Staggeringly basic boiler question.

Chapman Flack

I'm sure this either is a FAQ, or it should be....

When I look at a gas forced-air furnace, there are two ratings, input and output. That, I can get my mind around. Here I am looking at the specs for a gas furnace with a 60,000 btu/h input rating. The output rating, 55,000 btu/h, is how much the house gets. The rest goes up the flue. 55,000/60,000 = 92%, exactly the efficiency advertised at the top of the brochure. This makes sense. I like when things make sense.

Now I'm looking at my friends' boiler. The label has a gas ratings section, where it says gas input 125,000 btu/h, output 100,000 btu/h. So far so good, this makes sense, that's 80% efficiency.

But then there's this other label section that says I-B-R rating, hot water, 75,000 btu/h. What does this mean exactly? Is the boiler losing *another* 20% of the input heat somewhere? And if so, where? Where's it going? Or does this number tell me something else? If so, what?

How much heat goes IN is a simple concept, I can clock it on the gas meter. How much heat gets used for keeping the rooms warm is another simple concept, and the difference between those two ought to give the efficiency, to my way of thinking.... I don't yet understand why the boiler has *3* figures.

-Chap

Wayco Wayne_2

I know this one

The IBr rating takes into consideration the heat loss off of the pipes and losses from the boiler jacket. The insulation of boiler jackets are very much improved over the old days and if you insulate you pipes youcan improve the output also. It also strikes me that if your boiler and pipes are located in the house you're not losing the heat totally as it is still in the house. If the mechanical room is remotely located the IBR rating is more significant. WW

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Steve Ebels

Another way of saying it..........

Would be this. If a forced air system were rated in the same way, you would see a third rating that accounted for the heatloss of the attached duct system. This would take into consideration the amount of heat lost through tranmission of the duct itself and air leakage of the duct joints. This percentage would be substanially higher than the difference between DOE and IBR on a boiler.

There is still the debate, as noted above, that heat lost in either case is not really lost. It's still in the building envelope just not where you want it. Unless it is insulated and sealed, a duct system will always lose 15-20% more heat in this manner than a hydronic system.

Consider that a 1" pipe will carry the same BTU's as an 8 x18" duct. Now consider the surface area of the two and you can soon realize why the duct will lose far more than the pipe.

Along with that it must be noted that it will take about 3-4 amps of electricity to move the air as opposed to maybe 3/4 of an amp to do the same in the pipe. Water "carries" much more heat by volume than air, which is a very poor conductor of BTU's.

Just food for thought.. Enjoy

Paul Bock_2

the piping and jacket

losses are usually what keeps the basement above freezing. if you do an IBR heat loss, and the boiler is in the basement, you can ignore the basement,and not have to use the "cold floor" factor for the main level. Paul.