How to not be scammed by an HVAC Tech:
Be polite, offer the tech water or coffee, and have a flash light ready. Watch them while they are working, but keep your mouth shut, just observe. Offer to hold the flash light. Don't make distracting chit chat and don't ask the tech for a conclusion before he/she is ready. Have a note pad ready, don't take notes yet. You may not understand what he is doing but you can observe what areas he is working in and how much time he spends in one area. If you have some knowledge of heating and cooling or engineering don't tell the tech. You may hear an audible groan, if you don't he/she will be thinking "oh no not another engineer with a chip on his/her shoulder." The reality is if you could have fixed it you would have already, instead you called in an expert, if the tech is not an expert, you will have a chance to point that out later. For now just keep quiet. If the tech seems overly chatty about how you need to spend money on other "upgrades" like UV lights, just make the briefest comment you can about how you may be interested in talking about that after your problem is diagnosed. If the tech is nervous and tries to get rid of you by telling you to go adjust the thermostat, tell him flat out that you prefer to observe and he/she should feel free to make any adjustments. A true technician will be able to focus on the diagnosis of the system and reach a conclusion regardless of weather you are there or not. Once the tech reaches a conclusion he/she will approach you. Now is the time to ask questions. Some good questions are:
What do you think caused this part to fail?
How do you know it has failed?
What test did you preform to determine this?
That last question is a great one, it really tells you if you have a true tech or a bull sh..er
A true tech will be able to answer the question and even show you.
"Mr. Homeowner sir, you have a failed gas valve and thermostat."
"Oh, both parts are bad? How did that happen"
"Well..sir... the transformer here, creates 24 volts, now it has a little breaker on it, which you saw me reset, lucky for you it has a breaker other wise you might need a transformer as well. Ok, so it creates 24 volts which travels up to your thermostat and waits for instructions once there is a call for heat it travels back to the furnace and goes through a bunch of safeties, eventually it gets to the gas valve and energizes a coil which turns on the gas and gives you heat. Now your gas valve has an internal short to ground I can measure that here. But I know your thermostat has also failed because after I reset your breaker I could not get a call for heating, so I bypassed it here to test the rest of the system."
Now this is a good explanation, but it does raise some new questions and it does simplify some of the science (like transformers don't actually create 24 volts they transform), but the techs ability to simplify the language and help you see what happened is a real good sign that he/she knows what he is doing. Now he may be lying, but remember you observed him/her the whole time, so you can now ask questions to clarify, questions like, "Is this (pointing to a bundle of colored wires) where you bypassed the thermostat?"
His answer should further clarify and in effect teach you some thing:
"Yes the voltage travels up to the thermostat on the red wire and comes back to the furnace on the white wire, you can see on my meter here that the circuit is open even though your thermostat is calling for heat."
Now the tech has just raised the level of discourse. He is now letting you into his diagnostic process. Are you able to follow? This depends on you knowledge and his/her ability to explain. If either are lacking you have to start trusting, or asking for further clarification. These questions could go on endlessly but they often don't. There is nothing wrong with asking for an education as it relates to your problem. A good technician will be happy to oblige. One who does not know what he/she is doing will get frustrated, angry, or overly technical.
Be calm take a breath, ask him/her to do the same, and say you don't under stand.
If he starts to contradict himself start taking notes.
You may end up speaking with his boss or another company and you will need the notes.