Gravity return systems are simple and have remarkably long lives. Many have been in service for nearly 100 years. They work on a simple principle: If you let water stack in a vertical pipe, it will exert a pressure. These old systems use that pressure to help put the condensate back into the boiler.

Consider this. A steam heating system is like an old-fashioned balance scale.There's a "weight" of steam on both the supply and return sides.We call that weight "steam pressure," and it usually doesn't take out to make the system work. The pipe size determines the right steam pressure for any given system, and the original heating engineers made that decision years ago.

Whatever the pressure, when you make steam, you'll have more of it in the boiler than you will in the system piping. Steam is not like compressed air. The pressure is never the same throughout the system. It can't be,because steam condenses. You'll always have less at the ends of the mains than you do in the boiler.

And that's where the static weight of the returning condensate comes in.The original heating engineer figured this all out. In his gravity return system, he knew the weight of the condensate would combine with the "leftover"steam pressure at the ends of the mains. Together, the two forces would be enough to put the condensate back into the boiler.

Now, this is a beautifully simple balance of pressures that's worked well for years. But think about what happens when you add motorized zone valves to the mix.

Let's say you start with all the zone valves open. The boiler makes a bit of pressure and steam moves out toward the radiators. One zone reaches its set temperature, and a thermostat in the living space shuts the motorized valve.

Here's the problem. At this point, there's still pressure in the boiler,but there's no longer any "leftover" steam pressure at the end of that zone's main. There can't be, because the motorized valve stopped it back at the boiler. Suddenly, all you have going for you to "balance the scale" is the static weight of the water in the returns. Unfortunately,it's not enough.

So what happens? The water backs out of the boiler and up into the closed zone. That gives the boiler two choices! It can either shut down on low water or, if there's an automatic water feeder, it can take on fresh feedwater. Either option is bad for you because, eventually, that motorized valve is going to reopen. And when it does, you'll get a hefty dose of waterhammer as steam meets the backed up water in the main. Next, the backed-up water will rush into the boiler and flood it.

Contractors who are not that familiar with the ins and outs of steam heating usually blame either the low-water cut-off or the automatic water feeder for this problem. Neither is at fault, but since they're thinking that way,they'll never solve the problem.

If you put a check valve on the condensate return line, the water can't back out of the boiler. That's good! You'll think you solved the problem because the water will no longer disappear when a motorized zone valve closes. But, unfortunately, a check valve doesn't get you out of the woods, because the returning condensate can't open it. Remember, there's a limited amount of 'weight" on the system side of the "scale"when that motorized valve closes. It's not enough to open the check valve.

Since the condensate can't get through the check valve, the boiler will take on fresh water through its automatic water feeder. It won't need as much as it did before, but it will still need some. After a few cycles,the boiler will find itself flooded again. And then there's the waterhammer to consider whenever that valve reopens.

Don't be so quick to blame the feeder or the low-water cutoff if you're having water level problems. Take a hard look at those motorized zone valves.They are at the root of many steam problems. Don't let them fool you, too.

The only right way to use motorized valves on a steam heating system is to convert the system to pumped return. You do this by adding a boiler-feed pump to the main condensate return, and steam traps to the ends of the mains and the base of the riser drips. The key to successfully troubleshooting any steam heating system is to keep your eyes open, and to keep thinking about the balance between the supply and return sides.

ITT McDonnell & Miller representatives are well-versed in steam heating.Call them when you're faced with a tough problem.