Heater mats

Every freezer store must have a heating system that prevents groundwater under the store from freezing and pushing up the cold room floor, otherwise known as “frost heave.”  Cold stores, i.e., those where the air temperature does not go below zero traditionally did not have underfloor heating systems. However, South African palletized fruit stores running at minus 0.5 degrees Celsius for extended periods have experienced frost heave, as water including impurities can freeze at just below plus 4 degrees C.

There are several frost heave prevention systems in use around the world including forced hot air in 220mm diameter pipes, suspending the cold store floor on columns, heated glycol, and electrical heater mats, which are, by far, the most common method used in Southern Africa.

Frost heave in cold and freezer stores is something to be avoided. I saw one floor at a freezer store in George that had risen by as much as 900mms.

Upward floor movements of 15mms can start to affect the operation of mobile racking. Even in fixed racked freezers heave will make the racking unsafe and in automated stores it is disastrous. In recent years there have been too many cases of frost heave where it could have been avoided. Freezer mats are relatively inexpensive to install and will prevent frost heave if maintained. There is little that can be done to fix frost heave once it occurs. If the heat mat is repaired or turned on, the floor levels may drop over time. Where stores have been turned off for extended periods the underfloor ice will eventually melt, but additional weight is needed to push the floor down. I have seen heater mat cables installed as a last resort in the polystyrene insulation in drilled holes but am unsure if it had the desired result. The aim must be to prevent frost heave from happening or to pick it up quickly if it does.          

Heater mats normally consist of 3 circuits of standard underfloor heating cable, laid in the approximately 30mms thick sand bed under the first plastic barrier. As the heater mat design can vary depending on the size and shape of the store, cold store managers should understand how their heater mat has been installed.  The circuits terminate in a control panel which is fixed to the wall of the cold store close to the access door and about 2 meters from the floor. There are three cables so that even if 2 cables malfunction the third can still supply the necessary heat to stop the formation of ice.  The heater mat system includes a temperature probe which is normally placed under the floor at the center point.  The control panels are preset to activate the heater mats when the subfloor temperature drops below 7 degrees C. When the probe records underfloor temperatures of more than +7 degrees, the heater mat will turn off. If the 3 phase amp meters in the control panel show zero current, the digital temperature screen should show a temperature of above +7 and the red indicator light must be on.

On larger floors, multiple temperature probes are recommended as temperatures can fall below zero in an area far enough away from the probe so as not to register. It is also critical that the cables from the heater mat to the control panel are not cut or damaged. This can happen if they are close to the doorway and are confused with the sill heaters.  

If you manage a freezer or cold store, do you know where your heater mat control panel is situated? Underfloor temperatures taken from the heater mat probe(s) should be recorded on a weekly basis so that any change in underfloor temperature can be noted and watched. While underfloor insulation, mainly high-density polystyrene, is designed to prevent subfloor temperatures from approaching zero, it can become less effective over time. If 2 layers of insulation are laid it should be done so that the joints for each layer are staggered and not in the same place. Otherwise, the cold can have a direct channel to the subfloor. In addition, and especially in mobile racked stores, the freezer floor should be surveyed annually with the readings checked against the levels taken when the floor was originally laid.    

Should you have any questions regarding heater mats you can contact Maureen Cairney at cairneymaureen@gmail.com or phone her on 072 833 2602.