Abstract: A refrigeration system is provided that can be used to cool a plant growth chamber. The refrigeration system can include a compressor to compress refrigerant, a condenser to condense refrigerant, a compressed refrigerant line running from the compressor to the condenser, a throttling device, a condensed refrigerant line running from the condenser to the throttling device, an evaporator to evaporate liquid refrigerant, a throttled refrigerant line running from the throttling device to the evaporator, an evaporated refrigerant line running from the evaporator to the compressor, and, a by-pass line connected to the compressed refrigerant line after the condenser and running to the evaporator. A hot gas proportional valve provided inline of the by-pass line and a liquid proportional valve provided inline of the condensed refrigerant line are used to control the flow of refrigerant through the by-pass line.