Air Vents and Oversized Radiators

Years ago, my friend Frank Taverna (who happens to be one hell of a good heating man) and I looked at a steam problem in a small church in New York City. Five one-pipe steam radiators heated this church. These were huge radiators. Each put out 110 square feet EDR and each had a quick vent.

Beneath each radiator vent was a bucket. There were five of them. And they were all full of water! You see, when the boiler made steam, condensate would squirt from the quick vents. The water went up about 4 feet and was actually very pretty to watch – providing you didn’t stand too close.

But, understandably, the pastor didn’t share that feeling.

He told us that this had started all of a sudden and all by itself. Frank and I looked at each other and then explained that, with all due respect, nothing in steam heating “happens all by itself.”

“Father,” I said, “there is no Divine Intervention in steam heating. There has to be a cause.”

He liked that.

We probed a bit and learned that the spitting got very bad right after the parish handyman installed a day/night setback thermostat.

“But what could that have to do with the radiators?” the pastor asked. “The thermostat is over here and the radiators are over there.”

He didn’t see the connection, but we did. Before the handyman had installed the thermostat, the church was heated all the time. But now they let it go cold overnight. And when the steam finally came back up in the morning, it hit pipes that were ice cold. We were getting a lot more start-up condensate now than we did before. And that was the root of the problem.

But not wanting to make a judgment before seeing the entire system, we went downstairs to nose around. Frank and I checked out the size of the pipes against the size of the radiators. This is something you should always do when you troubleshoot one-pipe steam because too large a radiation load will increase the steam’s velocity in the riser and keep the condensate from draining. The in-rushing steam will hold the water in suspension. Instead of returning to the boiler, the condensate will be squirting out of the vents.

So we looked around, checked the figures, and realized the horizontal pipe leading to the vertical riser on the problem radiators was one size smaller than it should have been.

Now, we could have told the pastor that he’d have to have all that piping replaced. But had we said that, I’m sure the good Father would have asked the inevitable question: “How come it worked for so many years, boys?”

And then we would have told him, “Well, because you never let the pipes get so cold before. You have more condensate to deal with now, more of a pick-up load.”

I know he wouldn’t have believed us – even with our sincere faces and all.

Besides, increasing that pipe size sure would have been an expensive cure! But at the same time, we didn’t want to abandon the new thermostat and its energy-conservation benefit. So we decided to try something else.

I had Frank remove the quick vents and install five Hoffman #40 vents. I did this because the #40 has a notoriously slow but extremely steady venting rate. Hoffman designed it back in the old days to vent radiators slowly so they wouldn’t spit water.

My thinking was that if we could slow the venting rate, we’d also slow the rate at which steam entered the radiator. That would mean less condensate during any given minute. It would also mean slower velocity in the riser.

Considering the cost of the five vents versus the cost of re-piping five radiators, it was certainly worth a try.

So we put the Hoffman vents on and that was the end of the problem. The #40 vents never spit. Not once. To this day, they still haven’t spit!

When radiators are oversized as they were here, you can do a lot of good by slowing down the venting rate at the radiator and speeding it up on the mains. You can also use John Schultz’s technique of using two vents on the oversized radiator – one slightly higher than the other. We didn’t do that here because the church was heated only during services. Most folks keep their coats on so we didn’t have to get the place that warm.

You live and learn.

I learned this: Somewhere along the line, we went wrong with one-pipe steam. Give it some thought and I think you’ll agree.

The only reason we use fast vents on radiators nowadays is because there aren’t enough main vents to take the air out of the piping. In our happy-go-lucky rush to get the job done, we’ve managed to get everything completely backwards. It’s almost comical.

Fast vents vent fast. And because they do, they let steam enter cold radiators much too quickly. With all that steam roaring in, the condensate that rapidly forms inside the cold radiator has a tough time leaving.

That’s what causes the radiator to pound and shake and knock. That’s what causes the radiator to spit. Too much is happening at the same time. It’s too fast.

To put things back in the proper order we have to return to the old ways. Vent radiators fully as John Schultz advised, but don’t vent them too quickly. Slow the rate at which you produce condensate and that condensate will slip by the steam on its way out of the radiator without causing water hammer.

And since you’re venting the mains as well as the radiators, the steam will arrive at every radiator within a few minutes of steaming. That gives you the luxury of time. You can now afford to bring the radiators slowly up to temperature and avoid water hammer.

Give it a try. It works!

Want to learn more? Get a copy of The Lost Art of Steam Heating Revisited.


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