re: How do some think an HVAC thermostat controls the temperature of the air blowing out?

It’s ok … I’m just being an arse

Good night

I can’t help you understand thermodynamics other than this. The next time you have your thermostat set and its not cooling to your desired temp and your system is running on cool bury the thermostat to the lowest temperature setting and let me know how it works out for you.

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You are correct on a DX residential unit T-stat. But a commercial Terminal zone Unit can increase the CFM coming out of the box based on how far off set point the stat is reading, effectively cooling off the space quicker if you turn it way down.

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This fricking site

Sir this is a Wendy’s

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Vehicles ACs do basically work that way and pretty much everyone has operated a vehicle AC. So I give them a pass for not realizing that most residential units don’t really work like that.

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Sort of, the car is still producing the same cool or heat temp no matter what the setting once it has been running long enough.. Difference is that there auto systems blend in air to slow or increase the rate of change.

But it's not like if you set it to 78 that it will blow out 78 degree air. It doesn't work that way

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Idunno. My experience is as the ambient temperature of air going into the return air vent falls from say 80° to 68° the air coming out of the vent goes down a few degrees.

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Standard home ac units that are working should have a delta temperature from return to closest register of 15-20 degrees.

If it is lower than you probably have an issue

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The old Honeywell T-Stats that had a glass tube filled with mercury on a spring. Once the spring moved enough to move the mercury to the contacts the unit came “on”. Same for a modern t-stat.

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They had those on Air Force barracks. I can’t remember which was which now, but when the mercury was making contact either the air or heat would run continuously. With no contact the other would run.


Once we figured that out, we’d either wrap the mercury thingy with foil for continuous on, or pull it out for continuous on. We had the coldest barracks in the summer and the warmest in the winter.

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Well the thermostat has a temperature gauge built in.

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Irrelevant to the question asked

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Why do people say “turn the heat up” to make it warmer and “turn the AC up” to make it cooler?

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That is actually correct. You are confusing “turn the AC/heat up” with “turn the thermostat up”.

The latter is incorrect.

My house has a zone thermostat system. You need an engineering degree to understand it. I don't mess with it.