A method for determining whether water contained within a water pipe system in a building is at risk of freezing is disclosed. The method is carried out by a detector device comprising a processor. The method comprises determining a plurality of pipe temperature measurements using a temperature sensor. The processor analyses the pipe temperature measurements to detect whether a water draw has occurred. An exterior building environment temperature is compared to a predetermined freeze alert threshold. If the processor detects that (i) a water draw has not occurred; and (ii) the indicative temperature of the exterior building environment is lower than the predetermined freeze alert threshold, an alert is transmitted.