“Vapor/Vacuum” Do’s and Don’ts

Back in the days of coal and wood-fired boilers heating contractors used vacuum air vents to help them get the maximum efficiency out of their steam heating systems. They called these old systems “Vapor/Vacuum,” and the principle that made them work was a simple one: At very low pressure,steam takes up about1,700 times more space than water. When that steam condenses, it will create a vacuum if air can’t get back into the system.

The old-timers let the steam expand naturally. It pushed air ahead of itself, through the vacuum vents and out of the system. When the steam condensed in the radiators, it shrank to 1/1700th its size. Air couldn’t reenter the system through the vacuum vents because they have tiny checks valves at their outlets.

If the piping was tight, a deep vacuum would form throughout the system. The nice thing about a vacuum is that it lowers the boiling point of water. If the old-timer set it up right, a vapor/vacuum system could continue to make steam, even after the water temperature dropped as low as 140 degrees! The old-timers could take advantage of every bit of heat from the coal or wood fire as it burned down to embers. They wasted nothing.

Nowadays, however, most of us fire our steam boilers with gas or oil. Coal and wood-fired boilers are still around, but they’re the exception to the rule. While gas and oil are convenient fuels, they’re not a good choice for systems using vacuum vents because gas and oil burners cycle on and off.

This cycling creates problems in systems that have vacuum vents. The vacuum quickly forms when the burner shuts off. Any air that doesn’t get vented on the first cycle expands greatly, blocking the movement of the steam “vapor” to the radiators. And because gas and oil burners shut off completely between firing cycles, there’s no longer a hot bed of embers to keep the low temperature water boiling. When you mix vacuum vents with gas or oil, you usually wind up with uneven heat and callbacks. You also wind up with condensate that doesn’t return quickly enough from the system, and that can lead to water I level problems in the boiler.

At Hoffman, we haven’t made vacuum vents for one-pipe steam systems in about 15 years. Today, we make only one vacuum vent. It’s a main vent we call #76. We continue to make the #76 because there are still many two-pipe, vapor/vacuum systems out there that run on coal.

If you have a two-pipe, vapor/vacuum system running on gas or oil, you should be using our #75 main vent near the end of each dry return. The steam will push the air through the radiators, into the dry return and out the #75. The system won’t drop into vacuum. And as long as your radiator traps are working as they should, your old vapor/vacuum system will heat evenly at very low pressure. It usually takes no more than 12 ounces or so.

Steam traps are crucial to old two-pipe, vapor/vacuum systems. If you suspect your steam traps aren’t working as they should, test them with a contact thermometer or a temperature-sensitive crayon. You should see 10- to 15-degrees drop in temperature across the thermostatic radiator trap if it’s working.

If the traps are passing steam into the returns, you’ll have uneven heat, high fuel bills, boiler water level problems and water hammer noise. Steam traps are every bit as important on those old systems as they are on more modern systems.

You can repair those old steam traps with Hoffman Bear Traps or Hoffman Durastat elements. Our replacement parts are built to last for many years under the toughest conditions. They fit most old-fashioned steam traps, and they pay for themselves in no time with fuel savings and even comfort. Your customers will think you’re brilliant!

When you’re faced with an old steam system, think Hoffman. We have the parts and the specialized knowledge you need to solve those tough problems. And we’re always happy to help you because we appreciate your business. Thanks!

Reprinted from CounterPoint January 1995, Vol. 2, Issue 1