Few people understand HVAC. Although if you have anything to do with running a data center, you have to learn the basics.<p>I was once in a computer room at Stanford which had two large industrial AC/heat units. I was there to copy old backup tapes (alumni were saving some history) and had a lot of free time while the tapes spun. I noticed that one A/C unit was on heat, and the other was on cool. So they were wasting a vast amount of energy.<p>You're supposed to wire those units together so they cooperate. Their controller can do that.
Nobody knew that.
Before the pandemic, I had to replace my outside AC unit. I got three different quotes, each of which suggesting three different required changes, from electrical to new duct work.<p>I asked the first quoter about all the other work #2 and #3 suggested and he scoffed at it and said he did not want to keep his guys working in my attic one second longer than necessary in the August heat and that the electrical work was completely unnecessary.<p>Similar to dentistry, they're doing something useful but it's hard to know how much of it is actually valuable
It took some digging, but I managed to find the specifications for the temperature limit switch in my High Efficiency heating system... for $7.62 I was able to buy a replacement from Grainger, and get it installed.<p>I strongly suspect if I had called for service, I'd be told I needed a new furnace, even though this one is less than a decade old.
If you call an HVAC firm in Tucson, often they simply use the call to try to sell you a brand new unit, making disparaging comments about the condition of the current unit, and outright lying about its fixability.
I designed and installed a heat-recovery ventilation system in an Alaska house (new construction). I was aiming for five-star or five-star-plus rating but there was enough leakage that it only got four-star-plus. Still nothing to scoff at compared to the standard contiguous U.S. houses, but the amount of sealing at joints, electrical/plumbing penetrations, HVAC, and vapor barriers was pretty intense. Even the seal under the front door can be enough to lose a rating point if it's bad.<p>Balancing the HRV was done by some professionals once the install was done, and only required a manometer to adjust some wall vents and a baffle in the exhaust duct, which wasn't quite the same as heating/cooling calculations would be, since heating was hydronic and AC isn't really a thing in AK.
Contractors typically/frequently install AC condensers and furnaces that are too big for the house. These cost more and the contractors don't have to worry about call-backs for insufficient cooling or heat. Unfortunately they are much less efficient because they run for such short cycles.