HVAC repair starts getting into black magic territory. Giant, incredibly dynamic systems with hard physics, positive/negative feedback loops, control theory operating with very few inputs and very uncomfortable humans dripping sweat on the back of your neck while they watch you work.

I feel like everyone that frequents HN would have a good time spending a year working in that industry.

I've had a tangential interest in the physics / mechanics of air-conditioning since working in an office with two people on opposite ends of the "comfortable temperature" scale, and noticing that one of them opens their window in order to "solve the problem". I've since noticed various other people open windows to try to "fix" the air-conditioning temperature.

My assumption is that opening the window fucks it up for the whole system, but very few people are aware of this, and the office folks aren't made aware of it by the air-con techs.

Am I way off? Am I overstating the effect that an open window has on the system?

It will upset the building balance at a minimum. Air balancing techs are typically a different discipline from HVAC in large commercial spaces.

Leaving the window open and AC on in high humidity areas can freeze up the coil generating a service call, trick zone sensors, etc. If there is a large temperature difference the sharp gradient will not be comfortable.

I never worked on very large systems, mostly small commercial and residential. A buddy of mine supervises HVAC for a large commercial facility and we were talking about their makeup air. Basically it sounded like the building is effectively hermetically sealed until the makeup air vents open because of interior pressure or O2/CO2 levels. Seemed kind of creepy that there was some active system in charge of that. How do you know when it's not working?

The same CO2 sensors that call for make up air are hopefully monitored. Sick building syndrome.

Buildings are poorly sealed in my experience. In California code requires economizers on RTUs and they are almost always broken / stuck partially open. In the Inland Empire for example the actuators bind up from sand and often seize. Nobody wants to pay to fix just that, if they even know it’s busted.

I stay under 10 tons. Things are different in bigger buildings!

>Auto repair is 90% diagnostics and 10% wrench turning. A lot of newer vehicles have symptom matrixes with a process for repair. Lowering the skill floor for mechanics.

That’s unfortunate because nobody actually knows what is doing and everything is fine as long as it falls in the guidelines, if they found a problem that is not “in the books” they have no clue how to proceed and this problem not only applies to mechanic shops, i’ve seen the same situation in the medical field.

The skill floor is under the house: I have a 3-series BMW that was having odd engine hesitation but not throwing any codes. The dealership's mechanics could not fix it! They ended up throwing a ludicrous amount of parts at my car including some very expensive items such as a new transmission, new transfer case, new fuel injectors, plugs, coils, fuel pump, wiring harness. The bill, which was covered by the warranty, totaled over $10,000 and they still had not fixed the issue. They finally called in a guy from an Audi dealership and he fairly quickly diagnosed the problem as a faulty oxygen sensor. They had the car for nearly 60 days while I drove a brand new 4-series loaner car. So BMW did right by me, but holy crap I have never ever seen such lousy troubleshooting!