Leaving house with septic system over winter

I have had 2 different septic tanks freeze. One was a new system, put in the summer, used until late fall, we went south for the winter, and when we came back to use it, the top of the liquid in the tank was froze about 2 inches. I took a long pipe and broke up the ice and after we used it for a day it was thawed out. It was our fault because we came back to early and the ground wasn't thawed out yet. If they are used year around, the heat from the decomposing will keep the system from freezing. The other one was a home that had a high water table and the systems are supposed to be installed by code, as high as possible and the tops of the tanks are only 6" below ground. The furnace was a 96% furnace that has a condensate line going from the furnace to the floor drain. This is important to realize! In furnace usage, there will be a small dribble of water going down the drain and into the tank. Where the septic drain line exits the house, the water will freeze and make a dam and the water will eventually block the line. On that house, my son would come over every so often during the winter and turn the water on and flush the toilets and turn the inside faucets on and when he flushed the toilet, the water went down and a few seconds later came back up and went all over the floor. Good thing it was clean water. When we got back home in the spring, we had to have the line thawed with steam to open it up. Again, if we stayed longer and the ground was thawed out, there would be no problems. That was a very cold winter. Other years when we would us it later in the year and there was snow on the ground, the ground above the take would be clear of snow. On a commercial property that I own, the occupants don't use much water, and I just have the tank pumped in the fall and then the tank is just like a holding tank and when the ground is thawed the tank will be full and work normally. If you don't come back to early and start to use the system, you will be fine. Or if it is used all year around, you will be fine. Jim