An apparatus comprising: a flash tank configured to store a refrigerant; a first load configured to use the refrigerant from the flash tank to cool a first space proximate the first load; a second load; a third load; a first compressor; a second compressor; and a valve (140,145), during a first mode of operation: the second load is configured to use the refrigerant from the flash tank to cool a second space proximate the second load; the third load is configured to use the refrigerant from the flash tank to cool a third space proximate the third load; the second compressor is configured to compress the refrigerant from the second load and the third load; and the first compressor is configured to compress the refrigerant from the first load and the second compressor, and during a second mode of operation: the second load is configured to use the refrigerant from the flash tank to cool the second space proximate the second load; the second compressor is configured to compress the refrigerant from the second load and to direct a portion of the compressed refrigerant to the third load to defrost the third load and to direct the portion of the compressed refrigerant after defrosting the third load to the flash tank; the first compressor is configured to compress the refrigerant from the first load and the remaining portion of the refrigerant from the second compressor; and the valve (140,145) is configured to prevent the portion of the refrigerant at the third load from flowing to the flash tank until a pressure of the refrigerant at the third load exceeds a threshold; characterized in that the valve (140,145) is a regulating valve and is configured to control the threshold. a flash tank configured to store a refrigerant; a first load configured to use the refrigerant from the flash tank to cool a first space proximate the first load; a second load; a third load; a first compressor; a second compressor; and a valve (140,145), during a first mode of operation: the second load is configured to use the refrigerant from the flash tank to cool a second space proximate the second load; the third load is configured to use the refrigerant from the flash tank to cool a third space proximate the third load; the second compressor is configured to compress the refrigerant from the second load and the third load; and the first compressor is configured to compress the refrigerant from the first load and the second compressor, and during a second mode of operation: the second load is configured to use the refrigerant from the flash tank to cool the second space proximate the second load; the second compressor is configured to compress the refrigerant from the second load and to direct a portion of the compressed refrigerant to the third load to defrost the third load and to direct the portion of the compressed refrigerant after defrosting the third load to the flash tank; the first compressor is configured to compress the refrigerant from the first load and the remaining portion of the refrigerant from the second compressor; and the valve (140,145) is configured to prevent the portion of the refrigerant at the third load from flowing to the flash tank until a pressure of the refrigerant at the third load exceeds a threshold; characterized in that the valve (140,145) is a regulating valve and is configured to control the threshold. the second load is configured to use the refrigerant from the flash tank to cool a second space proximate the second load; the third load is configured to use the refrigerant from the flash tank to cool a third space proximate the third load; the second compressor is configured to compress the refrigerant from the second load and the third load; and the first compressor is configured to compress the refrigerant from the first load and the second compressor, and during a second mode of operation: the second load is configured to use the refrigerant from the flash tank to cool the second space proximate the second load; the second compressor is configured to compress the refrigerant from the second load and to direct a portion of the compressed refrigerant to the third load to defrost the third load and to direct the portion of the compressed refrigerant after defrosting the third load to the flash tank; the first compressor is configured to compress the refrigerant from the first load and the remaining portion of the refrigerant from the second compressor; and the valve (140,145) is configured to prevent the portion of the refrigerant at the third load from flowing to the flash tank until a pressure of the refrigerant at the third load exceeds a threshold; characterized in that the valve (140,145) is a regulating valve and is configured to control the threshold. the second load is configured to use the refrigerant from the flash tank to cool the second space proximate the second load; the second compressor is configured to compress the refrigerant from the second load and to direct a portion of the compressed refrigerant to the third load to defrost the third load and to direct the portion of the compressed refrigerant after defrosting the third load to the flash tank; the first compressor is configured to compress the refrigerant from the first load and the remaining portion of the refrigerant from the second compressor; and the valve (140,145) is configured to prevent the portion of the refrigerant at the third load from flowing to the flash tank until a pressure of the refrigerant at the third load exceeds a threshold; characterized in that the valve (140,145) is a regulating valve and is configured to control the threshold.