There was a blown fuse on the AC unit.
Good news that you found it.
Are you writing about the low voltage side of the AC unit?
If it is the low voltage side then it might be that HAI RC80 power stealing thing.
As RAL mentions above. This fuse really never goes.
Another initial fix to the RC80 power stealing was adding a common wire from the motherboard in the HVAC device to the RC80.
Have a look here.
Troubleshooting HAI thermostats
If it is a heating and cooling application, is the common wire installed? (note that this is about the RC80 and not the Omnistat2).
The RC-80 Thermostat was designed to work with most 4 wire HVAC systems. The transformer common (C) is usually not required, because the RC-80 can power steal (to maintain operating power when the heating or cooling relay is engaged) from the W (heat relay), Y (cool relay), and G (fan relay). If the heat, cool, or fan relay cannot supply 15mA to power thermostat without the relay activating, a common wire or the following procedure in this application is required.
Whenever feasible (if an additional wire is present or can easily be installed), it is recommended that a connection be made between the transformer common and the (C) terminal on the RC-80 Thermostat. In new construction installations, inform the HVAC contractor to run a minimum of 5 wires between the thermostat and the HVAC system, with one connected to the transformer common.
Note: The RC-80 requires an additional 3-conductor wire from the thermostat to the automation controller for communications.
In a scenario where the RC-80 thermostat resets when calling for heat or cool (and no common wire is present or can easily be installed), refer to procedure
TN: 1 and
TN: 2.
