Freezer sub-floor heating
Frost-heave prevention for freezer floors on-ground.
The earth beneath freezer floors more than a couple of metres wide will slowly freeze, causing lifting of the floor.
Beneath larger rooms without frost heave protection, the earth has been known to gradually freeze to a depth of at least 3 metres, despite the floor insulation. This causes a dome-shape upward heave to the freezer floor of more than 400mm at the centre. Although this can take years to develop, the structural failure of the floor is serious.
Will frost heave happen to all freezers on ground?
Provided that the floor incorporates 150 – 200 mm of polystyrene floor insulation or an equivalent, and the room is no more than 3 m wide, frost heave is unlikely. This may be increased to rooms up to 4 m wide provided that the sub base to the room is well drained to a depth of 600mm and even further if the floor is well elevated above the surrounding ground level.
How is frost heave prevented?
Three different methods of prevention are employed. The first is the use of a system of vented pipes laid at around 500mm spacing to provide for cross flow ventilation beneath the room. This method is not common because it requires unobstructed airflow across opposite sides of the room, free from other external obstructions or buildings.
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The second and most common method of prevention requires heating cables run at around 400 – 600 mm spacing. Heatec can design the layout and either supply the cable with layout plan, or complete the installation. The electrical loading depends on floor insulation and room temperature, but is usually very low – around 15 – 20 W/sq.m. Dual circuits with one cable not connected are usually employed. The cable loading is very low, preventing any form of cable deterioration, however most clients request dual circuits for the added peace of mind.
Mains voltage (230-240V) cable circuits are normally used, as there is no advantage or justification for using lower voltages.
A third method is the installation of polyethylene pipework either in or below the base concrete slab. A closed circuit of warm water or glycol mix is circulated through the pipe work runs at 600 – 800mm centres, using condenser heat captured from the refrigeration plant. This is more expensive to set up, but is justified for very large rooms eg. 500sq.m or larger.
The integrity of the vapour barrier below the insulation is very important. Failure can lead to water soaking into the insulation, dramatically reducing it’s thermal resistance. Sub-floor heating is installed at a low rate to balance the small rate of heat movement through the floor insulation. It is not designed with the capacity to cope with insulation failure.
Information required for quotation:
Inside floor area
Type and thickness of floor insulation.
Since the electrical loading beneath small rooms – up to 25 sq.m is low (eg 450W) the power is usually on continuously. For larger rooms, a sub-floor sensing thermostat is recommended for control and alarm.
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Facsimile: 617 3806 5457