HVAC is heating ventilation and air conditioning
I'm already a HVAC technician I went to college for it, my husband is one as well, we own a heating and cooling business, but when I go on service calls and the customer is a man they sometimes look at me like I shouldn't be doing the job I have...Even though I'm certified

It happens to women plumbers too. You have to learn to be confident and talk to them to reassure them. Use technical terms and describe your work as they would likely stand there to learn. If you look intimidated then it would not work. They just want to see confidence.
Not to get into politics but be a Hillary on the job!