Upon arrival we checked into the concern that the comfort controller was blank and system not operating. During diagnostics we noticed the 5 amp low voltage fuse was popped. No obvious low voltage short. Used a resettable 5 amp fuse and jumped thermostat wires to trigger operation and fuse did not pop. Replaced blown fuse with new 5 amp fuse. System started up and ran without blowing fuse. Checked system pressures and it is operating at manufacturers specifications.