I have some questions about the fuel efficiency of various A/C options.

1. Does it make a significant difference to fuel efficiency at what speed the fan is running?
2. Is fuel efficiency significantly affected by turning on or off "recirculate"?
3. What does "Max A/C" do? I though it just turned on recirculate, but since I've seen people say not to use it, I'm not sure.
4. Why is that "Max" A/C is not available with ECO mode on the TCH?

The basic theme here is that I thought that simply whether or not the A/C compressor is on far outweighs everything else - fan speed, recirculate, max mode. But given #4, I'm not sure any more.

1. Fan speed has no effect on fuel economy. At higher fan speeds the alternator may have to work harder, but I doubt there would be any noticable hit in FE.
2. Recirulate has NO effect, all it does is move a door so you get air from inside the vehicle instead of outside.
3. Max is the same as recirculate on most vehicles, but on Some hybrids(My Ford Escape) max will keep the Internal combustionn engine running at all times to keep the A/C going non-stop.
4. Not sure about the Toyota, but if it's like the Ford, see no. 3.

Hope this helps, Bud.

The actual power draw from the fan is not going to effect much, however, a higher fan speed will transfer cold air into the cabin (if the AC is running) more rapidly, and thus require the compressor to be engaged longer (in exchange for more cooling).

Not directly. Though when running AC, it is wise to use recirc, as it can keep the cold air inside the car longer, and actually cool the already cool cabin air further. It doesn't make sense to keep reintroducing hot air from outside. However, if AC is off, the reverse is true, the window glass will cause solar heating, so you'd end up recirculating hotter air, rather than allowing in cooler air from outside to replace it.

I believe it keeps the engine running more in order to run the bigger, belt-driven compressor, rather than switching to the elecrtic compressor. This is the opposite of ECO mode (engine off as much as possible)

I have the TCH. It doesn't have a MAX A/C mode, just a RECIRC button. ECO and RECIRC are not mutually exclusive. In fact I keep my A/C in ECO mode all the two and the system defaults to RECIRC.

The only time ECO shuts off on its own is when the system is in AUTO mode and you set the temperature to HI or LO rather than a numerical value (say 74 degrees).

AC in the Highlander is run off the electric motor, not the ICE - I suspect the same would be true in the TCH. This means that the ICE can shut off without shutting off the AC.

Yes, in the TCH the ac is run off the electric motor.

What about the rise in humidity when in recirculate mode? If the car is moving it could presumably cool the interior using a radiator like the engine itself is cooled. If you're in recirculate mode though the humidity will rise and you need to activate the compressor.

My '02 Civic has a humidity sensor in the cabin so I've always assumed it could switch the compressor on/off based on that as well as the cooling.

Edit:I should add that my '02 Civic has climate control so presumably it can be more clever in where it gets cold air for the cabin. If it cool it enough without using the compressor perhaps it will do so?

The above posts are right on the money. The TCH, like the Prius, does not have a belt driven AC. It is a directly driven electric compressor. Using the AC will affect your city fuel efficiency by drawing power from the battery pack, often very quickly, on a hot summer day. Of course, this means the ICE must run more often to charge the battery pack. I have noticed about 10% FE reductions here in Dallas over the recent 3 weeks of 100+ temps.

Alden Bowles
2005 Toyota Prius II HSD

Thanks everyone. Orcrone's post leads me to questions about ECO mode but I will take them to the TCH forum (if I don't find the answers by searching first)...