I noticed the other day that my 2010 Forester is only putting out heat on the driver's side, and cold air on the passenger side.  It doesn't matter what setting I use for the heater, it only blows cold air out the floor, defroster, or dash on the passenger side.  The driver's side seems to work fine though.  I'm guessing maybe it's a broken vacuum line under the dash somewhere, assuming vacuum is what controls the heater settings.  Any ideas where I should start looking?  

the actuators are electric. you might dig around at www.opposedforces.com for some helpful drawings.

after google, the world's most popular search engine is Youtube, might get lucky enough to find a helpful video.

 

It's only single zone climate correct? Meaning, the driver and passenger cannot have different temperature settings?

If so, it can't be a actuator as you have heat on the drivers side, just not on the passenger side. The actuator is working if you have heat on the drivers side.

2 possible issues. There is air in the heater core, or the heater core is restricted. 

Yes, it's single zone climate.

By air in the heater core I'm guessing you mean air in the cooling system, and I need to bleed it out?  Even if there was air in the heater core, why would it just blow heat on one side?  It seems like the whole system would be cold.  I'm not being a jerk, I'm just trying to understand.  Thanks!

Yes, air in the cooling system. The heater core is typically the highest point or very closely to it (stupidly enough) on most subarus, and therefore any air in the system collects in the heater core.

In my experience, typically a lack of heat only on the passenger size indicates a restricted core, but an air pocket in just the right side of the heater core could have the same symptoms. 

 

To see if there's air in the system, warm the car up. Once it's warm, while stopped in park, floor the gas pedal so the RPMs rapidly go up to about 3.5k or so. Then let it come back to an idle. Repeat until heat improves.  This will indicate there's air in the system.

To tell if it's a restricted core, just hold the RPMs up to like 3.5k steady in park. See if heat returns on the passenger side.

If I have a restricted core, do I need to flush it out to get it working again?

Another thought:  I just had the timing belt and water pump replaced.  I'm thinking maybe my mechanic didn't get all the air bled out of the system?  Sounds like maybe I should start there?

Thanks for the info guys!

Well I tried everything mentioned above and I still only have heat on one side.  I guessing that flushing out the heater core is my next step?

How long did you blip the gas up to 4k rpms? sometimes it can take like 3 or more minutes of doing that to get the air out.

 

But if it is a restricted core, typically you need to replace it. You can try flushing

 

Ahhh......  I held the throttle steady, I didn't blip.  I'll try that again.  Would it help to bleed the air out if I left the radiator cap off?

 

I tried the throttle blip for a few minutes and still nothing.  I'll try flushing next.

I'm guessing you have to about tear the whole dash out in order to get the heater core out?

My mechanic used compressed air to blow out by clogged heater core and heat was restored for me.  Not sure if that would work on your exact model, but it did for my 02 outback 2.5

I have exactly the same problem, cold air in the passenger side, there must be a specific answer to this...just noticed it today, we’ll see.