To make thing even more confusing, some of the ratings are shown as BTUH (British Thermal Units per Hour), while others are listed as Square Feet Equivalent Direct Radiation, or EDR for short (and here you'll find different numbers for water and steam boilers). Then we have this other column for Gallons Per Hour and another for Therms.

Okay, here's what's going on.

You have three basic columns. First there's Input. That's where you'll probably find the ratings in Gallons Per Hour or Therms because this column has to do with fire. What you're seeing here is the amount of heat that the fire is putting into the boiler. You PUT the fire IN and that's why they call it Input. Simple, right?

But, not all the heat that goes into the boiler winds up in the water. Nope, some of that heat goes up the chimney and is lost forever. There's also some more heat lost through the boiler's jacket, but this is one of those vague areas because if the boiler is inside the house can we really say that the jacket losses are gone for good? And there are some boiler manufacturers who will tell you that their jacket insulation is so fabulous that BTUs hardly ever chose to leave that way.

Next, we get to Gross Output. Gross is what's left over after the boiler has suffered the loss of heat up the chimney and through the. Now, this term can be a bit confusing because Gross usually implies that you're dealing with the whole enchilada, as in Gross Income (which means before taxes, right?). But in the World of Hydronics, Gross means, "what's left over" instead of "what you start with."

Or to put it another way, Gross means "after taxes." "Taxes," in this case, being the price you pay when you send some heat up the chimney and through the jacket. I know this doesn't make much sense, but if it did, everyone would be doing it, so look at the bright side. Just remember this. Gross Output is the amount of heat that rides on the water that's flowing out of the boiler. It's the heat that's available to the whole system (and maybe that's why we call it Gross).

Ready for the next factoid? Here goes. The difference between the Input and the Gross Output represents the combustion efficiency of the boiler. For instance, if a boiler has an Input of 200,000 BTUH and a Gross Output of 160,000 BTUH, that boiler would be running at 80% combustion efficiency. It's not hard to figure this out. Just divide the big number into the little number and then multiply the result by 100 to get a percentage.

And this brings us to Net Output. "Net" is what you're left with after taxes, right? What's important to know here is that Net Output is always going to be less than the Gross Output because there are two things going on out there in the system.

First, we have the piping losses. By this, I mean that it's going to take a certain amount of heat to bring the pipe from room temperature up to the temperature of the water that's flowing through the pipes. And this is where things can get a bit sketchy. Are those pipes inside the living space? And if they are is that heat really lost? Are those pipes insulated? And if so, how insulated are they? Is there 180°F (82°C) water flowing through that pipe out to baseboard convectors? Or is there 110°F (43°C) water running through that pipe to a radiant panel? All of these variables should make a difference, shouldn't they?