This is an interesting discussion.

Regarding the email below, I think that's a reasonable approach. I also agree with the previous posts in this thread.

To add some more to this particular discussion ... we know the two efficiencies are not the same, but I agree, getting good data on jacket/skin/radiation/convection (whatever you want to call them) losses or thermal efficiency values is sometimes difficult to get.

Sometimes, we get lucky and it's available. For example, Cleaver Brooks publishes a value of 0.25% of full load for the CBLE fire tube boiler product line.

However, I can see those losses being much greater than that for different boilers, depending on the specific boiler design.

Also, for hot water boilers, you may get lucky and find both the Combustion Efficiency and the Thermal Efficiency in the following AHRI publication (there may be a more recent version available, not sure):

[cid:image001.png at 01CE4B3C.1B9EDC80]

[cid:image005.jpg at 01CE4BCD.1DA0F630]

I think you can see from that publication that the difference between thermal and combustion efficiency is sometimes significant, depending on the specific make and model of the boiler.

Also, you can check the AHRI online product directory

http://www.ahridirectory.org/ahridirectory/pages/cblr/defaultSearch.aspx

Here's an AERCO example ... these seem reasonable; the Thermal Efficiency values are <= Combustion Efficiency values [cid:image007.jpg at 01CE4BCD.1DA0F630] Here's a Fulton example [cid:image010.jpg at 01CE4BCD.1DA0F630] Some of these are not making sense, since Thermal Efficiency is > Combustion Efficiency for several of the entries

We know (or at least I thought we knew) that thermal efficiency should always be less than combustion efficiency, never greater, since thermal efficiency includes the flue losses plus the jacket losses. If that's true, then why does both the online directory and the publication show thermal efficiency values that exceed the combustion efficiency values ???

Perhaps AHRI has a different definition of thermal efficiency versus combustion efficiency ? Or there are misprints in the AHRI online directory and publication ?

Interesting ...

It also doesn't make sense to me that ASHRAE 90.1 calls for Et for small boilers, but Ec for larger boilers. So we ignore the jacket losses for larger boilers, even though they are much greater? I think the issue is that the AHRI IBR testing (i.e. ANSI Z21.13) only covers boilers up to 2,500,000 Btu/hr, then it's the wild west after that, unless the larger boilers are covered by an ASME standard (not yet reference by ASHRAE) ?

One last interesting piece of info ... the AHRI IBR ratings per ANSI Z21.13 are based on entering water temp conditions of 80 degrees, even for non-condensing boilers, even though operation at that EWT condition would void the boiler warranty and destroy the boiler (i.e. condensing liquids dropping out in stack gas, then rusting out the stack and boiler). I've always been a bit curious as to why boilers are rated at conditions that they cannot operate at.

Any feedback on this would be appreciated.

Thanks! :)

Regards,

JAH

James A. Hess, PE, CEM, BEMP