One of my inspectors saw this and ask me about it. I said I never saw this before and said I ask the forum. The question is what are the two flexible pipes in the return ducts for as they go all the way to the back of the house through the attic and come out into the patio area. The flex pipe have shut off dampers in them and the units themselves are in a three car garage. Any ideas would be helpful. The seller did not have a clue and the units are 10 years old.
I would guess fresh air intake vents, pretty normal for age of building. Being partially in the attic I would hope they are sealed well at connections.
I am sure Charley. Dave A and others will be around to explain, I may be wrong, just trying to help out.
Why is it in the return and not above the units.
They are in the return side so it can be conditioned before entering the interior of the home. Hot or cold. Also if the vent was on the supply side it would be blowing out instead of drawing in
Why would you bring in unconditioned outside air and pipe it through the attic and into the return. It doesn’t make sense.
He is saying they might be trying to introduce fresh outside air into the home. NOT fresh air for combustion. Is that the hang up? Hope that helps!
The reasoning is to bring in fresh air, help balance the air in the system, instead of recirculating the same air over and over.
https://www.smud.org/en/residential/environment/solarsmart-homes/images/air-sealing-02.jpg
Ventilation for Indoor Air Quality](http://brightgreenstrategies.com/158/)
Rare around here but I have seen that as well.
Agree fresh air aux.
Get used to it. It’s coming to the Code Department near you! 
The “shut off” is a balancing damper.
This is not just for fresh air.
There are a dozen uses for this application.
Radon is one.
Adding another “sucking” device to a sucking house with a Radon problem, just plain “Sucks”!
A poor man HRV comes to mind.
Come to think of it, the positive pressure created in the home by such a system especially if the furnace circulation fan is left on 24/7 should reduce Radon level…
Considering that HVAC leakage is the primary cause of building depressurization, which in turn is the cause for Radon to enter the house…
Adjusting the HVAC system to prevent the air infiltration is the fix to many problems.
Most likely a radon reduction system or if people want fresh air introduced to the house.
I stopped asking, why do people do what they do, a long time ago.
If it’s for radon, they need a means to turn it on and off and a device present to show that it is working as intended. That is according to the Florida DOH
Fresh air intake. Very common in commercial but some residential have it. Tell the buyer to keep it closed at all times. I do not recommend it at all. It will make the unit work harder to cool (95 degrees outside air being drawn in) and with the humid Florida air you should see how quickly it will corrode the coils
We used to recommend that back when CMHC Canadian Mortgage Housing Corporation was making it look like a viable solution to IAQ problems. Turns out it was not. After some years it is now even questioned that a normal HRV should be used for Air quality issues that has next to nothing for filtering. This has been a problem since many are installed and then forgot about. However introducing air for bringing down CO2, CO, Particulate and of course Radon is a viable thing to do with a HRV or ERV.
Thanks guys.
Not to mention excess humidity, if properly balanced of course.
Some information on hard-connected duct is attached.
Fresh air is needed if the house construction is tight. There needs to be a minimum number of air changes, ASHRAE suggests 0.3 air changes per hour in a house, more is better, but will cost more too in heating season. Inside air is often more ‘polluted’ than outside air, higher humidity, CO2 from occupants, cooking fumes, off gassing fake wood products, various chemicals used for cleaning, grooming, radon, whatever, if air changes are prevented by tight construction such as 6 mil taped poly vapor barriers, well sealed windows and doors etc.
So fresh air must be brought in. You can open a window, but who is going to do that when it is minus 20? Its a YMMV thing, in milder climates, windows are open, there is plenty of fresh air leaking in to older homes.
I personally would not want to draw my fresh air out of the attic, who knows what is going on up there? There are plumbing vents, chimneys or B vents, leakage of inside air through the ceiling, dust and more dust, I would want my fresh air to come from outside, some place far away from any kind of exhaust, including vehicles.
Bringing fresh air in through the return air is a great solution if you have a forced air furnace, as the air is conditioned, including filtered by the furnace filter.