Did you only have $657 of income tax before the energy credit (look at lines 16 through 22 of your Form 1040)? The credit is non-refundable, meaning that if your income tax is only $657, it can't give you credit of more than $657.
As a side note, unless your furnace or water heater is a "heat pump", the maximum credit is only $1200. The extra $2000 is only for a "heat pump".