I once did an experiment with a foot long 2" PVC pipe and it took over 12 hours to freeze solid in my freezer which is set to 0 degrees F. So it takes quite a long time for water to freeze in pipes because of the latent heat in the water. Add to that the heat of the equipment on the pad, the pad itself and the house it sits next to, there is a lot of heat that needs to be removed before the water can even start to freeze.
Also, I have a theoretical model which includes radiation, convection and conduction thermal transfer and it gave me pretty much the same answer as the test above. Using the model with an air temperature of 20 degrees F and a water temperature at 32 degrees already, it takes about 25 hours for water to freeze in a 2" pipe with no wind blowing. At a 10 MPH wind, it would take about 15 hours.
The model had given confidence several years ago, to turn off my freeze protection and I haven't used it since. During this time of year, we regularly get below freezing for up to 8 hours at a time and the pipes have never even begin to freeze. But even more amazing, on an especially cold night where the temperature reached a low of 22 degrees or so in the morning, I checked the pump basket, the clear check valve and the SWG and ice had not even started to form in any of those items. What was also interesting is that the air temp was 22 degrees but the water temperature sensor read 32 degrees in the section of pipe I couldn't visually check but that also told me that the pipe had not frozen yet or it would have been lower than 32 degrees.
