How An Undersized Steam Boiler Can Still Heat A Building by Dan Holohan


An engineer called the other day to ask if I had any output ratings for some oddball steam radiators he needed to measure before he could specify a replacement boiler. These radiators were a bit strange but I managed to find a rating for them in one of my old books. 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?" I asked.

"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 33-1/3 percent. All the original radiation is still here. I don't see any capped risers anywhere."

"Are the pipes insulated?"


"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'm now specifying 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 building's installed radiation and piping somehow manage to heat the building?

The answer is . . . It depends.

Here, let's back up for a moment. Steam is a gas that really wants to turn back into a liquid, right? It will give up its latent heat energy to anything that's colder than it is. And 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 EDR rating.

We measure EDR (Equivalent Direct Radiation) 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 looking for because the boiler's ability to produce steam must match the system's ability to condense steam. EDR is the radiation's ability to condense steam, expressed as a number. If you have, say, 1000 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 literature.


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