If you are able to dedicate a room to the rack -> I closed the heater vent to the room and put a towel under the door to hold in heat, and then got a space heater (electric, ~$40) from Home Depot that has a built-in thermostat that turns it on and off. It helped enormously to bump the ambient rooms temps from my usual 68deg in winter to ~76deg, with about a $35/month increase in my electricity bill. It may not be the most energy-efficient setup, but it worked for me.