Harold Kestenholz
You have a lot of ‘magic’ in your description. There is nothing in your description that gives an idea of the load you are to heat, the size of the slab you are heating, whether the slab is insulated, the length of the baseboard elements upstairs, etc. In other words, you do not give a description of the heat sink. A garage floor would be 4 or more inches of mass that is usually uninsulated from the ground. A two-inch slab in a heated house on an insulated raised floor takes many hours to get to 70F.
People raised on warm air heat have no concept of what it takes to heat several tons of concrete. Your 30,000 btuh of electric heat would not heat the slab any faster if you attached a 350HP Chevy engine to a fire hose pumper.
