A: Dear Valued Home Depot Customer, We have reached out to the manufacturer and their answer is,“1/8 inch is reference to the input opening”.
A: There was non stop hissing for the duration of the heat cycle.
A: The length of time in question is not dependent on the vent. It is dependent on the system. Thus it will be different in every system and in every radiator; depending on pipe length and diameter. Allow me to explain. When the water in the boiler reaches 212 F the water flashes to steam. Stream is extremely powerful and leaves the boiler at roughly 55 miles per hour. The steam needs to push and clear all the the air in front of it out of the way. This is the hissing noise you hear. The hiss is made by a combination of the air and the steam that has condensed back into water, as it was cooled by the pipes, being forced out of the system by the steam. Eventually when all the pipes have been heated and all the air pushed out of the way, steam will reach the radiator. This can take some time, even though the steam leaves the boiler fast. This is because the cold pipes pulling energy out of the steam will slow it down and cause it to condense. When the steam does reach the radiator the vent locks up or closes. Since no more air can now escape, no more steam goes to that radiator. Instead the steam pressure will pick the path of least résistance to the next radiator that is not yet satisfied. When all the vents are closed, pressure in the system will build and the boiler will shut off. That is assuming the thermostat has not already been satisfied and shut the system off. When all the heat is given up by the radiators and the all the steam has condensed back into water, the vents open. This releases the vacuum and allows the water to flow back from the radiators to the boiler. Then the entire process can happen over again.