Heating/AC Question

Myth Busters did one episode testing this theory.

They tried windows rolled down, they tried using the vent fan, they tried with A/C.

In the end, it makes no difference at all.

 

That's about right. Also, set the fan to your choice of speed (or OFF).

 

That's about right. Also, set the fan to your choice of speed (or OFF).

 

I generally use AUTO year round. In winter, the AC will dehumidify the air helping keep you windows clear of moisture; I usually set the temp to 74. In summer, I set the temp to 72 which I find comfortable. If it's really hot or really cold out, I set the temp high or low to get quicker results.

 

The advantage of AUTO is that you don't have to think about it. Heat or A/C will depend on what the car thinks is needed based on internal temperature and the current temperature setting. Fan speed on AUTO runs at what is expected as needed to get to the desired temperature. Faster for wide variance...slower for narrow variance.

I have a remote start on my Prius, so I can "start" the car properly...not that remote A/C nonsense that came with higher trim levels. This lets me heat or cool the car for 12 minutes from a distance (remote A/C only cools based on available battery power). My biggest problem is that I have to set the system to defrost the windshield if I want it to get the primary benefit of running during winter. However, my experience is that regular A/C warming the car for 12 minutes goes a long way to defrost all the windows, so it's all good.

The only reason I manually set fan speed for defrosting is if something is causing a fogging issue. Once the Prius reaches the set temperature, the fan blows slow, and it might not be enough circulation if you have several people in the car. Selecting windshield defrost bumps up the fan speed automatically.

 

I'm not sure why you think those are the only two alternatives.

There's a separate A/C button on your dash (down by the lower right of the display). With the A/C turned on by that button, auto mode will use the A/C, or not, as conditions demand. With A/C turned off by that button, you can still use auto mode, it will just auto-control the fan speeds and vents and heat, and not use the A/C in normal conditions.

(Note that when you select a defrost mode, the system can still add A/C for you even if you haven't asked for it, but in the normal modes it won't run the A/C unless you have it enabled by the A/C button.)

-Chap

 

i use manual year round. gives much more precise control, but requires some attention.

 

My grand life challenge has always been to identify the things I don't need to precisely control....

-Chap

 

I use manual, too. One think I don't like, when switching between AC ON/OFF, I need to adjust temperature. In OFF I set coldest temperature (LO), when go AC ON, I set 74-76.

 

i leave the temp on low and fiddle with the fan setting.

 

..but then when you turn AC ON, it runs extra hard, right?

Off topic: bisco, how many miles you get on ev (average) and what speed when ICE kick in on freeway?

 

no, that's if you hit auto. i don't use auto. when i hit the a/c button, the fan speed doesn't change.

i'm getting about 15 miles around town. i try not to use ev on the freeway, unless i know i have enough, but the engine kicks on somewhere between 60 and 65mph. the speedo isn't that accurate. the book says 62, but the speedo is usually 63-64.

 

VovCA is correct, at least by the (admittedly brief) observations I've made on my car with a ScanGauge. The compressor power is determined (at least to a large extent) by the difference between the set temperature and the cabin temperature. If you set the temperature to LO, then it runs the compressor at very high power (as observed through the ScanGauge, using the A/C Watts gauge), even if the fan is on a low speed setting. So I'd recommend adjusting the temperature to get the fan speed you want.

 

What you may not know, and I don't believe has been said so far, is that in the Prius the A/C compressor is electric and not "driven" by a belt on the gas motor (ICE) as in normal cars. Thus, there is not the economy "hit" that you seem to be concerned about. What you will notice in cold weather is that the ICE will need to run to warm the car (engine warms the coolant, which in turn warms cabin air), so your fuel economy will still go down in winter, but it is due to the need to warm the cabin.

 

Well, it's not driven by a belt on the engine, but the power isn't coming from nowhere. It is electrically driven, using power from the HV system, charged by MG1, driven by the engine. What is nice about the Prius compressor is that it's variable speed, so it can be run at a wide range of power levels, from about 1/3 HP to around 3 HP, depending on how much cooling is needed, and independent of the engine speed. That is an improvement over the belt drive. Still, that 1/3 to 3 HP is being supplied by burned gasoline.

(Now, the HV system can also recover some power from MG2 when braking or on a downhill; on a long downhill you could think of getting your A/C for free ... on a conventional car too, with the engine and belt-drive compressor being driven by the transmission. But in the bigger picture even then, regen braking or a downhill is only applying energy that came from burning gasoline to reach the speed or elevation that you're braking or coasting downhill from.)

Regarding the compressor power and fan speed, if you are watching the compressor power when the fan is not on auto, you should find that the compressor does not reach its highest power levels with the fan speed reduced, even if the temperature error is large. At low fan speeds, the compressor power has to be limited to match the rate of warm air being blown over the evaporator ... otherwise the evap coil would grow too cold and be blocked eventually by frozen condensate. So, to some extent, you can limit the A/C compressor power consumption by keeping the fan speed down.

-Chap

 

my apologies to vovca, i thought he meant the fan speed. yes, in manual with the temp set to low, the compressor runs at top speed. the fan runs at whatever speed you set it to.
that's why i like it. i get instant cold, and i don't get the stupid fan funning all the way up to high.
maybe it's less efficient, but i prefer cold air and low/quiet fan, it's very effective at cooling.

 

Just to reiterate, if you watch the compressor power consumption on a ScanGauge or the like, you will see that even with the temp set to low, the compressor will not hit its highest power levels if the fan is at a low setting. The compressor will run as hard as it can given the limited airflow from low fan speed, but that won't be full power, unless the fan setting is also high enough to keep up.

-Chap