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Undersized Steam Boilers

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Author
Dan Holohan
Published
July 16, 2009
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A guy called me to ask if I had any output ratings for some oddball steam radiators he needed to measure before he could replace the boiler. I looked them up and told him. He thanked me and hung up, but then called back about an hour later.

“I’m sorry to keep bugging you,” he said, “but this doesn’t make sense.”

“What doesn’t make sense?”

“Well, when I add up all the radiation in the building and then figure in the pick-up load for the piping, I’m coming up with a total EDR that exceeds the rating of the boiler that’s here by about thirty percent.”

“Are you sure you measured all the radiation correctly?” I asked.

“I went by the numbers you gave me for the oddball radiators, and I already had the ratings for the rest. Yes, I’m sure that the numbers are right.”

“What did you use for your pick-up factor? I asked.

“I allowed the standard 1.33,” he said. “All the original radiation is still here. I don’t see any capped risers anywhere.”

“Are the pipes insulated?”

“Yes.”

“Then size your replacement boiler by the load you calculated,” I advised him.

“But the old boiler is nearly a third smaller than the one I’m figuring. And it’s been heating this place for years,” he went on. “It’s going to be tough explaining to the owner that I now have to put in a bigger boiler. How am I going to justify it?”

Good question, eh? What would you do?

Here’s an even better question: Can a steam boiler that’s undersized for the installed radiation and piping still manage to heat the building?

It depends.

Here, let’s back up for a moment. Steam is a gas that really wants to turn back into a liquid. It will give up its latent heat energy to anything that’s colder than it is. Once the latent heat is gone, you no longer have steam. Because of this, we can say that to be properly sized, a steam boiler’s ability to produce steam must match the system’s ability to condense steam. That’s why we spend so much time measuring radiators when it’s time to replace an old steam boiler. You have to get that total Equivalent Direct Radiation rating.

We measure EDR in square feet. Radiator manufacturers publish these ratings in their sales literature. In the case of older radiation, you often have to search for the literature, but it’s worth the hunt because the boiler’s ability to produce steam must match the systems ability to condense steam. EDR is the radiation’s ability to condense steam, expressed as a number. If you have, say, 1,000 square feet of Equivalent Direct Radiation, the boiler should have a Net rating of 1,000 square feet EDR. You’ll find that rating in the boiler manufacturer’s sales literature.

But you’ll also find a couple of other ratings for the same boiler. First, you’ll see the Input rating. That’s the amount of heat that’s leaving the fire and entering the combustion chamber. Right after Input, you’ll see the Gross Rating. Sometimes the manufacturer will substitute the term, “D.O.E. Heating Capacity” for “Gross” but they mean the same thing. The Gross rating is the amount of heat that’s left over after the boiler loses some heat up the chimney and through its jacket. The difference between Input and Gross indicates the boiler’s combustion efficiency.

The next rating on the chart is Net. That’s the actual radiation load, expressed in square feet EDR. If you take the boiler’s Net rating and multiply it by a factor of 1.33, you’ll get the Gross rating. In other words, the manufacturer is assuming that the amount of steam the boiler will need to heat the pipes that lead to the radiators will be about one-third of the total radiation load. When you size a replacement steam boiler, you must figure both the piping and radiation loads because both piping and radiation have the ability to condense steam.

Think of it this way. Suppose you were to remove all the radiation and piping from a building and weigh it on a big, industrial scale. Let’s say you come up with a total of 15,000 pounds of iron and steel. Now, in sizing that replacement steam boiler, your job is to bring that 15,000 pounds of metal from room temperature to steam temperature, which in this case is 215 degrees F. (One square foot EDR equals 240 BTUH when there is 70-degree air around the radiator and 215-degree steam inside the radiator).

Can you see this in your mind’s eye? You’re going to need a certain amount of heat to get the job done. How much heat you’ll need depends on an engineering term called Specific Heat. To avoid a lot of mumbo-jumbo, let me just say that Specific Heat is simply a ratio of the amount of heat it takes to raise the temperature of something by one degree, as compared to water. For instance, it takes one BTU to raise the temperature of one pound of water by one degree Fahrenheit. Since steel pipe and cast-iron radiators are denser than water, it’s going to take more heat to raise each pound of these metals by one degree than it takes to raise the temperature of water by one degree. If that 15,000 pounds of metal is currently 50 degrees Fahrenheit, and you need to get it up to 215 degrees, you could figure out exactly how much heat you’ll need to get the job done. Just get yourself a book that lists the Specific Heat of iron and steel and work out the numbers. But the manufacturers have saved you the trouble of having to do the math by giving you those EDR ratings and pick-up factors.

But let’s get back to that undersized steam boiler. The guy who called told me that the boiler had managed to heat the building for years. How could it do that?

Here’s how: Consider that 1.33 pick-up factor for a moment. This represents the difference between the boiler’s Gross rating and its Net rating. That’s the amount of steam the piping will condense when you first start the boiler. But once the piping gets up to temperature, the pick-up factor drops out of the equation.

To understand why, let’s make believe the pick-up factor is like the starting winding in an electric motor. That winding is there to give the motor the torque it needs to start the rotor spinning. But once it’s up and running, the motor’s centrifugal switch drops the starting winding out of the play, right? Well, the same thing happens with the pick-up factor in a steam system. Once the pipes are hot, you don’t need that load anymore. The pick-up factor is the steam system’s “torque.”

And this is how that undersized boiler managed to heat the building for all those years. It came on and ran for a long time. During that time, the people who were in the rooms furthest from the boiler noticed that their radiators weren’t getting hot as quickly as the radiators in the rooms that were closer to the boiler. This is because the system has the ability to condense more steam (on start-up) than the boiler can produce. And the problem is especially noticeable during the spring and the fall because this is when the weather isn’t as severe. The system starts and stops more often. The pipes don’t stay as hot as they do during the winter.

Someone in the building decides to move the thermostat to the coldest room. That allows the burner to run even longer. This, of course, wastes fuel because the people with the hot radiators will open their windows. And as the burner runs longer, the steam eventually manages to heat all the piping. That takes the pick-up factor out of the equation. Suddenly, the boiler is large enough to heat all the radiators.

The end result is that the undersized steam boiler manages to heat the building. But it does it at greater expense than it would if it were properly sized because the burner has to run for a much longer time. The people who own the building have no way of knowing this because they have nothing to which they can compare their current fuel bills. When a contractor puts in a boiler that’s sized to the correct piping-and-radiation load (even if it’s larger than what’s there now), that new boiler won’t run as long, especially during the spring and the fall, and that’s when the building owner will see the savings – even though he now owns a larger boiler.

Makes sense, doesn’t it?