What does a cold room defrost cycle do?
A cold room defrost cycle melts the ice that builds up on the evaporator. The evaporator is the finned coil inside the room where a fan blows the room air across the cold refrigerant pipes. Danfoss's technical article on refrigeration controllers states the aim plainly: to give back an evaporator clear of ice, so heat moves from the room air to the refrigerant without a layer of ice in the way. Climate Flow Limited installs, services and repairs cold rooms across London and surrounding areas. This guide explains how the defrost works, which controller settings run it, and who should change them.
If the evaporator is not defrosted regularly, ice can restrict the airflow through the coil or stop it altogether, and cooling falls off, as Danfoss explains in its article on adaptive defrost.
Off-cycle, electric and hot gas defrost: what is the difference?
The difference is where the heat to melt the ice comes from, and that depends on how cold the room runs. The method is chosen when the system is designed.
Off-cycle defrost
In a chiller room working at around 5 °C, the ice can be melted by pausing refrigeration and letting the evaporator fans carry on circulating the room air. Whether a particular chiller room can rely on it depends on the room's design and the load it carries.
Electric defrost
In a freezer room working at around -18 °C, the room air is too cold to melt the ice, so a heat source is needed. The same Danfoss article names the two common ones. The first is an electric heater mounted close to the evaporator fins.
Hot gas defrost
The second heat source is hot gas from the refrigeration system itself, circulated inside the evaporator tubes. Danfoss's guide to hot gas defrost on low temperature evaporators describes the sequence in outline. The liquid supply to the evaporator, the cold liquid refrigerant that normally flows into the coil, is shut off. The suction valve, the valve on the pipe that carries refrigerant out of the coil, closes. The fans stop, and a hot gas valve opens to let hot refrigerant gas into the coil. When the defrost finishes, the valves return to their normal positions and the fans restart once the water left on the fins has frozen again. The valves and the controller run this sequence; it is not something done by hand.
How does the controller end a defrost?
A controller can end a defrost on temperature, with time as a backup. A defrost sensor fixed to the evaporator ends the defrost once the coil reaches the temperature set for it. This setting is often called the defrost termination temperature. As a backup, a maximum defrost time can be set in the controller, which ends the defrost if the sensor fails or something similar goes wrong.
Both settings matter. Danfoss notes that a defrost that is too short can leave ice on the evaporator.
How often should a cold room defrost cycle run?
There is no single interval that suits every cold room. The interval is set in the controller for the equipment and the way the room is used. With interval defrost, the controller starts a defrost at a fixed interval. Danfoss explains the setting with an example: enter an 8 hour interval and a defrost starts every 8 hours. That is an example of how the setting works, not a recommended interval for any room.
Getting the interval wrong cuts both ways. Danfoss points out that defrosts that are too infrequent can leave ice building on the evaporator, while too many defrosts run for too long can affect food quality.
What happens after a defrost: drip time and fan delay
After the defrost comes the drip time: a set period that lets the melted water run off the coil before refrigeration restarts. The controller can also hold the evaporator fans off after the defrost until the defrost sensor has fallen to a set fan cut-in temperature, the coil temperature at which the fans are allowed to start again. Both are controller settings, and both are part of the defrost cycle as much as the heating step is.
What are the signs a defrost schedule is wrong?
A defrost schedule that does not suit the room tends to show in the room itself: ice on the coil, weak airflow, or a room that will not hold its temperature. When an engineer looks at a defrost problem, these are the points they check:
- Ice on the evaporator coil between defrosts
- Airflow from the evaporator fans, and whether ice is restricting it
- Whether the room holds its set temperature through a defrost and after it
- The defrost sensor, its position on the coil and the termination temperature set for it
- The maximum defrost time and the defrost interval in the controller
- The drip time and the fan delay after defrost
Temperature matters because of the law on chilled food. Under GOV.UK's food hygiene guidance on chilling and freezing, chilled food must be kept at 8 °C or below, a legal requirement in England, Wales and Northern Ireland. A chiller room that warms during each defrost leaves less margin under that limit.
Who should change cold room defrost settings?
Defrost settings should be changed by an engineer qualified to work on the refrigeration system. UK rules, set out on GOV.UK's page on F gas qualifications, require anyone working on refrigeration or air conditioning equipment containing F gas to hold their own qualification to install, service and maintain it, check for leaks and recover gases, even if they work for someone else. The defrost settings sit alongside the rest of the system's controls, and a wrong value can leave ice on the coil or warm the room, so this guide gives no step-by-step for changing them. Climate Flow's engineers are F-Gas certified.
When to call an engineer
Call an engineer when ice keeps coming back on the evaporator, when the fans seem to move less air than they did, when the room will not hold its temperature, or when stock warms during defrosts. Tell them the make of the controller, the defrost type if you know it, and what the room is doing.
Our cold room installation page covers new chiller and freezer rooms, and the commercial refrigeration services page covers the repair and maintenance of the refrigeration behind them.
For an example of the work, see the walk-in cold room and freezer for a food preparation area at King's Cross St Pancras Station. If you are choosing between a chiller and a freezer room, our guide to how cold rooms and freezer rooms are built explains the difference.
A cold room defrost cycle set for the room is designed to clear the coil and let the room get back to temperature.
This guide is general information, not a survey or a quote for your site; every system is different, so an engineer needs to see yours before any work is priced.