Two possible explanations:
- The freezing temperature of the glycolated water is too close to the evaporation temperature. As it approaches its freezing point, glycolated water undergoes a very significant increase in viscosity and "layers" of glycolated water stop flowing. The heat exhange drops and the glycolated water slowly freezes in the heat exchanger. The freezing point of glycolated water must be at least 5 degrees lower than the lowest temprature likely to be reached in the exchanger.
- The circuit contains a lot of sludge. This sludge will settle in the areas where there are the greatest losses of load and the coldest areas, so generally in the exchanger. The only alternatives are either to drain and clean out the system or to mount a bypass filter to remove the sludge.