The 5 amp 24V control power fuse blows immediately upon selecting cool mode. The unit works perfectly in fan mode (air handler fan) and heat pump mode. I disconnected the 24V wires to the contactor fuse still blows. Disconnected the two 24V leads to the transfer valve (switches from A/C to heat pump), fuse blows. Replaced thermostat, fuse blows. Burned up two transformers 240V/24V replaced with one with a breaker. I don't know anything about the Low Voltage Defrost Relay, could it be that?