Main Vent Testing

BVMH

I would like to know how to determine if the main vent on my one-pipe steam furnace is operating properly. I've attached a pic of the current main vent that looks to me like it is relatively old.

Fred

It's a Hoffman #75 vent. If it closes when steam hits it it is good. Take it off and shake it. It should rattle. If you hold it upright you should be able to blow into and through it. If you turn it upside down and blow into it, it should be closed and not allow air to blow through it.
The bigger issue, if that is the only vent on a main, is probably that it is not enough venting for that main, unless the main is very very short.

BVMH

I checked the vent per your instructions and it is definitely faulty. The main is approx. 90ft in length from the header all the way back to the main vent. Can you recommend the correct size of the vent to install? It is the only main vent on the system.

gerry gill

Try a Barnes & Jones vari vent..Maybe two of them together.

Fred

I know @gerry gill loves the B&J vari vent but they seem difficult to get, especially if you only want one or two. If you can't get them, the Barnes and Jones Big Mouth is an excellent main vent also. You probably need two of them for a main that long. Amazon carries the Big Mouth but I think they may be out of stock. If you call Andrew at Barnes and Jones, he may be able to ship either one of them to you:
Andrew
Phone # 1-781-963-8000, Extension 206

EBEBRATT-Ed

One way to determine if you have enough venting is by the boiler steam pressure. The lower the pressure you run at the better