The purpose of air vent on pipe works of infrastrucure
No. You vent air from a sewer line.
Install an automatic air vent on the line
Attach an auto air vent at the highest point on the line
If by vent pipe you mean the venturi pipe it sounds as if it is plumbed backwards, in a spa jet you have 2 feed conections, one for water (the bottom pipe) and another for air (top pipe) if plumbed incorrectly water will back up out the venturi line (vent pipe)
Look at where the cold air is coming from and see if it can be stopped, if not insulate the pipe.
purging is the process of purging an air to the unburned fuel oil, before it be fired.
A direct vent appliance uses a two pipe system which takes combustion air from the outside through one pipe, and then exhausts flue gases out the other pipe. These system do not use room air for the combustion process.
reticulated system pipe line colour code
Yes. You need to open the plumb, & you have an air vent there.
In order for water to flow down a pipe, there must be a vent to allow the air to escape. Above a toilet, on the roof, there is usually a pipe or a vent hood, which is connected to the drain pipe. When the toilet is flushed, the sound of water draining can be heard issuing from this vent. In addition to the above answer, the air rushing through the water may be the cause of the gurgling sound, which is the air escaping through the vent to prevent the build up of air pressure inside your water system.
Shouldn't be a problem. You may need to extend the pipe at the sink a bit to put in air vent so that the drain can vent. It is a checkvalve that lets in air when the sink drains. Or you can tie into the existing vent stack if possible.