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Boiler Sizing


Dan Holohan
July 16, 2009
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When steam heating was new, back at the turn of the 19th century, coal was the fuel of choice. It burned hot, and it burned long. A good coal-fired boiler would stay lit for eight to 10 hours on a load of coal. Those early boilers contained cast iron grates that were similar to what you have in your barbeque, but much thicker and far sturdier. The person tending the fire (and it was almost always the woman of the house) had to know how to start the fire and spread the coals so that the air could enter from below and keep the home fires burning.

And I know that it was the women making the fires because my antique engineering books tell me so. The writers of those books made a big deal about this gender thing, and most suggested that a smart heating engineer choosing a boiler for a house will add up to 75% more capacity to the coal-burning grate area than is actually required to get the job done. Can you guess why?

It was because the man of the house might try his hand at building a fire on the weekends.

The writers of the old engineering books flat out wrote that the men didn't know what the heck they were doing when it came to making a proper coal fire (it was women's work). The writers figured that if they gave the husbands more room in which to play, there would be less chance of a lousy fire on the weekends, and fewer complaints to the heating engineer.

Seventy-five percent extra space in the combustion chamber for the husband, because he didn't know what he was doing.

How about that?

And there's something else you may not know, something that had a huge impact on the development of house heating back in the early days of the 20th century. It was the Spanish Influenza pandemic of 1918-19. The flu killed 50 million people worldwide.

Roll that number around in your mind for a moment.

Fifty million people.

This was the worst disaster in human history, but you probably never learned about it in school. It was an event so horrific that it was literally erased from the common memory. It killed so many people during that winter that the editors of The Ladies Home Journal magazine coined a new term during the summer of 1919. They decided to rename the parlor the "living room" in honor of those who survived. "Parlor" had taken on such a sad connotation because that was where most families laid out their dead.

The Spanish Influenza disappeared as suddenly as it had arrived, but it affected people for years afterward. During the summer of 1919, no one was allowed out on the street unless they were wearing a surgical mask. If you go on the Internet, you'll find photographs of Major League Baseball games where every player is wearing a mask, and so is every fan in the stands.

This was the scariest virus of all time, the perfect virus, and it was airborne. People became deathly afraid of the air in their homes, in their workplaces, in their houses of worship.. They began to open their windows to let in fresh air, and this had a dramatic effect on central heating. You can see it in the engineering books published after 1920. The authors wrote of the Fresh Air Movement and cautioned engineers to specify boilers and radiators that will be large enough to heat the building on the coldest day of the year, with the windows open.

If you base the size of your new boiler on the label that's on the old boiler, there's a very good chance you'll be sizing a boiler that's large enough to heat your home with the windows open. You'll also be accommodating a long-dead husband who didn't know how to make a coal fire on the weekends. These are vestiges of American Heating History. There's no reason why you should have to embrace them, though.

But wait, it gets even wackier. The Dead Men oversized those early boilers for good reasons, and they installed them well so that they would last for decades, which they did. What changed as time went by, however, was the fuel. Coal was never cheap, and once the oil burner arrived in the late 1920s, oil heat looked very attractive. The Spanish flu had vanished, and people got on with their lives. Oil was cheaper than coal, and it was an automatic fuel. That made quite an impression on the woman who had been keeping the home fires burning. Imagine, a boiler that takes care of itself! And oil is a lot cleaner than coal. Switch to oil, and there would be less housework. And you can reclaim the space in the basement where the coal bin used to be. Get rid of the coal dust, and your kids would be healthier. Yes, oil was the way to go!

The challenge for the oil dealer, however, was how to size the new oil burner for the old coal-fired boiler. To make the switch in fuels, he would have to remove the coal grates from the boiler, fill the base of the boiler with sand, and then mount the new oil burner on the cleanout door of the old boiler. It was more art than science.

And to make it even tougher, there was no reliable conversion formula to go from coal to fuel oil because the heat value of the coal varied depending on the type of coal and where it came from. The oil dealer didn't want to pick a burner that would be too small to get the job done because then his customer would complain, so he figured in a generous safety factor.

You following this? We begin with a coal-fired steam boiler that can heat the house with the windows open. To this we add 75% because of the husband, and then the oil dealer adds a margin for safety to that.

And that's the way Americans heated for years. Too hot? Open the window. Oil was cheap, and many of those boilers lasted through the 1960s and into the early 1970s.

And then 1973 rolled around, and with it came the first OPEC oil embargo. The price of fuel spiked, and we all sat in long lines at the gas stations, and people began looking for ways to save energy. Those drafty old steam boilers that once burned coal and now burned oil had to go. Many people switched to natural gas in the early '70s, and boilers went in quickly because everyone was in a rush to save energy.

The contractors doing the fuel conversions usually sized the replacement gas-fired boiler based on the size of the nozzle in the old oil burner. But keep in mind how that oil burner got sized as you mull over all of this. And consider, too, that the gasman is also going to add a generous safety factor when he does his sizing. He was concerned because the modern steam boilers were much smaller than the beasts they were replacing. That's part of what made them more efficient. They didn't contain as much metal or as much water. The gasman worried that he might not have enough water in the boiler to keep the boiler running while waiting for the condensate to return from the system. So he usually bumped up the boiler by at least one size.

Okay, let us go over this one more time. We begin with a boiler that can heat the house with the windows open, and if necessary, the roof removed. We nearly double that size because of the husband. Then the oil guy enters the picture and tosses in his safety factors. And then comes the gasman, who does the same.

Which brings us to the 21st century. It is probably time to replace that boiler again. So, how do you feel about sizing a new boiler based on what is in the basement right now?

I would feel the same way.