Mankind has been heating water for a long time and the boiler industry in particular has been around for a few centuries. Like most things over time, we have managed to learn and improve the product. As the industry went along it developed terminology to describe the equipment and its actions. Now I remember a youngster talking about car engines with my buddies and we were all bragging about our engines sizes in cubic inches. My kid does the same thing with his buddies today but they talk about the size in terms of liters. The engine basics are the same, just the unit of measurement changed. Our boiler industry also uses different terminology today than it did yesteryear and when we talk about capacity, there’s probably a whole lot more terms then we really need. For example, I went to a job site the other day to help evaluate the heating system for a new boiler. I met with the utility guy, the consulting engineer, and the old time contractor. The three were engaged in a spirited discussion when I arrived tying to establish what capacity the new boiler needed to be.
The problem was the utility guy was talking therms input, the engineer was talking gross output, and the contractor was talking square feet. Now I’m no encyclopedia but after a “few” years you pick up some things. I made a few quick conversions for these folks and in the end all three were really talking about the same capacity unit. Now you may think this is easy stuff but you’d be amazed how many people have a problem with this. Just when I thought I was a smart guy that day, I got back to the office and some guy wiser than me sent in a request for a boiler quote and noted the required output in Kilocalories. After looking at that I remembered it was past lunchtime.
To make this whole lingo thing easier I thought I’d make up a few simple tables to give cross-reference data. Now, before you jump down to the tables we need to discuss a few things. Number one, when we are dealing with boilers that use a fossil fuel (gas, oil, etc.) as an energy source, you need to understand that the device is not 100% efficient. By that I mean all the energy that is contained in a given part of fuel is not completely available as usable output. Most boilers today are able to convert 80% or better of that input energy into usable output. That 20% or less that isn’t usable is lost in exhaust energy up the stack or radiation losses through though the boiler jacket into the surrounding space. Now if the boiler is located in an area that you have to heat anyway, the jacket losses don’t really count. In my house I utilize the jacket losses by placing my shoes on top of the boiler’s jacket. Nothing like a warm pair of shoes to slip into in the morning. Anyway when we look at a boiler we have a fuel input value and a boiler output capacity value. Number two, the industry tosses us a curve ball with output values by listing a gross and new number. The best way to think of the gross value is if you measured the energy output right at the outlet of the boiler with nothing connected to it. This is the maximum or gross output that the boiler can produce. Now some smart guy’s years ago realized that all the gross energy produced by the boiler is not available for the radiators. Just like the jacket losses of the boiler, the piping also has radiation losses. You can heat up your shoes on the piping also, especially if the piping is not insulated. SO the net boiler rating is simply a factor applied to the gross output that allows for piping losses and pickup. What’s pick-up? You want the boiler not only to be able to be able to produce enough energy to equal the heat load but exceed that so you can heat the building from cold conditions. So these guy’s years ago came up with an allowance of 1.33 for steam boilers and 1.15 for hot water boilers. Tip; use the net ratings when you are sizing up a job.
One last thing, in order to make the table information mean something we need a reference point of energy. I used the most common term for this reference “BTU”. This stands for British Thermal Units. Look it up if you want but it’s defined as the amount of energy required to raise one pound of water, one-degree Fahrenheit at sea level. Have fun folks.
|TABLE A – FUEL INPUT DATA|
|FUEL TYPE||APPROXIMATE ENEERGY VALUE|
|NATURAL GAS||1,000 BTU’S PER CUBIC FOOT|
|NATURAL GAS “THERM”||100,000 BTU’S|
|PROPANE||92,000 BTU’S PER GALLON|
|MANUFACTURED GAS||525 BTU’S PER CUBIC FOOT|
|#2 FUEL OIL||140,000 BTU’S GALLON|
|#5 OR #6 OIL||150,000 BTU’S PER GALLON|
|COAL||13,000 BTU’S PER POUND|
|WOOD||14,620,000 BTU’S PER KILOWATT HOUR|
|ELETRIC||3,413 BTU’S PER KILOWATT HOUR|
|TABLE B – OUTPUT CAPACITY DATA|
|TERMINOLOGY||APPROXIMATE ENERGY VALUE|
|1 BOILER HORSEPOWER||33,475 BTU’S|
|1 MBH||1,000 BTU’S|
|1 POUND OF STEAM||970 BTU’S @ 212 DEGREES F.|
|1 SQUARE FOOT OF STEAM (EDR)||240 BTU’S – EQUIVALENT DIRECT RADIATION|
|1 SQUARE FOOR OF WATER (EDR)||150 BTU’S – EQUIVALENT DIRECT RADIATION|
|1 KILOWATT HOUR||3,413 BTU’S|
|1 KILOCALORIE||3.9 BTU’S|
|1 KILOJOULE/M (3) @ 15 DEGREES C.||.0268 BTU’S PER CUBIC FOOT|
|BEING ABLE TO REMEMBER ALL THIS||PRICELESS|