Sizing a defrost heater for a −40 °C blast freezer is three separate calculations that have to agree with each other: the heat the frost actually needs, the watt density the tube can carry at that temperature, and the cycle time the cabinet can afford. Get one of them wrong and the cabinet runs badly.

By Jake, Product Manager · Published 30 October 2026 · 11 min read · Last verified: 2026-10-30

Line One: The Frost Load, Which Is the Only Number That Starts the Calculation

Every defrost heater sizing calculation begins with the mass of ice that has to be melted, and that mass is a function of the air the evaporator has been handling, not of the cabinet’s cubic metres.

Where the ice comes from

Because the frost load scales with the moisture throughput and the coil temperature difference, a coil that is 5 K colder than specified can carry substantially more ice than the same coil at the design temperature, and the heater sized for the design load then becomes the limiting component in the whole cabinet.

The energy the ice needs

The energy required to turn the frost into water and carry it away is a latent heat calculation, and it is the one part of this that is not a matter of judgement. Mass times latent heat of fusion gives the energy in the ice itself, and you then add the sensible heat of raising the meltwater from the evaporator temperature to the drain temperature. What matters for the heater is the rate: divide the energy by the defrost window and you have the power the heater has to deliver while it is energised.

Because the heater is the component that has to deliver that rate within a window the cabinet cannot extend, the power requirement and the cycle time are one calculation rather than two, and specifying them separately is how the mismatch gets in.

The part people forget

The heat has to get from the heater surface to the ice, through whatever is in between. A defrost heater that is buried against the coil transfers less of its output to the ice than one with airflow across it, because the air between them has to move for the heat to travel.

Because a heater buried in a stagnant layer can deliver its full electrical output and still fail to clear the frost, airflow across the element is a design input to the sizing calculation rather than an installation detail.

Line Two: Watt Density, and Why It Is Only Half the Specification

Watt density is the number a buyer quotes and the number a supplier answers with, and it is the number that leaves out the temperature. That is the whole issue.

What diameter actually changes

Watt density is watts per metre of tube, and the surface area of that metre depends on the diameter. A 10.7 mm tube carries the same 400 W per metre as a 6.5 mm tube while presenting roughly two and a half times the surface area, which means a lower surface loading for the same heat output.

Because the surface loading sets the sheath temperature, and the sheath temperature is what determines whether the element survives the thermal cycling of a defrost cycle and the −40 °C ambient, the diameter choice is a durability decision and not a dimensional one.

Why the temperature changes the answer

At −40 °C the heat transfer conditions around the heater are different in two ways. The air is colder, so it takes more of the element’s heat away by convection at the surface, and the ice on the coil is colder, so the conduction path back to the melting front is different. A heater that clears a domestic evaporator comfortably can be marginal in a blast freezer even at identical watt density, because the same electrical input is being disposed of into a colder environment and a harder thermal path.

Because the heater must do two jobs at once — melt the ice and shed its own heat — and both jobs are harder at −40 °C, the watt density that works at ±0 °C is not the watt density that works at −40 °C.

Where the 300–400 W per metre figure comes from

Our published power for cold room and cold storage defrost heaters is 300 to 400 W per metre, and the same band is used for the drain pan defrost heater on unit coolers. That band is a manufacturing and process range: it is the band in which our tube diameters, our magnesium oxide fill and our element winding produce a sheath temperature the sealing and the lead wire can survive.

Because the watt density range is a consequence of what the element construction can hold, and because it was derived for cold room and air cooler duty, using it unchanged for a blast freezer assumes the same heat transfer conditions and it does not have them.

Line Three: Cycle Time, and What the Cabinet Can Actually Afford

The defrost window is not a free variable, and the third calculation exists to check that the heat can be delivered inside the window the cabinet’s product temperature allows.

What limits the window

A blast freezer running at −40 °C has essentially no thermal margin. Every minute the compressor is off and the heaters are on, the cabinet’s product temperature rises, and the food inside has to survive that rise. In a −18 °C freezer there is room to manoeuvre. At −40 °C there is almost none.

The failure mode nobody writes down

There is a compounding effect here that is worth stating explicitly, because it is the difference between a heater that is slightly undersized and one that is significantly undersized. Residual ice on the coil is an insulating layer. The next cycle has to melt the ice that was left as well as the new frost, so a heater running at the limit accumulates a deficit that shows up weeks later as a cabinet that will not hold temperature. The symptom appears to be a compressor problem, and the fix appears to be a compressor. It is neither.

Why hot gas and hot water exist

The alternative to an electric defrost heater is hot gas or hot water defrosting, and both exist because they move the heat rather than generating it. Hot gas uses the compressor’s own discharge, which is free once the compressor is running. The trade is that they require the cabinet to be designed around them, and they are hard to retrofit into a cabinet that was built for an electric element.

Because the defrost window at −40 °C is bounded by the product and cannot be extended, the choice of defrost method is usually made at cabinet design stage, and retrofitting a heater into a cabinet designed for hot gas is a different project than ordering a heater for a new cabinet.

The consequence for the enquiry

Which means the numbers a supplier needs are not the wattage alone. They are the evaporator coil surface or the air cooler size, the cabinet set point and the product temperature, the number of defrosts per day, the volume of air through the coil, and the available defrost window. Our published guidance before an inquiry asks for exactly this kind of data: the drawing or a real picture, the heater size, power and voltage, and any special requirements.

Because the sizing calculation has three inputs that the cabinet designer holds and the heater supplier does not, the numbers have to arrive with the enquiry or the quote is a guess dressed up as a specification.

Frequently Asked Questions

Which Approach Should You Take?

Run the frost load, the watt density and the cycle time, and make all three agree before you choose a diameter. A blast freezer defrost heater is a three-part specification, and the failure mode of each part is different.

Because the third failure is the slowest to appear and the most expensive to diagnose, and because it is entirely a sizing error, the margin belongs in the power figure rather than in the service plan.

Our defrost heater range covers the evaporator, walk-in freezer, cold room, unit cooler and domestic refrigerator versions, with a Cr20Ni80 nickel-chromium core in a stainless steel pipe with high-density magnesium oxide insulation, IP65 sealing, insulation resistance at or above 100 MΩ, CE and RoHS certification and a service life of 8,000 hours or more. The cold room and cold storage heater is the one built for the conditions a blast freezer creates.