Littelfuse released a new solid state latching relay CPC1601M which is very suitable for thermostat application.
It is a non-isolated, low operating current,1-Form-A solid state latching relay, available in a small 3 × 3 mm2 DFN package. The relay can obtain operating power from the open-circuit load or system power supply. If powered from the load, the relay takes zero power from the system supply, thus helping to extend battery life. When power is supplied by the load, the relay opens periodically to obtain power via the open-circuit load voltage. In most applications this interruption is transparent to the load. Related evaluation board is avialible already with details as below,