Forester Aircon question

[Tweaksta]

Here's one for you all!

In my SF GT I have the basic rotary dial to control my HVAC temperature.

With the aircon off, I drive with the dial at 12 O'clock in Winter and fully cold (ie: 8 O'clock) position in Summer.

With the aircon on, I start with it fully cold then gradually move the dial towards 11 o'clock and sometimes 12 O'clock as the core gets colder.

My question is: Does the Subaru HVAC system have the smarts to 'block' hot air from mixing with the cold air from the aircon? Is it mixing hot and cold to get the desired temperature? Or does a solenoid close off the hot air entry when the aircon is on, meaning that the Subaru aircon system is smart enough to use compressor duty cycle to regulate coldness?

 

[Kevin]

Good question! I'm talking thru my hat but at a guess it does both. With my climate control the a/c compressor cycles on/off so that is one temp regulator, but I've also noticed that when I turn off the a/c function (but leave fan running) I hear the sound of solinoids moving flaps about, so maybe it is closing off the hot air mix.

 

[simxs]

Does the Subaru HVAC system have the smarts to 'block' hot air from mixing with the cold air from the aircon?

The airflow goes like this:

1. in through the intake, either from outside or inside the car depending on 'recirc' setting
2. through the a/c evaporator (the thing that makes the air colder if the a/c is switched on)
3. through the heater matrix and heater by-pass each in varying degrees, directed by a baffle, the position of which is controlled by the temp control dial (manual HVAC) or an ECU (climate control)
4. to the outlets, directed by a baffle, the position of which is controlled by the mode (face, feet, screen) dial

(I'm not 100% sure where the fan sits in this, but it's probably between 1 and 2.)

Basically, the air is cooled by the A/C evaporator (if the A/C is on) and then some proportion if it is optionally heated up again, depending on the temperature you select.

Is it mixing hot and cold to get the desired temperature?

As above, yes.

Or does a solenoid close off the hot air entry when the aircon is on, meaning that the Subaru aircon system is smart enough to use compressor duty cycle to regulate coldness?

No. If my interpretation of the specs in the service manual is correct, if the A/C is switched on the compressor simply turns on once the evaporator output air reaches 3 degrees C, then turns off once it has dropped to 1.5 degrees C. It is not dependant on what temperature the driver selects.

I've also noticed that when I turn off the a/c function (but leave fan running) I hear the sound of solinoids moving flaps about, so maybe it is closing off the hot air mix.

I've noticed this with the climate control too. I'm pretty sure that when you turn off the A/C it is reducing the proportion of air going through the heater matrix so the output air stays vaguely the same temp as before even though the evaporator won't be cooling the air any more.

Climate control (as opposed to manual HVAC) is pretty complex and probably right up there with systems like ignition timing and fuel injection in terms of the number of inputs and control algorithms required to make it work. (On my car at least, it is far from perfect by the way - certainly not 'set-and-forget' as it is touted to be.)

 

[simxs]

the compressor simply turns on once the evaporator output air reaches 3 degrees C, then turns off once it has dropped to 1.5 degrees C.

Looking at the spec again, there is actually more to it than this as there are pressure switches within the system, suggesting that the compressor operation also depends on the refrigerant pressure within the A/C system, which would make sense. Either way though, compressor duty is not dependent on the cabin temp dial - this simply controls the proportion of air going though the heater matrix.

 

[Tweaksta]

Thanks Simon, well answered buddy.

Essentially where this is leading to is fuel economy. When we get into a car on a hot day we like to 'blast' the car with the coldest setting possible until the cabin is comfortable. Due to the fact that I am a skinny bugger I then like to reduce either (a) the fan speed, or (b) the temperature setting.

Since the fan only consumes a couple of amps I would guess the load on the alternator is negligible so turning it down to a lower fan speed once cold really doesn't affect much.

A possible way to reduce fuel consumption with the aircon on is to intercept the compressor wiring and pulse it with your own controller - such as https://autospeed.com.au/cms/title_Digital-Pulse-Adjuster-Part-1/A_2471/article.html perhaps?

Theoretically this should work.....thoughts?

 

[simxs]

Essentially where this is leading to is fuel economy.

Fair enough too.

I remember an early eighties Volvo my grandparents used to have which had a dial to control the A/C rather than just an on/off switch. Max it out if the car is really hot or turn it right down if you only want a bit of cooling. Some Japanese cars in the eighties and nineties had an 'eco' setting for the A/C which is along the same lines but this seems to have fallen from favour. Maybe people were confused by having to many options. Still, would be nice to control the A/C directly rather than always have it work flat out, only to then have to heat the air up again because it's too cold for comfort.

Interesting device that pulse adjuster. Do you want to put it on the solenoid that controls the A/C clutch?

 

[Rally]

Not wishing to be negative, but how much do you expect to gain from this, if fuel economy is the driving factor? Since fuel economy is a concern, why not perform a test. Fill the car with fuel and drive to the places as you normally would, and have the A/C on all the time. Re-fuel and do the same test as near as possible to the original, with the A/C off the whole way, and compare the difference. Of course, this only gives a total difference, as with the system cycling the A/C will still come on, but for less time. So start with dividing the difference in 2 to be more realistic. Then find out how much the savings are and you will have a better idea if it is worth it.