Vaccuum on Steam Boiler

Vince Marino

Our situation is that our plumbing company is working on a large steam boiler at a single family home and removing the pepcock at the bottom of the gauge glass connection. The problem is that it is pulling a vacuum instead of letting water out. On the gauge the needle sometimes goes to vacuum instead of pressure as well. What would cause a steam boiler to have a vacuum like this? Thank you for your time.
-Vince Marino
Vince Marino Plumbing & Heating, Inc.

Steamhead

Vince, what type of system

is that boiler connected to? One-pipe? Vapor?

Your system probably has vacuum-type air vents on it that let air out, but not in. Besides causing the petcock to suck air when you open it, vacuum can expand any air that's left in the system. This can block the flow of steam.

I'd find and replace any vacuum vents on that system. If it does not use air vents on the radiators, look for vacuum vents on the mains and dry returns. If that's a Vapor system with a Float Trap/Air Eliminator, look for a vacuum check on the top.

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Paul Reid

When steam sucks

steam boiler at a single family home ... it is pulling a vacuum instead of letting water out. On the gauge the needle sometimes goes to vacuum instead of pressure as well. What would cause a steam boiler to have a vacuum like this?

It is "supposed" to do that.

A high-class one-pipe steam system will be setup for vacuum/vapor operation. After you build a good fire and vent all the air out, you can let the fire die down. Steam pressure will drop from say +1psi to -10psi (vacuum), the water will still be boiling at 150 degrees F. You get gentle heat from luke-warm radiatiors, and instant pick-up. Without vacuum operation, the radiators are either stone-cold or boiling-hot, and you get no new heat until the boiler water comes back to 212 degrees.

This works really well for coal, which naturally dies-down after the initial morning blaze of heat. You want a draft damper to keep the coal and water simmering at that 140-200 deg. boil according to thermostat demand.

It works, less naturally, with oil/gas/electric fires which are on/off.

It does not work if there is ANY leak in the system. A whisper of air getting back into the system quickly turns it back into a normal 212 deg. steam system. I have never managed to get any of my homes to run as vapor consistently. I have seen vacuum but it leaks out in an hour or so, even when there is no noticable water loss working at positive pressure. Never had the energy to re-pack every valve, and replace all the vents.

Since you are faced with a working vacuum system, treat it right. All your work really tight, like gas-pipe, so it can sit there for hours and not suck a droplet of air. There may be special vents rated for vacuum operation. I don't think you need any other special care for oil/gas firing. (Coal would be different, but that's one for the Dead Men to answer.)

-PRR