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Discussion Starter · #1 ·
Does turning off the rear HVAC actually disable any major power-sucking components, or does it just stop airflow (maybe turning off some kind of fan)?

Over the last month as temps got more spring like, I've been keeping the back HVAC turned off and I think my tank-tank mileage has gotten somewhat better. My driving patterns are pretty random, and its hard for me to really tell if whether I've just been doing more highway miles and less short-haul, or whether disabling rear HVAC has actually done something.

I'm not talking miracle improvements, like maybe 1-2 mpg per tank, but noticeable.
 

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Does turning off the rear HVAC actually disable any major power-sucking components, or does it just stop airflow (maybe turning off some kind of fan)?

Over the last month as temps got more spring like, I've been keeping the back HVAC turned off and I think my tank-tank mileage has gotten somewhat better. My driving patterns are pretty random, and its hard for me to really tell if whether I've just been doing more highway miles and less short-haul, or whether disabling rear HVAC has actually done something.

I'm not talking miracle improvements, like maybe 1-2 mpg per tank, but noticeable.
There is a separate rear compressor, so I think you are correct. I turn off the rear climate control when nobody is back there. It's going to be hard to see the MPG difference if your tank-to-tank average fluctuates even a little bit, but I suppose we can feel fairly confident that shutting off the rear does save some fuel, however small the amount is.
 

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I live in Houston where it's always hot and humid. AC is a must. So run the front and rear AC constantly and over the last 4,000 miles (trip B) I am averaging 25.8mpg. So it might affect it but I don't think it does make much of a difference. AC compressors are much more efficient these days.
 

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Discussion Starter · #4 ·
There is a separate rear compressor, so I think you are correct. I turn off the rear climate control when nobody is back there. It's going to be hard to see the MPG difference if your tank-to-tank average fluctuates even a little bit, but I suppose we can feel fairly confident that shutting off the rear does save some fuel, however small the amount is.
The parts manual only seems to indicate one belt-driven compressor for the whole car, with separate piping to the rear.

So at best turning off the rear climate control mostly means less electrical load (no blower) and some reduced cycling of the belt-driven compressor. I mean if we're generous, about maybe 2 HP reduced engine load.
 

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The parts manual only seems to indicate one belt-driven compressor for the whole car, with separate piping to the rear.

So at best turning off the rear climate control mostly means less electrical load (no blower) and some reduced cycling of the belt-driven compressor. I mean if we're generous, about maybe 2 HP reduced engine load.
Thanks for looking that up. I assumed that since there is a condensate drain both front and rear that there must be two compressors.
 

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I'm wondering if it's a dual clutch compressor with two separate "channels"? There's enough piping for that, I think.
 

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I live in Houston where it's always hot and humid. AC is a must. So run the front and rear AC constantly and over the last 4,000 miles (trip B) I am averaging 25.8mpg. So it might affect it but I don't think it does make much of a difference. AC compressors are much more efficient these days.
Based on fuelly.com, your car is a unicorn. I’m not sure how you do it but my average mpg been stuck at 20.8

http://www.fuelly.com/car/subaru/ascent
 

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It's ridiculously easy but boring. I'm at about 20 overall, but by choice. I've run tanks at 22-24mpg even with our horrible traffic.

I live in Houston where it's always hot and humid. AC is a must. So run the front and rear AC constantly and over the last 4,000 miles (trip B) I am averaging 25.8mpg. So it might affect it but I don't think it does make much of a difference. AC compressors are much more efficient these days.
Based on fuelly.com, your car is a unicorn. I’m not sure how you do it but my average mpg been stuck at 20.8

http://www.fuelly.com/car/subaru/ascent
 

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Based on fuelly.com, your car is a unicorn. I’m not sure how you do it but my average mpg been stuck at 20.8

Subaru Ascent MPG - Actual MPG from 132 Subaru Ascent owners
I tried using fuelly and gave up because I always forget about it. I am glad you posted this because it is very much on par with what I have been seeing and have had moments were I thought I was the only one. The Ascent is a great highway cruiser, but in city traffic with constant stop and goes it takes a toll on MPG when the Turbo is constantly spooling up.
 

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I tried using fuelly and gave up because I always forget about it. I am glad you posted this because it is very much on par with what I have been seeing and have had moments were I thought I was the only one. The Ascent is a great highway cruiser, but in city traffic with constant stop and goes it takes a toll on MPG when the Turbo is constantly spooling up.
Yup. My commutes are primarily traveling on highways but the traffics in morning and afternoon reflect more like the "city mpg".
 

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Thanks for looking that up. I assumed that since there is a condensate drain both front and rear that there must be two compressors.
Actually, that's what we were originally told, since there was a second drain in the rear.

However, what it does is route piping to the rear. Just as in the front, the piping will tend to sweat, and they route the drain for that to a single spot. There is a separate heater for the rear, as previously mentioned.
 
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