I was reminded of another pet peeve, the protectionism in virtually the entire HVAC industry. There are a handful of online dealers now that will sell to anybody but prior to that it was virtually impossible for an individual to buy a furnace, boiler, ducting or even replacement parts like gas valves and such. I've heard people say it's about safety but I don't buy that. I can walk into any trade counter, electrical, plumbing, lumber, roofing, etc and buy anything I could possibly need to build or maintain a house, they aren't going to give me the discounts they'll give their regular guys but they're happy to take my money and give me the item I need. I can walk into an automotive supplier and buy any parts they sell for maintaining a car, truck, van, even bus or semi including fuel system parts, high voltage ignition, brakes, suspension, anything. I can even buy medical supplies and parts for medical equipment, a little harder to come by but I've never had a supplier refuse to sell me something. I can buy a gas water heater at any hardware store or home center, I can buy a gas range, or any other appliance, all readily available. All of this stuff is readily available to anyone, except for HVAC. Try buying a furnace, AC condenser, most ducting or anything else and those places will flat out refuse to sell unless you are a licensed contractor.
I have never encountered any other group of tradesmen that are so openly hostile toward the DIY crowd and I can think of no other reason than protecting of their own and feeling entitled to a profit. They act like the work they do is extremely difficult and beyond the abilities of ordinary people but it is not THAT hard and it would be much easier if all of the materials and equipment were readily available like they are with other trades. It requires an attention to detail and combines several other trades but it is something that virtually any engineer or advanced DIY enthusiast ought to be able to do if they feel inclined. Plumbing, electrical, highschool level physics and chemistry, basic math, it's not rocket science. Some of the guys who practice the trade are real craftsmen but a lot of them are just goobers and I've seen some really idiotic sloppy work done by supposed professionals.