One of the easiest things you can do to raise the temperature of your 'feed box' during the late winter/early spring,  is to use passive solar heating.  I use this box on top of the inner cover with a jar of 1:1 feeding syrup through an access hole in the inner cover.  If you paint this side of the box and expose it to the southern side, you gain some extra heat to warm the syrup.   As you can see from my experiment today I have gained an extra 4 degrees.  This may not sound like much, but it could be critical to helping the bees to get some feed early in the season.   Any extra help during this critical time of the colonies life - could effect its survival.

Solar Passive Heat Experiment:  Black vs. White Painted