ECN Forum Forums General Topics of the Trade General Discussion Area 60 Hertz issue with portable generator

I'd start with suspicion concentrated upon the CONTROL CIRCUIT.

Modern HVAC controls are digital-analog. They may well be drawing upon power sources entirely different from the HVAC unit, itself. (Think Smart Houses, etc.)

This is entirely different from the ancient days, when a thermostat was entirely powered off of a line running from the HVAC unit, and low voltage at that.

So, the first question must be: is the control circuit being properly fed?

I'd look to see if the customers are trying to exclusively power up critical loads -- and the circuit feeding the 'thermostat' is not recognized by the homeowner as being a critical circuit.

There are now so many variations on HVAC controls -- including DIY retrofits -- I can't hazard a guess as to what you're dealing with.

But, that's where I'd start. It's usually the last thing that a homeowner considers -- if ever.