A lesson in steam heating that will save you a headache
A steam boiler has to produce the amount of steam the building will need on the coldest day of the year.
Do you know who invented the motorized zone valve used on older steam-heating systems?
You know what I mean? They’re the ones that shut off the steam heat in entire sections of a building. They’re the big valves you see up there near the boiler-room ceiling. They’re not original to the job. Do you know who came up with those?
It was Satan.
I know this because so many of these ridiculously misapplied valves wind up in houses of worship. The knucklehead who installs them says, after the problems start: “Well, I was just trying to save the congregation money. I don’t think these problems have anything to do with the valves.”
And Satan smiles.
Think this through with me. A steam boiler has to produce the amount of steam the building will need on the coldest day of the year. The boiler is like an evaporator and the piping and radiators are like a condenser. If a boiler’s ability to produce steam doesn’t match the system’s ability to condense steam then none of this is going to work.
So the Dead Men sized the two to match each other and there was peace in the valley and in the houses of worship for many years. Satan considered giving up and moving on to more-promising prospects for misery.
But then the price of energy rose and as I said, some well-meaning knucklehead showed up and talked the folks in charge of the house of worship into zoning the system with those cursed motorized valves. The boiler, and let’s say it’s sized to let loose a million Btu worth of steam volume each hour, now meets the zone valves. Let’s imagine there are three of them because the knucklehead is really trying to save the congregation money. With all three zones open, life is good. The steam leaves the boiler at full power and has plenty of room to run.
But then one of those motorized valves closes because the knucklehead is trying to save the people money. At that point, one third of the piping and radiation load disappears, which means the boiler is now way oversized because when a zone valve closes no one stops by to change the boiler. It keeps firing full blast.
So the 1 million Btu/hr. races into the piping that is now too small to accommodate it. The steam will do this because it’s a gas; but it will not do this at the design pressure drop, which typically is 1 oz. of loss for every 100 ft. of travel. Nope, now the steam is going to suffer a much greater pressure drop and that means there won’t be as much pressure left when the steam reaches the end of the main. Uh, oh.
Satan heads back to the building, smiling.
Making a big mess
Consider the way a gravity-return steam system, the sort found in so many houses of worship, works. It’s that leftover steam at the end of the mains that combines with the condensate that’s stacking in the vertical pipe between the end of the lowest steam main and the center of the boiler’s gauge glass. We call that space Dimension A and it has to be at least 28 in. high because that will create 1 psi of static weight between the end of the steam main and the center of the boiler’s gauge glass.
That water weight combines with the leftover steam pressure at the end of the steam main to put the condensate back into the boiler.
But if there is excess pressure drop caused by too much steam trying to flow through too little pipe (because of the closed motorized valve), the leftover steam will be too weak. There won’t be enough pressure to combine with the water weight in Dimension A to push the condensate back into the boiler. The condensate will rise inside Dimension A and spill over into the steam main where Mr. Steam will start hammering it into the elbow that’s at the end of the main.
This usually happens during the holiest part of the service.
Just for laughs, and because this is what happens in real life, let’s close another zone valve and keep the same boiler running. We’re now shoving 1 million Btu/hr. into a piping system sized to handle one-third of that load. The velocity of the steam increases and the water starts to prime out of the boiler with the steam and enter the piping. The water hammer gets even more violent and flying monkeys appear. The boiler is cycling and wasting the fuel that the knucklehead promised the congregation they would save.
But wait; there’s more.
While the boiler is running in this psychotic condition, let’s remember there is atmospheric pressure within the closed-zones’ steam pipes. That’s because those pipes have main vents near their ends and since there’s currently no steam inside the mains because of the closed valves, the vents will be wide open to the atmosphere.
But there is steam pressure inside the boiler, right? So now we’re looking at a hydronic see-saw. There’s pressure on one side and no pressure on the other side. The steam pressure in the boiler will shove the water backward and up into the closed mains. It will probably fill them right back to the closed valves.
The boiler, of course, notices this and tries to go off on low-water, but there’s an automatic water feeder on this boiler, and it jumps into action to maintain a minimum water level and keep the burner firing.
At some point, the zones that were off come back on and this is when Mr. Steam meets Mr. Water, and right at those motorized valves.
Freddy Kruger shows up with a sledge hammer.
The house of worship lifts off its foundation with the pounding water hammer. The water in the steam mains screams back into the boiler in a rushing gush, flooding that boiler up into its header.
Someone calls the knucklehead. He shows up and curses the company that makes the automatic water feeder.
Satan grabs his knees and roars with laughter.
The knucklehead drains the boiler and adds lots of chemicals to fight the corrosive effect of all that fresh water. He explains to the congregation they’re going to have to start buying more chemicals. The congregation asks why and the knucklehead tells them it’s because the system is, um, old.
The congregation asks the knucklehead why they never had any of these problems before he installed the zone valves. He explains that sometimes stuff just happens.
What can you do?
Satan nods. He likes this guy.
Did you know they had motorized valves during the 1920s? I see them in old product catalogs. The Dead Men had the good sense to use them on their hot-water systems but not on their steam systems. They were smart.
They liked making Satan more miserable than he already is.