Basically for every degree F below 77 degrees, you lose about 3% production.
But, on the positive side, colder water produces lower TDS so it has its advantages. Colder water is more dense so treats better.
Given a choice I would take colder water any day. If its too cold and GPD suffers is much better to add a booster pump than to try warming the water. Some people attempt to temper or blend hot and cold and this is the quickest way to ruin a membrane there is. RO membranes cannot take any water over 113 degrees or it melts the TFC membrane fabric. Remember being in the shower and someone flushes the toilet or starts the washing machine? You got your tush scalded and thats the same thing that happens to a membrane. Toast.
Others try using an aquarium heater in a bucket of water with some tubing coiled up in it but that does not work either for several reasons. One is the BTU's required to heat even one gallon of water one degree is tremendous and when you consider water is passing through the tubing at better than a quart a minute a heater just can't keep up. Add to that the fact plastic tubing is a horrible conductor of heat and cold, it does not work as a heat exchanger at all.
A booster pump like the Aquatec 8800 is the only way to really overcome cold water by increasing pressure. Again play with the calculator and see how increasing pressure makes up for cold water.
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