It looks like someones idea of combustion air for your gas appliances all though I have never seen anyone make one out of flex before and it looks a little goofy this should be made out of steel ductwork and piped straight and square to the right area call your mechanical inspector he will tell you how he wants it run

Here is a link that might be useful: Air Conditioning and Heating Repair Made Easy

That's exactly what it is. It doesn't have to be connected to anything, but if you wanted to have outside air, there might be a kit that hooks directly to your burner.

The combustion air duct is required by code and simply provides outside air for combustion for gravity type furnaces/boilers. Sealed combustion units bring in their own supply of combustion air and do not need additonal duct. In a gravity type furnace/boiler, once the air and fuel combust, the spent gases go up the flue and that air has to be replaced. Some installers connect the combustion air duct to the return duct on the furnace which is under a slight negative pressure, drawing the air in and creating a slight positive pressure in the house and reducing infiltration. The fresh air is directed to occupant comfort instead of directly to the burner and out the flue. Connecting to the return eliminates the cold air pouring onto the floor and creating discomfort. You are not allowed to install dampers of any kind in a combustion air duct. The combustion air duct should not be connected any closer than 10 feet (duct length) from the furnace due to extreamly cold air in the winter hitting the heat exchanger which has resulted in cracking heat exchangers. 10 feet should be adequate time for mixing of return air and outside air. The outside air/combustion air duct needs to be insulated to eliminate frost in the winter and condensation in the summer. As always, check with your local inspections department to verify if this is acceptable.

Hope that all helps,

Dan Martyn

We call it a fresh air intake. You don't need it stretched like that. Let it drop straight down and set the end inside a five gallon bucket. Keep the end about 3" to 6" from the bottom of the bucket. That prevents too much cold air spilling onto the floor but still allows make up air for the appliances. Some times a loop is made at the bottom which also prevents excess cold air spilling out. Check with your code to see what is allowed.

Is there an elephant stuck to the other side of that wall? That's where I'd look first look for the answer....

Looks to me like that basement as its set up in the pic is big enough to not be considered a confined space so that would rule out a combustion air duct. Also the fact that its insulated flex tells me thats a fresh air duct thats supposed to be tied into that return. Hopefully with a damper to tweak the amount of frsh air being drawn in. The flex is used because regular metal pipe would condense in that warm space. Thats not how you run combustion air unless that installes knows absolutely nothing about combustion air codes.

That is exactly how our codes work except you requier 2
pipes. One for combustion air and one for makeup air.
That " tube " could be mounted a lot better to make it look good. Our new home building codes make the house so tight that sometime 4 tubes are used. If the installer is any good 2 of the tubes are hidden. Trust me. When you have the drier,bath exhaust,Genair range, kitchen exhaust,
fire place and or wood stove going you NEED 4 tubes. It's
our code. If you have an air to air heat exchanger add 4
more holes. Just in case you wonder. All new home constuction must have a minimum of 2X6 outside walls. The home must have inside and outside house wrap and all outside wall electrical outlets must have plastic air seals. Oh Yah. the inside house wrap joints must be glued.
All inside stapels on the house wrap must be caulked.That is our code. I can see they are going to change this code very soon. It's killing too many firemen or the firemen will have to relearn their job.

Thanks for all of the replies. Just a side-note: The basement is going to get finished, so this tube is going to be enclosed in a small space between the new basement bathroom and the stairs. I am wondering if this was designed with that in mind - and if not, what modifications to it may be necessary when the basement is finished?

It's meant as an air supply to your heating unit. I would totally box off that heating room unless you want cold air in the basement.