Why Steam Boilers Short-Cycle

I figure there are only two things that can start and stop a steam-heating boiler. One is a thermostat of some kind. It might be an ordinary one, the sort you’d find in a small house. Or it could be a central controller that feels the temperature of the outside air and then decides how long the boiler should run on that day. I see those in apartment buildings.

The only other thing that can start and stop a steam boiler is a pressuretrol. I like that name because it’s easy to understand. It’s short for Pressure Control. You don’t need much pressure in a steam system, and it’s the pressuretrol’s job to watch over that pressure. You can trick it, though.

When I was young and most eager to learn, I would go to local trade shows and ask the reps who sold boilers how to set the pressuretrol. It has a cut-in scale on its front that moves between one-half psi and 10 psi. It will give you whatever you desire. And when you remove the cover you’ll see a small white wheel tagged DIFF, That’s short for differential.

You take the cut-in pressure and add the differential pressure to get the cut-out pressure. It works like a sine wave, which only brought more questions to my mind.

So I’d ask the boiler reps how to set this thing and why it had to go up and down like that, and most of them would tell me to set it for whatever it needs and it goes up and down because that’s the way it works. They would then nod confidently.

Such helpful advice was this. It was only after years of reading technical books and old heating magazines that I learned that the pressuretrol bobs up and down like that because that’s what allows the air vents to reopen once they close on steam temperature. You see the steam pressure inside the pipes and radiators blows closed the little floats inside the air vents, and unless that pressure drops, the vents won’t open. And if no venting takes place after the first cycle, most of the air won’t leave the system. And where there is air, steam will not go.

When that happens,  the burner short-cycles. This usually inspires some hapless person to raise the pressuretrol setting in an attempt to force the steam into pipes and radiators that are filled with air that can’t get out. This makes the burner run longer, which wastes fuel, but that just makes the hapless person raise the pressure even higher. And there are lots of hapless people in this world.

Think with me. Every steam system is wide open to the atmosphere. Before each cycle, all the pipes and radiators are completely filled with air because the air vents are wide open. Those vents work in both directions – out and in.  The boiler fires and steam forms. It races out of the boiler, shoving the air ahead of itself and toward those air vents. Steam is lighter than air so the two won’t mix. The steam is like a plunger. It pushes the air down the line. If the vents are working, the steam will reach the radiators and the rooms will get warm. This satisfies the thermostat, which then shuts off the burner. All’s well with the world.

But suppose the air vents are clogged or missing and some hapless person has replaced them with pipe plugs because plugs cost less than vents. Now the steam pushes the air ahead of itself, but the air can’t get out. The two gases, steam and air, work together to build pressure as they compress each other. The pressuretrol senses this and shuts off the burner. But the thermostat is still calling the burner on because it’s in series with the pressuretrol. If one says stop, the other must listen.

Now the building is cold because the radiators contain air and not steam. The steam has nowhere to go. It sits inside pipes that may be uninsulated because we’re all afraid of asbestos these days but we also don’t want to pay for fiberglass insulation to replace the asbestos. The steam condenses within the bare pipes, dropping the system pressure. The trapped air expands back into the space left by the shrinking steam. It becomes a gaseous tug of war. The pressuretrol senses the drop in pressure, and since the thermostat is still pining for hot radiators, the burner restarts. Steam returns and squeezes the air back into the radiators. The air still can’t vent so, once again, the pressure builds and the pressuretrol shuts off the burner. That’s short-cycling.

So the hapless person raises the pressuretrol setting even higher. This causes the burner to run longer because it needs to squeeze the air even tighter. The radiators sill won’t get hot but the short-cycling seems to be less now because the burner hardly ever shuts off.

Think some more with me. The only way you can raise pressure inside of any vessel, be it your car’s tires or a steam-heating system, is to fill the vessel completely with a gas and then add some more gas. That’s Boyles Law. The volume of gas at a constant temperature varies inversely with the pressure exerted upon it. That’s what you see on a boiler gauge. The pressure goes up because the system is completely filled with a gas, be it steam or air, and someone just added some more gas.

People will often say they think something is wrong with their steam system because the pressure gauge never moves. I’ll ask them if the building is warm. They usually say that it is. I’ll tell them this means the steam is reaching the radiators, and at a pressure so low that the gauge can’t sense it. Steam at 0 psi contains all the Btu needed to heat the people. The only pressure we need is enough to overcome the resistance to steam flow, which is typically one ounce of loss per 100 feet of travel. It’s not the pressure that heats the building; it’s the latent heat the steam gives up when it turns back into water.

But if the air vents aren’t venting, the pressure will rise, and the tug of war between the steam and the air will begin, and that’s what causes the burner to short-cycle and waste fuel.

Can you see it in your mind’s eye? If you have a steam boiler that’s short-cycling and wasting fuel get the heck out of the boiler room and walk the length of the pipes. While you’re walking, whisper to yourself, “If I were air, could I get out?” Look for the main vents. Look for the steam traps on the radiators because before they can trap steam they need to vent air. Are those traps working? Look for float-and-thermostatic traps on the steam mains and steam-riser drips. Are those traps working? Did a hapless person double-trap those return lines? Double traps can keep the air from escaping the system because water will get caught between the two traps.

Wander and wonder. If you were air, could you get out? Answer that question and you’ll cure just about every short-cycling burner problem you’ll ever meet.