All steam systems were originally coal-fired, and thus the systems were designed for a constant fire. But in the 1950s, oil burners came in to use. Oil burners let the Missus of the house finally drop the coal shovel and, one by one, coal-fired boilers were converted to oil. She was thrilled. But the steam system wasn’t. You see, an oil burner works much differently than a coal fire. The coal fire was constant. The oil burner (and later gas) cycles on and off. Usually once an hour during the winter. This change required a complete transition in the way the system vented itself. But for the most part that change never occurred. Why? See #1 above. They were all dead, remember? Who was around to know or care?
Those old steam guys were pretty clever. You see, they knew that when water became steam it expanded in volume 1700 times, and when it condensed, a vacuum was formed. They also knew that water at atmospheric pressure boils at 212º. But with negative pressure (a vacuum) water can be made to boil with a temperature as low as 180º and still make steam. Very efficient. And very clever, as I said. So the systems were designed to operate in a vacuum.
But what happens when we replace coal with an oil or gas burner? The burner shuts off every hour remember? So what happens to the vacuum when the burner shuts off? Well, when the burner shuts off the vents open and that loses the vacuum. Simply put, steam systems can no longer operate in a vacuum with modern burners. That’s long gone (unless one wants to go to considerable expense installing a vacuum pump).
To compensate, this conversion necessitated a drastic change in the way steam systems vented. Which of course largely never happened…
Picture your steam system when the boiler is not running. All those miles of piping and all those great big radiators are filled with air aren’t they? When the boiler fires, all that air needs to get out so that the steam can get in and begin to heat the home, right? They can’t coexist. The quicker this happens the more efficient the system. They didn’t need or use big vents in the coal days: who cared if it took an hour to get the air out? The fire was lit in the fall and generally stayed lit until spring. Sure, the initial venting took a long time, but because the fire remained lit, the vents remained closed, and the air stayed out until spring. Six months of air-free vacuum with one venting cycle.
But today our burner comes on and shuts off every hour. Which means every hour of every winter the burner fires and consumes fuel chasing the air out so the steam can get in. The bigger the main vents, the faster the air can get out. The faster the air gets out, the more balanced and efficient the system runs.
Except you don’t have large main vents. You still have the same size vents they used in the vacuum era. In a way it’s like saying your steam pipes are clogged (with air), and the clog is preventing the steam from getting to each of the radiators quickly and at the same time. Those tiny little coal-era vents on your returns in the basement are woefully inadequate for a modern system. And they are costing you a lot of money and comfort. Comfort because with slow venting, the radiators closest to the boiler are going to get hot first, long before the ones furthest away will.