Coldroom Heater Transformers: How They Work and Why They Fail

If you have door frame heaters, threshold heaters, or heater mats fitted to your freezer coldroom, there is a transformer somewhere in the circuit. It is one of the most overlooked components in a coldroom installation, and one of the most misunderstood when something goes wrong.

This guide covers what low voltage transformers do, why the industry uses them, how they work in practice, and what causes the internal wiring to scorch and become brittle over time.

What Is a Low Voltage Transformer?

A low voltage transformer is a step-down device. It takes the mains supply (230V AC in the UK) and reduces it to a lower working voltage, typically 17V, 19V, 24V or higher, depending on the heater circuit it is feeding.

In coldroom applications, transformers are used to power:

• Door frame heaters (the heating element embedded in or around the door frame that prevents condensation and ice build-up)
• Threshold heaters (floor-level heating strips that stop ice forming at the base of the door opening)
• Modular heater mats (usually 25mm polyfoam board with heating elements used under floors of freezer coldrooms and coldstores to prevent frost heave)

The transformer is usually housed in an enclosure mounted on the face of the coldroom or on top of the ceiling panel. It runs continuously or is controlled by a thermostat or timer, depending on the installation.

Why Low Voltage?

The short answer is safety and practicality.

Door frame heaters sit behind metal heater covers, which can become damaged through general use and impact. Threshold heaters are particularly vulnerable in high-traffic areas where trolleys and cages are regularly used, as the plates above them take repeated knocks and loading that can compromise the heater beneath.

Running a 230V circuit around metal door frames, or beneath a floor that may be wet and subject to heavy traffic, creates a genuine risk. The heater element is also compressed by the door frame on every closing cycle. Low voltage significantly reduces the danger of electric shock in the event of insulation failure or accidental contact.

There is also a practical benefit. Low voltage heaters can be made thinner and more flexible than mains-rated elements, which makes them easier to fit into door frames, thresholds, and tight rebates without compromising the seal or the door alignment.

How They Work

A transformer works on the principle of electromagnetic induction. Two coils of wire, the primary and the secondary, are wound around a shared iron core. Alternating current in the primary coil creates a changing magnetic field in the core, which induces a voltage in the secondary coil. The ratio of turns between the two coils determines the output voltage.

In a step-down transformer, the secondary coil has fewer turns than the primary, so the output voltage is lower than the input.

One important consequence of this is the relationship between voltage and current. The mains supply side of the transformer operates at 230V but draws relatively low current. The low voltage output side operates at a fraction of that voltage but carries significantly higher current to deliver the same wattage. A transformer supplying a 60W heater mat at 24V will draw around 0.26A on the mains side but push 2.5A through the secondary circuit. At higher loads the secondary current rises accordingly. This is not a fault or inefficiency. It is simply how transformers work, and it has direct consequences for how the secondary wiring and terminals age over time.

The transformer itself has no moving parts and requires no maintenance in normal operation. What it does produce is heat. Even a well-specified transformer running at full load will generate some heat as a by-product of the conversion process. In a sealed or poorly ventilated enclosure, this heat accumulates.

The heater circuit connected to the secondary side also generates heat at every point of resistance in the circuit, including terminals, connectors, and any section of cable that is undersized or has developed a poor connection over time.

Why the Internal Wiring Scorches and Becomes Brittle

This is the question that comes up most often when a transformer is opened up for inspection after years of service.

The wiring inside the enclosure, and the link wires connecting the transformer to the heater circuit, are subject to a combination of stresses that standard PVC cable is not designed to handle indefinitely.

Continuous energisation. Unlike most electrical circuits that switch on and off, heater mat transformers are often live for extended periods, sometimes continuously. Every hour of operation adds a small amount of thermal stress to the insulation.

Localised resistance at terminals. Even a well-made terminal connection has a small amount of contact resistance. At low voltage and high current, this resistance generates heat at the connection point. Over time, terminals loosen slightly through thermal cycling (heating and cooling), which increases resistance further and generates more heat. This is a self-reinforcing process.

Ambient temperature. The enclosure is not always in a cool location. In summer, or in poorly ventilated spaces, ambient temperatures can be significantly higher than the rated operating temperature of standard PVC insulation. PVC cable insulation is typically rated to 70 degrees C. In a warm enclosure with a transformer running at load, local temperatures at terminals can exceed this.

Cable type. Standard flexible PVC cable is adequate for many applications but is not ideal for continuous high-current, elevated-temperature service. Heat-resistant flexible cable, rated to 105 degrees C or higher, maintains its insulation integrity significantly longer in these conditions.

The result of these combined factors is gradual hardening and discolouration of the cable insulation, starting at terminals and working outward. In older installations, the insulation can become brittle enough to crack when flexed, exposing bare conductors. This is not a sign of a faulty transformer or incorrect specification in most cases. It is the expected end-of-life condition for a circuit that has been working hard for many years.

What to Check When Replacing the Unit

When a transformer reaches this condition, replacement is straightforward, but it is worth taking the opportunity to address the underlying factors that accelerated the deterioration.

• Check the heater load current against the transformer VA rating. An undersized transformer runs hotter and ages faster.
• Replace any heat-affected cabling and link wires, not just the transformer itself.
• Re-terminate all connections and check that terminals are secure and correctly torqued.
• Use heat-resistant flexible cable (rated to 105 degrees C or higher) for all connections within the enclosure and at the heater terminals.
• Ensure there is adequate ventilation around the enclosure. A louvred enclosure or a simple ventilation gap can make a significant difference to operating temperature.

Taking these steps at replacement stage gives the new unit the best chance of a long service life.

Manufacturers and Specifications

The most widely used low voltage transformers in UK coldroom installations come from a small number of specialist manufacturers. Thermal Dynamics, Loheat, and Thermatek are among the names most commonly found in commercial refrigeration applications, producing units designed for continuous duty in coldroom installations.

Many transformers used in this application are adjustable, offering multiple voltage tapping points rather than a fixed output. Common settings range from around 17V to 24V or higher, allowing the installer to match the output to the length and resistance of the heater tape. A shorter run of heater tape has lower resistance and draws more current at a given voltage, so reducing the voltage setting prevents overloading. A longer run may need a higher setting to maintain adequate heat output.

When specifying a replacement, the key figures to match are the output voltage range, the VA rating (which must meet or exceed the total heater load), and the duty cycle (continuous or intermittent).

If you are unsure which transformer is correct for your installation, the heater mat or frame heater will usually have a label showing the voltage and wattage. Divide the wattage by the voltage to get the current draw, then select a transformer rated comfortably above that figure.

Summary

Low voltage transformers are a small but critical part of any coldroom heater installation. They step mains voltage down to a safe working level, allow the use of thin, flexible heater elements, and run continuously in often warm and confined conditions. The mains supply side draws modest current, but the low voltage secondary side carries significantly higher current to deliver the same wattage. Over time, the combination of that high current, localised terminal heat, and continuous operation degrades cable insulation in a predictable way. Scorched and brittle wiring is a sign of age and accumulated thermal stress, not necessarily a fault in the original specification. Replacing like-for-like with the correct VA rating, upgrading to heat-resistant cable, and ensuring adequate ventilation will give the replacement unit the best possible service life.