An extra clever system would include every temperature controlled appliance in the house. Heat could be exchanged between the hvac, water heater, refrigerator, and oven.
When the oven is done cooking it can dump heat into the water heater (and or furnace in the winter). The fridge and HVAC could dump heat into the water heater before pumping it outside in the summer.
When the oven is done cooking it can dump heat into the water heater (and or furnace in the winter). The fridge and HVAC could dump heat into the water heater before pumping it outside in the summer.