Back in the old days when most folks burned coal, vacuum was just about everyone’s friend. That slow and steady coal fire produced a steam that gently nudged the air from the system.
Back then, the radiator air vents probably had tiny check valves in their outlets. Those checks would let the air out, but they wouldn’t let it back in. When the steam condensed in the radiators, a naturally induced vacuum formed and the water would start boiling at temperatures much lower than 212° F. This made the most of the diminishing coal pile.
But then when the oil burners arrived, the system problems arrived with them. The burner now came on instantly and at full force. And when it shut off, there wasn’t much heat left in the boiler. The vacuum formed before the steam could push all the air out of the system, which led to very uneven heat.
Vacuum was an unwelcome visitor, and the old-timers solved the problem by getting rid of those vacuum air vents on the radiators and near the ends of the steam mains in favor of standard air vents, which let the air back into the system when the burner cycle ended.
Vacuum still causes problems in steam systems, but in different ways. Consider float and thermostatic (F&T) traps. They allow air and condensate to pass, but shut tightly against steam. If you have a two-pipe gravity-return steam system with dry (above the boiler waterline) returns, you need F&T traps at the ends of each steam main. Their job is to make sure steam doesn’t work its way up into the return side of the radiators. The only other time you would use F&T traps with steam heating is if you had a condensate or boiler-feed pump putting the condensate back into the boiler. Here, the trap’s job is to keep steam from pushing through to the pump’s vented receiver. You put one F&T trap at the end of each steam main, just before it drops to return to the pump. There should always be a straight, downhill run from the outlet side of those traps to the inlet of the receiver. No water should be allowed to accumulate and cause a water seal to form in this piping. If it does, air won’t be able to vent back to the receiver and out of the system. You’ll have very uneven heat.
Unfortunately, the boiler-feed pump’s inlet is probably positioned higher than the return lines. That means you have to lift the end of the return main to make it into the receiver, and that lift will create a water seal. Air won’t vent through that water seal.
To solve this problem, you may be tempted to install main vents on the inlet sides of the end-of-main F&T traps. That will get rid of the air, and it seems to make sense. The distribution will definitely be better, but another problem will now appear because F&T traps discharge water at saturated steam temperature. That means that if you’re running, say, 5 psig at the boiler, you’re going to have 225° water leaving that trap. Some of that condensate will flash back into steam as it enters the return line. Then, because of the water seal, it will almost immediately condense, shrinking 1,700 times as it does so. That leaves you with a vacuum that is going to lift the water in the return. If your trap is low enough in the system, the condensate will back right up into it. You’ll have water hammer inside the trap, and that’s not good.
To avoid this potential problem, always install the main vent on the outlet side of the F&T trap rather than on the inlet side. In that position, the vent will not only get rid of the air, it will also act as a vacuum breaker when it opens. Problem solved!
Another place where unwanted vacuum shows up in a steam-heating system is on jobs that have motorized zone valves at the boiler. These are normally on the takeoffs from the boiler header. When the zone valve closes, the steam between the zone valve and the boiler will condense, leaving you with a vacuum inside the boiler. If there’s a boiler-feed pump on the job, the water in the pump’s receiver will quickly flow into the boiler and flood it. You have to see this happen to believe it. You’ll swear someone is filling the boiler with a high-pressure hose. It happens that quickly, and the vacuum is the culprit.
The solution is to install a vacuum breaker anywhere in the piping between the zone valve and the boiler. Do this and you’ll look like a genius.
Then, there are thermostatic radiator valves. You’ll see them replacing the worn supply valves on radiators. TRVs promise to keep those oversized radiators from overheating, but vacuum can mess up that promise. The challenge is that most of this work seems to get done during the winter. And it’s almost always accompanied by steam-trap work. The problem with winter work is that you can’t do everything on the same day. Most of these systems have been neglected for many years, and there always seems to be some steam scooting around in the returns where it doesn’t belong. So, let’s say you install 50 or so TRVs, and you also replace the failed elements in the steam traps. You do this during the winter because that’s when most of the heating work gets done. Here’s what can go wrong.
First, steam will pass through the wide-open radiator valve and enter the radiator. The trap will close on temperature. So will the TRV.
Now, the radiator is closed on both sides and the steam is condensing. That forms a vacuum inside the radiator. The cooling condensate gathers around the trap element and opens it. Unfortunately, there’s steam in the return line because you couldn’t possibly do all the traps at the same time. The steam senses the vacuum in the radiator and claws its way up into the trap where it meets the water surrounding the element.
You can destroy trap elements as fast as you can install them when you do this type of work in the winter. And if the traps aren’t working, the building isn’t heating. The solution is to do the work during the months when the heating is off.
And yes, I know that’s easy to say.
Here’s another place where vacuum shows up as an unwanted visitor. We both know that many steam systems still contain asbestos insulation. Sometimes, it’s wrapped and safe; other times it’s not. Now stop for a minute and think about what can happen if people have the insulation removed from the boiler room piping, but not from the steam mains.
The burner fires and the boiler makes steam. The steam leaves the boiler room and enters the mains. It doesn’t condense that much inside the insulated pipes. The burner comes up to a high limit and shuts off. Once that happens, the steam in the uninsulated boiler-room piping begins to condense much faster than it condenses in the system piping. The vacuum forms at the boiler and you now have a lower pressure in the boiler room than you have in the piping. The steam stops moving out to the system. Instead, it turns around and heads back toward the boiler. As it does, it’s going to suck air back into the system through the air vents that are still open. That’s really going to mess up your distribution.
If you suspect this may be happening, go to any vent and hold a lit match near its outlet. If vacuum is forming somewhere in the system, the vent will pant, pushing the flame toward and away from the match as the system tries to make up its mind. You can’t solve this with more or faster air vents. You need insulation on that near-boiler piping. That’s the solution.
Oh, and I learned all of this the hard way. I hope you won’t have to do the same.