another to keep the system running smoothly.
| Equipment type | Installation location | Main functions | Typical applications |
| Defrost resistor | Directly on the evaporator | Melts the ice on the evaporator's aluminium fins completely | Small and medium cold stores, blast freezer cabinets, freezing tunnels |
| Door frame heater | Around the cold store door seal | Prevents condensation and ice forming at the doorway | Every cold store with a dedicated insulated door |
| Hot gas defrost | System refrigerant pipework | Uses the heat in hot discharge gas from the compressor to defrost | Large industrial refrigeration systems using NH3 |
Cold store defrost heater usually uses a nichrome element or a stainless-steel-sheathed heating rod. A door heater, by contrast, uses silicone-sheathed cable wrapped around the seal so the door does not freeze shut or damage the rubber gasket. On large projects, hot gas defrost may be preferred to save energy, but the capital cost is higher and it generally only suits systems with a cooling load of 200 kW or more.
At Tân Long we build high-quality Cold storage defrost resistor into our control cabinetsystems, fully automating the defrost process so the evaporator stays free of ice and heat transfer performs at its best.
Types of industrial cold store defrost heater
Depending on system design and operating temperature, contractors today generally choose one of the following heating devices:
Glass tube defrost heater
This is the line Cold storage defrost resistor is the most widely used in small and medium cold stores in Vietnam, thanks to its consistent performance. A standard unit consists of a heat-resistant glass tube enclosing a nichrome element, sealed at both ends with a dedicated plastic or ceramic cap from which the heat-resistant leads emerge.
- Specifications: Ratings run from 150 W to 500 W at 220 V AC, with a resistance of 96 Ω to 323 Ω. Glass tube lengths range from 300 mm to 1,200 mm.
- Characteristic: Its greatest advantages are low cost and simple replacement during industrial refrigeration. Its drawback is that the glass tube cracks easily under a sharp knock or a sudden temperature change (cold water landing on a hot surface).
Silicone rubber heater
The Cold storage defrost resistor uses a heating element sheathed in high-grade silicone rubber, which can be bent to follow the actual shape of the equipment. Silicone withstands a punishing temperature range from -60°C to +200°C and is completely waterproof in high-humidity conditions.
Tân Long's engineers usually prefer this type for air blast freezer or for awkwardly shaped locations such as refrigerant pipework, solenoid valves and cold store door frames, where local ice build-up must be prevented.
Cold store defrost heater comparison table
The table below brings together the technical figures to help you select the right device for your cold storage project:
| Criteria | Glass tube | Silicone rubber cable | Flat heater |
| Rating (W) | 150 to 500 | 50 to 800 | 100 to 1,000 |
| Voltage (V) | 220 AC | 110/220 AC | 220 AC |
| Temperature range | -30°C to +100°C | -60°C to +200°C | -30°C to +120°C |
| Mechanical strength | Low (fragile) | High (flexible, resilient) | Medium |
| Price | Low | Medium | Medium to high |
| Main applications | Small and medium cold stores | Complex evaporators, store doors | Freezer store floors, flat surfaces |
| Ease of replacement | Simple | Made to order by size | Medium |
Fitting the right Cold storage defrost resistor not only maintains cooling capacity but also protects the life of the compressor package. At Tân Long we supply genuine components together with a scheduled maintenance service, so your system runs as reliably as possible.
Defrost heater construction and working principle
The components in detail
Cold store defrost heater in glass tube form is the most widely used component in Vietnamese refrigeration systems, thanks to its consistent performance. A standard unit comprises four main parts:
- Protective glass tube: Made from high-temperature borosilicate glass, which is non-conductive and withstands extreme temperature swings. This sheath keeps the heating element from direct contact with moisture and water during defrosting.
- Heating element: A Ni-Cr (nickel-chromium) alloy of high resistivity, giving even heat output. This is the core of the Cold storage defrost resistor, converting electrical energy into heat efficiently.
- Insulating end caps: These hold the heating element in place and provide electrical isolation. At Tân Long, premium units replace the plastic with ceramic for better heat resistance and mechanical strength.
- Heat-resistant leads: These connect the device to the central control circuit. They are usually sheathed in PTFE or silicone so they do not melt under the heat radiated from the element.
Heat generation and the automatic defrost cycle
During the operation of a cold store, moisture condenses and freezes on the evaporator surface. Once the ice is thick enough to reduce heat transfer, the system automatically triggers a defrost cycle through the Cold storage defrost resistor.
The process begins when the controller opens the compressor circuit and energises the heating element. The heat raises the coil surface temperature from sub-zero to around 0°C to 5°C within 10 to 20 minutes, melting the ice completely so it drains away through the tray. When the sensor reads a safe coil temperature (typically +10°C to +13°C), the circuit opens automatically to end the heating and prepare for the compressor to restart.
How the defrost timer and defrost thermostat work together
To Cold storage defrost resistor to work efficiently and safely, coordination between the timer and the thermostat is essential:
- Defrost timer: This is the timing brain, deciding how many defrost cycles run each day. In the modern systems Tân Long installs, we usually build this module into the PLC control cabinet so the defrost frequency can be optimised from actual pressure sensor readings.
- Defrost thermostat (cut-out sensor): This important protective device cuts the supply to the Cold storage defrost resistor the moment the ice has melted. A failed thermostat is the leading cause of a burnt-out evaporator or wasted energy, and technicians encounter it regularly during maintenance and warranty work on industrial refrigeration systems.
Getting these components working accurately not only keeps the store temperature stable but also extends the life of the whole refrigeration system equipment.
Cold store defrost heater specifications
Specifying the wrong heater rating is a common cause of incomplete defrosting (ice left behind) or excessive defrosting (heat exchanger tubes distorted by high temperature). The table below sets out the Cold storage defrost resistor specifications Tân Long's engineers apply in practice on major projects such as NECS and Meito Vietnam:
| Store / equipment type | Rating (W) | Voltage (V) | Resistance (Ω) | Length (mm) | Thermostat cut-out temperature |
|---|---|---|---|---|---|
| Chill room 0°C to +5°C | 150 to 250 | 220 | 193 to 323 | 400 to 700 | +13°C (+55°F) |
| Cold store -10°C to -18°C | 250 to 400 | 220 | 121 to 193 | 600 to 900 | +10°C to +13°C (+50°F to +55°F) |
| Freezer store -25°C to -35°C | 350 to 600 | 220 | 80 to 138 | 800 to 1,200 | +10°C (+50°F) |
| Domestic blast freezer cabinet | 100 to 200 | 220 | 242 to 484 | 250 to 500 | +13°C (+55°F) |
| Door frame heater | 30 to 100 | 220 | 484 to 1,613 | 1,000 to 3,000 (cable) | No thermostat, runs continuously |
Technical notes: Resistance (Ω) and power (W) are related by P = V²/R. At 220 V, a 200 Ω element gives around 242 W. When measured with a multimeter, any value between 80 Ω and 700 Ω indicates a serviceable element, depending on type.
How to test a cold store defrost heater properly
To keep an industrial refrigeration system running continuously and avoid an iced-up evaporator, regular testing of the defrost components is extremely important. Below are the two methods Tân Long's engineers use to establish a unit's condition during professional maintenance and warranty work.
Testing with a multimeter
This is the most accurate way to determine whether a Cold storage defrost resistor is still working or has failed. The procedure has 5 standard steps:
- Step 1: Isolate the entire electrical supply to the system. Wait at least 10 minutes for the element to cool to room temperature, so the measurement is accurate — resistance always varies with temperature.
- Step 2: Carefully remove the component from the evaporator. On industrial freezer stores that are already iced up, defrost manually first so the protective glass sheath is not broken during removal.
- Step 3: Set the multimeter to resistance (Ω). Select the x100 range on an analogue meter, or auto-ranging on a digital one.
- Step 4: Connect the two probes to the two leads of the Cold storage defrost resistor. Make sure the contact points are clean and free of dirt or oxidation.
- Step 5: Read the value and compare it against the standard table below:
| Measured value | Technical conclusion | Corrective action |
| 80 Ω to 700 Ω | Device working normally | Check the timer and thermostat as well |
| 0 Ω or close to 0 Ω | Component short-circuited | Replace Cold storage defrost resistor |
| OL or ∞ (infinity) | Component open circuit | Replace the device immediately |
| Fluctuating value | Poor contact or a hidden break in the element | Replace as a safety measure |
Visual inspection of the heating element
This method identifies physical damage quickly without special instruments. The store operator can look directly at the heating element inside the glass tube:
- Normal condition: The heating element inside the Cold storage defrost resistor should be silver-grey, with evenly spaced coils and no sign of distortion.
- Signs of failure: Dark marks on the glass tube, melted spots, or gaps between the coils all indicate that the element has burnt out or broken.
Catching failures early keeps refrigeration equipment performing at its best and saves a considerable amount on running costs. If you have difficulty carrying out the check yourself, contact Tân Long's technical team on the hotline for a site visit within 2 hours.
Causes of failure and how to spot a faulty cold store defrost heater
Spotting defrost system problems early not only protects the refrigeration compressor but also saves a significant amount on running costs. Below are the common causes and the symptoms seen on site.
A broken or short-circuited heating element
This is the most serious fault, directly affecting the performance of the Cold storage defrost resistor. The technical causes include:
- Sudden thermal shock: When cold water from the defrost process falls directly onto a hot element, the sudden expansion breaks the heating element inside. This usually happens when the drain is blocked or the component has been fitted out of position relative to the evaporator design.
- Moisture absorption and insulation breakdown: In a high-humidity chill room (above 85% RH), the insulation at the element's terminals degrades easily, causing a short circuit.
Faults in the defrost thermostat or timer
In many cases the Cold storage defrost resistor still measures a normal resistance yet the evaporator remains iced up. The fault then usually lies in the control devices:
- Failed thermostat: If the thermal relay sticks in the normally open position, the electrical signal never reaches the heating element.
- Faulty defrost timer: The defrost cycle is not triggered at the right time, letting frost build up beyond what the heater can deal with. This is a system-level fault that Tân Long's maintenance and warranty team regularly resolves for customers.
Signs that the evaporator is too heavily iced
You can recognise the problem from the following visible symptoms:
- Ice build-up: If the ice is more than 5 mm to 10 mm thick after one operating cycle, the defrost system is definitely in trouble.
- Cooling performance drops sharply: When the evaporator is blocked with ice, the air flow through it falls below 30% of design. The cold store then cannot reach its setpoint even with the compressor running continuously, putting the stored goods at risk.
If you notice any of these signs, carry out a technical inspection immediately to avoid cascading damage across the whole industrial refrigeration system.
Choosing the right cold store defrost heater for each application
Calculating and specifying the defrost system correctly is the key to maintaining evaporator heat transfer. Depending on the operating temperature range, the engineer needs to be specific about rating and materials.
Deep-freeze stores at -18°C to -35°C
In deep sub-zero conditions, frost builds on the evaporator quickly and hard. The Cold storage defrost resistor therefore needs enough power to melt the ice completely within 20 to 30 minutes.
- Calculation figures: The standard rating is usually 0.5 W to 0.8 W/cm² of evaporator surface.
- Materials and construction: Prefer a glass tube element or a high-temperature silicone heating cable at 220 V. The system needs a thermostat cutting out at +10°C to protect the evaporator against overheating.
- Special note: For industrial freezer stores running at -35°C on NH3, Tân Long usually recommends hot gas defrost rather than electric heaters, for better energy use and more effective defrosting.
Chill rooms at 0°C to +5°C
Chill rooms run at high humidity (usually above 85% RH), so the Cold storage defrost resistor must be completely moisture-proof to avoid any electrical leakage.
- Technical characteristics: The required rating is lower, only 150 W to 250 W, but the defrost frequency may be 4 to 6 times a day.
- Recommended material: A silicone heating cable is the best choice, for its flexibility and its excellent moisture resistance in the condensing conditions of a chill room.
Selection criteria: rating, material and brand
To ensure the durability of your refrigeration equipment, component selection should rest on quantified figures rather than instinct:
- Confirm the three key figures: Before ordering, check the rating (W), the voltage (V) and the actual length (mm) carefully. Fitting the wrong voltage burns the element out instantly.
- Quality standards: Tân Long recommends using only CE- or UL-certified components from reputable manufacturers.
- Tân Long's practical experience: Across more than 500 design and construction projects nationwide, our engineers find that 60% of iced-up evaporators are not caused by a failed heater at all. The cause is usually a timer set to the wrong cycle or a thermostat fitted in the wrong position.
Checking the entire control circuit is therefore a mandatory principle in professional maintenance and warranty work, and is what keeps a system running stably and reliably.
Defrost heater replacement and maintenance procedure
Steps for replacing a defrost heater safely
Replacing a defrost heater yourself requires basic electrical knowledge and an understanding of cold store systems. Without that expertise, call an experienced technical team to avoid the risk of electric shock or damage to the evaporator.
The standard procedure has 6 steps:
- Open the cold store's supply breaker and confirm it is dead with a voltage tester.
- Defrost manually with a fan or warm water to melt the ice around the old heater, so it does not snap during removal.
- Record the mounting position, orientation and wiring of the old heater before removing it.
- Disconnect the wiring and remove the old heater. Measure its resistance again to confirm it has failed.
- Fit the new heater in the correct position and wire it per your notes. Check carefully that there is no electrical contact between the heater body and the evaporator's metal tubing.
- Restore the supply, trigger a manual defrost (bypassing the timer) and watch the heater for the first 10 minutes to confirm it is working correctly.
Recommended maintenance schedule
Maintaining the defrost heater cannot be separated from maintaining the whole defrost system. Tân Long recommends the following schedule:
| Category | Frequency | Content |
|---|---|---|
| Check defrost operation | Monthly | Watch a full defrost cycle and time the actual defrost duration |
| Measure resistance with a meter | Every 6 months | Measure and record the resistance, comparing it with the previous reading |
| Check the thermostat | Every 6 months | Measure the actual cut-out temperature and compare it with the setting |
| Check the defrost drain tray | Monthly | Clear any blockage and check the drain heater cable |
| Replace the heater | Every 3 to 5 years, or on failure | Replace according to service life, or when resistance falls outside the acceptable range |
A cold store defrost heater is a small component that determines the stability of the whole refrigeration system. Three technical points matter most. First, the heater always works together with the defrost timer and thermostat, so fault-finding means checking the whole circuit, not the heater alone. Second, heater selection must rest on three mandatory figures — wattage, voltage and a length that matches the evaporator design. Third, measuring resistance with a multimeter every 6 months is the most effective proactive maintenance there is; a value outside the 80 Ω to 700 Ω range (depending on type) is the signal to replace it before it fails completely.
Tân Long — with more than 20 years designing and building industrial cold storage systems from north to south — provides scheduled maintenance of defrost systems and advises on selecting and replacing the right heater for each type of store. Contact our engineers: (+84) 933 357 058 (Mr. Thinh) for direct technical support.
Frequently asked questions about cold store defrost heaters
How often must a cold store defrost heater be replaced?
A defrost heater has an average service life of 3 to 5 years under normal operating conditions (a freezer store defrosting 2 to 4 times a day). A store running 24/7 below -25°C (-13°F) with 6 defrost cycles a day will wear heaters out considerably faster. Measure resistance every 6 months and replace when the reading deviates more than 20% from the original specification.
What wattage of defrost heater suits a cold store?
There is no fixed figure. The rating depends on evaporator area, store temperature and the required defrost time. The rule of thumb: 0.5 W to 0.8 W per cm² of evaporator surface. A 50 m² cold store with an evaporator of roughly 20 m² of heat transfer area typically uses a total of 1,000 W to 1,600 W, split across 2 to 3 heaters wired in parallel.
Does a constantly iced-up evaporator always mean a failed defrost heater?
Not necessarily. A constantly iced-up evaporator can have three causes: a failed heater, a defrost timer set incorrectly (too few cycles), or a defrost thermostat stuck open. In addition, a store door that does not seal (a split gasket) lets humid outside air in continuously, icing the coil faster than the defrost cycle can cope with. Check the whole defrost circuit before deciding to replace the heater.
Does the power have to be off to test a defrost heater?
The power must be isolated before measuring. A defrost heater runs at 220 V AC, which is extremely dangerous if touched by hand or probe while live. A multimeter can also only measure resistance accurately when no voltage is applied across the component.
What does a cold store defrost heater cost?
The price depends on type and rating. A common 200 W to 400 W glass tube heater costs between 50,000 and 200,000 VND depending on brand and size. Made-to-order silicone heating cable costs more, from 200,000 to 1,000,000 VND per set depending on length and rating. Replacement cost (including labour and parts) runs from 500,000 to 2,000,000 VND depending on how complex the system is and where the store is located.
Frequently asked questions about cold store defrost heaters
What is a cold store defrost heater?
It is a heating element mounted on the evaporator and the drain tray, used to melt the frost on the heat transfer surface on a regular cycle so the evaporator retains its heat transfer capability.
Why does frost form on a cold store evaporator?
Because moisture in the air condenses and freezes on the cold evaporator surface. That moisture comes from air entering when the store door is opened and from the goods inside.
What happens if defrosting does not run on the right cycle?
A thick layer of frost obstructs heat transfer and blocks airflow, so the store cannot reach temperature, the compressor runs continuously, power consumption rises and the risk of liquid returning to the compressor increases.
How do you test a defrost heater?
Isolate the supply, then measure the insulation resistance and the element resistance with a meter and compare with the manufacturer's figures. Also check whether the evaporator is completely clear of frost after a defrost cycle.
What are the signs of a failed defrost heater?
Frost still on the evaporator after the defrost cycle ends, ice in the drain tray, an abnormally long defrost time, or the heater branch breaker tripping when the defrost cycle starts.
Besides an electric heater, what other defrost methods are there?
You can defrost with hot gas taken from the compressor discharge line, or with water. Which method suits depends on the store's temperature range, the scale of the system and running costs.
Related pages
- What is AHU? Structure, operating principle, AHU parameters
- What is HVAC? Principles, structure and research documents of HVAC
- What is COP? The energy efficiency coefficient in refrigeration engineering
- What is a refrigeration system? Current types of industrial refrigeration systems
- industrial refrigeration systems designed and built by Tan Long
