Don't Size Heating Equipment with Rules of Thumb

rules of thumb sizing

The World Headquarters of, our humble company, used to be in a tiny store in beautiful downtown Bethpage, here on the Isle of Long. We then moved to another, larger, store, this one just around the corner. It's was a much nicer place.

The new landlord was so happy to get us as his tenant that he decided to have the storefront rebuilt by a guy who was a professional in such matters. The old storefront had a big air conditioner hanging over the door. You know the kind that drips on your head when you enter? I mentioned to the landlord that it was important for the new storefront to also have a space for an air conditioner (one with a drain line). He said that this would not be a problem. The storefront professional was a friend of his, and he really knew his stuff.

So we trusted these guys and went off on a seminar trip. When we returned we found the new storefront in place. The professional had left me a bit less than 10 vertical inches in which to install the new air conditioner. That's a mighty skinny air conditioner, I thought, and this thought was confirmed when I went to the local appliance store to seek the advice of yet another professional.

"How big is your new office?" the professional salesman asked. I gave him the dimensions and he tapped the numbers into a calculator, along with a rule-of-thumb constant that I'm sure he feels has served him well over the years. I saw the total come up as 28,000. "You need thirty-thousand Btus," he said.

"It said twenty-eight thousand on the calculator," I said.

"We round it off," he said. "Never hurts o have a bit extra, especially in this hot weather."

"Oh," I said.

"But none of that matters because the unit you need won't fit in the hole you have over that new door. That guy who installed your storefront was a moron. I hate to tell you this, pal, but you're screwed."

Salespeople on the Isle of Long can be that charming.

"That leaves me with a window unit?" I said. "What's the biggest one you have?"

"We got fourteen-thousand Btus. That's it, but it ain't gonna be enough. You'll need two of them. Got two windows?"

"Yes," I said.

"Want me to write it up?"

"I'll get back to you," I said.

I went home and figured the load myself. Our store has an apartment above us, and there are stores on either side. We also have an old-fashioned canvas rollout awning over the front window, the sort they used in the days before air conditioning. It does a remarkable job of cutting the heat gain, and that's why so many modern European buildings (owned by folks who are very conscious of energy) have metal shades or awnings on the outsides of their windows. If the direct sunlight can't get in, the place stays cooler. Smart.

I did the calculations and came up with 15,000 Btus, half of what the guy in the store insisted I needed. So much for rules of thumb. I went to a different store, bought a 14,000 Btu window unit, stuck it in the window and the elves have been cool and happy ever since.

Since then, I've been wondering about all the consumers who don't have access to heat loss/gain software, and who don't really understand how air conditioning works? They trust the guy in the store. And I wonder what this does to America's energy usage. What if everyone in the country eventually winds up with an air conditioner that's twice as large as it needs to be? Even if it has an Energy Star rating, it's still twice as big as it needs to be. That can’t be very efficient.

And how about boilers? Some years ago, a guy asked me to look at the heating system in his house. He had just bought the place. It was in a northern New Jersey town where money goes to live. This house was about the size of an elementary school, and it had perched up there on the hill for more than 100 years. It had gravity hot water heat, and the horizontal pipes in the basement were eight inches in diameter. And they were screwed pipes. Imagine that. I thought about the Dead Men who had built this system, and I wondered how big their wrenches were, and where the heck they had stood to get the proper leverage. How do you catch a thread on an eight-inch screwed pipe that goes from here to the other side of the basement?

The American Radiator Company had made the boilers. They had originally fired coal but had long ago been converted to burn oil. There were two of them, and each fired 500,000 Btuh. Keep in mind this is a single-family home.

I asked the owner if he was going to have the boilers changed and he told me that he would like to do that because the oil truck drove up his long driveway every ten days or so to make a delivery. This was understandable since the draft through one of these old boilers is enough to blow off your hat. You could yodel inside one of these things.

"Has anyone done a heat loss calculation on your home?" I asked.

"Yes," he said, and he showed me a quote for a single boiler, sized at a remarkable 1,300,000 Btuh. "The guy wrote down what was on each boiler and then gave me this."

"Did he go upstairs?" I asked.

"No. He said it wasn't necessary. If these boilers have been here for a hundred years they must be right. That's what he said."

"And he told you that you needed just one boiler?"

"Yes, he said that two boilers can be a problem because there’s twice as much stuff to break down."

"But each boiler that you have now has a rating of a half-million Btus," I said. "What's the deal with the extra three-hundred thousand Btus?"

"He said that it never hurts to have a little bit extra, especially in this cold weather."

So I did a heat loss calculation and came up with 375,000 Btuh on the design day. Guess what? One of those old boilers was a standby to the other. People who are wealthy get to have as much redundancy as they'd like.

And why were both boilers firing? I figure it was because someone doing service on a cold day years ago decided to run both boilers. What the heck. And besides, it doesn’t take long for the abnormal to become normal around here.

So I specified two, 175,000-Btuh boilers for the guy and explained about the joys of outdoor reset when you have to deal with an old gravity hot water systems. I also told him about the wonders of primary-secondary pumping.

He had the system installed that way, and for most of the following winter he heated his castle with a single 175,000-Btuh boiler. When it got really cold, the other one kicked in to help out.

The knucklehead who wanted to sell him 1,300,000 Btus worth of boiler didn’t get the job.

Rules of Thumb come with a heavy price, and it's usually the consumer who pays.


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