What does that setting actually do?

On AUTO the blower runs only while the system is heating or cooling. On ON it runs continuously. People set it there for even temperatures or for filtration, and both of those are real — but in a humid climate there is a cost that is never mentioned.

• In cooling, it makes the house more humid. When the compressor stops, the coil is still wet. A blower that keeps running blows that moisture straight back into the house instead of letting it drain away.
• It also runs the blower motor a great deal more hours than it otherwise would.
• If you want even temperatures, the honest fixes are balancing and duct work, not the fan switch.
• If you want filtration, a variable-speed system running a low continuous speed is a different thing from an old blower stuck on full.

A house that is allowed to drift towards outdoor conditions while nobody is in it loses less heat, or gains less, than one held at the same temperature all day. The saving is real and it is one of the few free ones.

• The saving grows with the size of the setback and the length of it. A two-hour setback is not worth the bother; an eight-hour one is.
• In cooling, a deep setback in this climate lets the house take on moisture, and the system then has to pull it back out. Moderate is better than extreme in summer here.
• It does not make the system work harder in any damaging sense; a system recovering is a system running normally.
• There is one important exception on a heat pump, below.

It does not work. A thermostat is a switch, not a throttle: the system runs at one capacity until the setpoint is reached, and setting 65 instead of 74 does not make it produce cold any faster.

• What it does do is run past the temperature you actually wanted, so the house ends up colder than you meant and you paid for the extra.
• On a system with staging or a variable-speed compressor it can also force a higher stage than the house needs, which is less efficient.
• If the house genuinely takes hours to pull down, that is a sizing, airflow or charge question and worth measuring.
• The same applies in reverse in winter.

On a heat pump, emergency heat switches the heat pump off entirely and heats using the electric resistance elements alone. It exists for when the heat pump has failed, and it is the most expensive way the system can possibly run.

• It is not a “heat faster” button and it should not be used as one.
• If you are using it because the house is not warm enough, the correct step is to find out why the heat pump is not keeping up.
• Left on for a season it produces a bill people remember for years.
• A system that needs it genuinely is a system that needs a visit.

Smart thermostats are good at scheduling and good at reporting. What they are not is a fix for a system that is wrongly sized, badly ducted or incorrectly configured, and they are sometimes fitted without checking what the equipment underneath expects.

• The commonest problem we see is a thermostat that does not have the right wiring or configuration for the equipment behind it — which shows up as staging that does not work properly.
• Learning schedules work well in a household with a regular pattern and badly in one without.
• The reporting is genuinely useful: run times tell you more about a system than almost anything else a homeowner can see.
• If yours was fitted by somebody who did not check the equipment settings, that is worth an hour of somebody’s time.

It feels like it should work, in the way that closing a door on an empty room feels like it should. It does not, and on a ducted system it makes things worse.

• The blower still moves the same air. Closing registers raises the pressure it is working against, which reduces total airflow and makes the whole system less effective.
• Higher duct pressure increases leakage from every joint in the system, including the ones in your attic.
• The closed room still loses heat to the rooms around it, so you have not really removed the load.
• Rooms that genuinely need different temperatures need zoning or a separate system, which is a real answer rather than a free one.