I use a Kasco circulator to maintain an area of open water during the winter. It runs less than 50% of the time (temperature and timer controlled). It does and excellent job of keeping an area open all winter long. We currently have 28 plus inches of ice.
The circulator affects the whole (5 acres) pond as witnessed on a calm spring morning. It will put a ripple on the whole 5 acres of surface area. You can set the angle of the circulator so that it primarily affects the surface or alternatively to move water from bottom to top like an aeration system does (or anywhere in between).
