Hands In Pockets, Please
Have you ever found yourself in some dank basement staring at (and totally appreciating!) the simple beauty of some old one-pipe, gravity-return steam heating system? You're down there in the gloom, wondering who the heck did this work so many years ago. You're sliding around on a sloppy floor, breathing rank air, and marveling at how something so wacky could have lasted so long. Ahh, the joys of ancient engineering. You marvel. You wonder. You appreciate.
And then you decide that this system, which has gotten by without you for nearly a century, should have motorized zone valves.
Why do you do this?
Is it because life isn't quite exciting enough?
Is it because you don't know when to keep your hands in your pockets?
Is it because you are determined to save energy?
Whatever. Know that you are doomed on the day that you make this fateful decision. Doomed, doomed, doomed!
And here's why.
A gravity-return steam system uses a combination of two forces to put the returning condensate back into the boiler. First, there’s the steam pressure at the end of the mains. Now, this pressure will never be as great as the pressure that’s inside the boiler because as steam travels through the pipes it loses some of its energy to friction. What you wind up with at the end of the main depends on the size of the pipes and the boiler’s load. This was all figured out years before you were born by the Dead Men.
The other force that works to put the returning condensate back into the boiler is gravity. Gravity combines with the “leftover” steam pressure at the end of the main to create a force that’s greater than the pressure inside the boiler. I'm a big fan of gravity because it's one of those things that you can usually depend on to be there for you. The Dead Men allowed for enough vertical space between the end of the lowest steam main and the boiler’s water line to give the returning condensate a place to stack up. From there, gravity takes over. It’s all very simple, and very elegant in its own wacky way.
But now, you've decided to add some motorized zone valves to the boiler side of the steam main. You're doing this to save energy. It seems to make sense with a steam system because it makes so much sense with hot-water-heating systems. And how different can those systems be? So you do it.
Now, as long as those motorized valves are wide open, everything will be just fine. But as soon as one of them closes, the “leftover” steam pressure that you were depending on to be at the end of the main suddenly vanishes. Now, all you have going for you is gravity, and there’s not enough of that to put the returning condensate back into the boiler because there isn't enough vertical stacking space for the water to build static pressure. So the condensate backs up into the main and lays there. It's waiting for the motorized valve to reopen. And when the valve does reopen, Mr. Steam comes raging through the valve at about 60 miles per hour. He meets Mr. Condensate and knocks him on his keister and then sends him flying down to the end of the main where Mr. Condensate hits with enough force to knock the building off its foundation. You can hear it in Hoboken.
The folks in charge of the building (and these are the same folks who are supposed to pay you) begin to call you names. This is not good.
But it's not your only problem. Keep in mind you have a burner on that boiler that’s sized to provide steam for the entire building. When any motorized zone valve shuts down, the firing rate doesn’t change, does it? Suddenly, you have more steam volume than the pipes can handle. Too much steam volume means the steam will move at a higher velocity than it ought to be moving, and at the higher velocity, the steam will suck some of the water out of the boiler. That, of course, leads to water hammer in the zones that are calling for heat.
And this, of course, leads to even more name-calling on the part of Those Who Pay Invoices.
As the water goes flying into the pipes, the boiler begins to drop into a low-water condition. Bound to happen, right? The burner shuts off, but before the condensate can return, the automatic water feeder kicks in and adds water to a system that doesn’t really need more water. When the condensate finally returns from the system, the boiler floods.
You'll blame the automatic water feeder and make some angry phone calls to the manufacturer's representative. You'll do this because everyone loves to scream at those guys.
Meanwhile, you’re back on the job every other day, trying to figure out what to do next. Finally, you decide to have the contractor install a check valve in the wet return. You figure a check valve will keep the water from backing out of the boiler. This idea came to you in a dream.
On the day you install the check valve, the water stops backing out of the boiler. You think you’re out of the woods. You're not. Now none of the condensate can get back into the boiler because there’s not enough pressure to open the check valve. The water hammer continues its psychotic banging whenever any zone valve opens. You look at your hands and wonder why you didn't leave them in your pockets.
Next, you specify a boiler-feed pump and wonder who will pay for that, but that is a bridge to be crossed in the future (albeit the NEAR future).
As soon as the pump goes in you realize that you have opened the wet returns to atmosphere by hooking them up to that vented receiver. Great gobs of steam vomit into the boiler room from the vent. You wonder why you didn't become a stockbroker instead of an engineer. You should have listened to your mom.
You decide to specify one huge master trap at the inlet to the boiler-feed pump. This has never once worked in all of heating history, but you think you might be the lucky one. Your faith is strong.
Unfortunately, your trap is weak in spirit. It allows steam to work its way into the formerly wet returns. Now, you have more water hammer than you had before. Worse yet, whenever all the zone valves close on a boiler that’s filled with steam, a vacuum forms inside the boiler. This causes the water that’s in your new boiler-feed pump to flow into the boiler and flood it.
You curse the totally innocent automatic water feeder and call the rep. If you can't get paid, at least you can scream at the rep.
And then again you wonder again why you didn't leave your hands in your pockets.
Next time you get jammed up like that, try this. And know that it's a Band-Aid, not good engineering. Run a quarter-inch line over the top of the cursed motorized valve and use that line to bleed some steam pressure past the closed valve. This will raise the pressure downstream of the motorized valve and it just might increase your chances of getting the condensate back into the boiler. It might also get you paid. Keep the boiler pressure as low as possible. The size of the bypass line is small enough to keep much steam from getting through, so overheating shouldn’t be a problem. If you’re living a clean life, that is.
The bypass won’t solve the problem of the burner being oversized when some of the zones are closed, of course. You'll need a modulating burner that will back off when some of the motorized zone valves close to solve that conundrum. You also need someone to pay for that. One step at a time though.
I was reading an old book the other day. You know what? They had motorized valves way back when. The Dead Men just knew that they shouldn't use them on gravity return steam systems.
So they kept their hands in their pockets.