when i dream of my new system (what, you don't dream of AC systems?) it has a way to switch over, 100%, to outside air if needed. So... like i said earlier, i feel like I'm constantly running the AC (perhaps at 75) when the outside temp is 65 (or even 55). Seems like a system would be very efficient, financially and "green-ly", if there was a way to have it simply use the fan(s) to pull the stale/warm air out and replace it with clean/cool air from the outside. And then, if the outside is not cold enough (which would be often) i would kick in the AC and do a "50/50" type situation.
Weelll...... There's a problem with that approach. HVAC systems don't just deal with temperature and fresh air: another very big part of HVAC is controlling humidity. Your studio should have a fairly constant relative humidity of around 40%. Your instruments (especially ones that use wood or other natural fibers in their construction) will swell and shrink as the humidity in your room changes, and both the tone and tuning will therefore change... Probably not what you want when you are trying to repeat takes from yesterday, when it was sunny and dry out, but today it's raining and foggy! Some gear and some mics also like to have constant humidity...
Fortunately, HVAC systems take care of that. There's two types of heat that air conditioners get rid of: latent heat, and sensible heat. "Sensible heat" is the type of heat that you are talking about when say the air is hot or cold. Latent heat is the type of heat you are talking about when you say the air is "muggy" or "clammy" or "dry". When the warm, moist air in your room moves through the coils of the HVAC unit, the first thing that happens is that excess moisture in the air condenses on those cold coils. The simple act of that water changing phase from gas to liquid, means that heat was released. That heat is carried away by the coolant in the system. So the system removes a lot of
heat from the air... but the
temperature of the air did not change yet! The only heat that was removed at this point, was "latent" heat: the heat that was stored in the gaseous state of the water in the air, but then released as the water changed phase to liquid. So heat was removed, but the temperature didn't change. Then, now that the air that has been relieved of its excess water load, as it continues to move over the rest of the coils in the HVAC unit, it can cool down: That's the "sensible heat" part of the equation.
OK, so what does all that mean? Firstly, if you forget to take BOTH of those heat loads into account when you dimension your HVAC capacity, you could end up with a system that is only able to remove the humidity (latent heat), but doesn't have enough "power" to also remove the sensible heat. So the air comes out drier, but still warm. And if you then pump in warm air from the outdoors, the room will end up getting hotter, not colder, even though the HVAC system is running at full capacity, maximum speed, and 100% duty cycle. It will wear out real fast like that, since it is overloaded, and even so still can't handle the job.
"So!" you say to yourself "I'll just get one that is bigger than I need! Has extra capacity, beyond what my room needs!". That's just as bad. If the unit has too much capacity, it will dry out the air too much, cool it too fast, then go into standby with just the fan running but no compressor... and stay like that for a long time, until the air gets moist and warm again. In other words, the duty cycle will be very short (just a small percentage of the time actually doing it's thing, then the rest of the time just moving air). Which means that the temperature and humidity in the room will swing wildly from extreme to extreme, all the time! A sudden blast of icy, dry air that yanks the temperature and humidity way down, then a slow build up again to high humidity and high temperature... rinse ... repeat....
The point being? Don't try to do stuff that won't work! Yes, it's nice to think you are saving power, and saving money... but in reality you are not. Instead, follow the guidelines to correct size your HVAC system: It needs to have the capacity to deal with the highest latent heat and sensible heat load that your studio will ever conceivably have, on the hottest, dampest day in summer with a dozen musicians jamming breathlessly, working hard and sweating hard and breathing hard (which greatly increases both the temperature and humidity in the room....), plsu all the gear turned on and running at max levels, plus beer and pizza (more heat, more humidity)... and the HVAC system needs to be able to deal with that, without overloading or running beyond about 90% duty cycle... but it also needs to be able to deal with the situation where it is just you in there, sitting quietly, listening, with minimum gear and lights on, on a cool dry day in winter, late at night. The system has to be designed to deal with both extremes: enough capacity to handle that highest load and still keep everyone cool with the humidity under control, while also not freezing your butt off at the other extreme.
So, the VENTILATION part of the HVAC system needs to be adjustable (preferably automatically, controlled by sensors and a system controller) such that it always dumps the right mount of stale air and also provides the corresponding amount of make-up fresh air from the outside world, as needed for each situation. You could do it yourself, manually, sure, but you'd soon get bored rushing around checking the temperature, humidity and CO2 levels in each room, then adjsuting the dampers, fan speeds, and air flow rates... then adjsuting it again when the pizza arrives, or when Joe fires up his new 10,000 watt bass amp, or three musicians suddenly take a break in the green room, or a couple of WAGs arrive unexpectedly and want to sit in on the session in the control room... I think you get the picture! That's not something that you want to be trying to handle manually. Just install a system controller, sensors in each room (and outdoors), and let the controller do its job.
So, that's my rant for the day on why your plan might sound good on the surface, but isn't practical, and wouldn't work anyway... Sorry!
I've found some very inexpensive inline ducts that have a motorized louvers, and (in my mind, at least) this would be easy to set up in two places, to allow for this sort of a design.
Probably not! See above...
As a point of reference, when I design a studio from scratch, I often end up spending as much time on the HVAC system as I do on the entire rest of the studio! Lot's of things to take into account, and juggle... It's big. Way bigger than most people realize.
Example: right now, as i type this, the AC is set for 75, and in 3-4 hrs it will be close to 60 outside.. and then i should be able to switch over to 100% fresh air to cool the room... that's my goal.
If you had a proper system, designed and programmed correctly, you would leave the temperature set at 70 ALWAYS; and allow the system to do it's thing: It will decide when it can use more outside air... but it will consider the humidity of that outside air, and decide if it would waste more power in removing that humidity than it can save by not needing to cool the air as well... If you bring in humid air (you live in LA! It's ALWAYS humid!) then that needs to be dehumidified (latent heat removed), which means the compressor has to run to do that, even if the air doesn't need cooling! It still has to to the work of cooling the coolant, pumping it through the pipes to the indoor unit, so the cool humid air can flow over it to have its excess humidity removed, but then NOT be cooled additionally... all of that takes electrical power to accomplish: it would probably have been more efficient go just circulate the warm-but-already-dry air that is in the room, and cool it a bit WITHOUT needing to first remove the latent heat, because it is already at the right humidity.... If the humidity is high, it takes a lot of power to remove that latent heat, but the slight extra sensible heat from cooling recirculated room air isn't such a big deal, probably....
Just follow the usual guidelines. With a good HRV (or ERV) and a good AHU / Heat Pump combination, with sensors and controller, it will be very efficient, and you won't have to worry about it. It will be a "set and forget" system. If you use one of the ultra-efficient "inverter" HVAC systems, then it's going to be very, very efficient. I guarantee that you would be totally unable to make it more efficient by trying to manually adjust the make-up air ratio... and very likely you would make it far less efficient, as the system would have to fight against your decisions all the time, to fix what you messed up....
more to come!
Yup!
Edited to add... :
feel like I'm constantly running the AC (perhaps at 75) when the outside temp is 65 (or even 55).
Then your system is likely either not designed correctly (inadequate capacity for dealing with high latent heat), or IS designed correctly and is doing its job perfectly: removing latent heat but NOT cooling air that doesn't need cooling.
If the compressor is running at a high duty cycle (eg, more than 70% "on"), then likely it is a design issue: wrong system for that job. If it is running around 30% to 60% duty cycle, it's probably doing just what it is supposed to do.
- Stuart -
) - being the pro he is!
" 
That is, indeed,the way to do it! 
) to me that would indicate two things: insufficient fresh air intake / stale air exhaust, and lack of humidity control. It might also indicate that your HVAC system is too small for the load you are placing on it (not enough BTU/hr capacity). Plus, it also indicates that it desperately needs a thorough cleaning and maintenance visit from a qualified HVAC technician, who will not only change the filters and check the coolant pressures, but will also clean out all the grunge from the fans, cooling cools, and registers, then spray the entire air path with anti-bacterial mist...