Okay guys, I am involved with septic installations. Different states have some different rules on the designs, but here are a couple thoughts on this: Septic tanks get heat not only from warm waste being introduced to the tank, but also from microbial activity within the tank. The tank is also in the ground, at least partially, below the frost line so it won't likely freeze solid. What can happen with a tank on a vacant house is a freeze on the top of the liquid like a lake freezing. The ice can plug the inlet and/or outlet pipes which are at the top of the liquid level. Deeper tanks, snow, leaves, straw can all help prevent this by insulating the ground. In this situation water will go down the drain, hit the ice, fill all the waste pipes and then back up into the house coming up in the drains. Running some hot water won't likely be enough to thaw this ice before the back-up occurs. Newer systems typically have the tank lids above grade. Open it and look inside. The pipe between the tank and house can't freeze up on a vacant house because there is no water there. A functioning drainfield on a vacant house won't freeze because there is no water there either (it has soaked into the ground). Mounds are trickier because liquid is pumped from the tank to the mound and then some of the liquid from each dose runs back into the tank again. The liquid is spending more time in a cold pipe and ice could build up with each dose. On a vacant house with a conventional system, if you look in the tank and it isn't frozen, you should be good to go. If it is frozen, I would not run water from the house. How to attack it would depend on the thickness of the ice.