heating cap. vs. net IBR

Brad White_134

If you are talking steam, your fuel input divided by your combustion efficiency is your gross output. The I=B=R rating de-rates that, dividing it by 1.33 to account for piping and pick-up from a cold start. In other words the I=B=R rating is about 75% of the gross.

Here is an example: The boiler of your desires has a fuel input of 125 MBH (125,000 Btu's per hour). The combustion efficiency is 82.5% so your gross output is roughly 103.1 MBH. The concurrent I=B=R rating would be about 77.5 MBH.

This boiler puts your 86 MBH radiator capacity (358.3 SF EDR is that right?) between these numbers.

If you are confident that your piping is well insulated and in a warm place (not running through an unheated garage for example) you may favor your selection closer to the gross output number. It is conservative.

By the way, I=B=R ratings for hot water boilers are similar but use a 1.15 divisor versus the 1.33. I presume this is because there is residual heat and mass in the water-filled piping; the steam always starts from air-filled pipes near room temperature.

For much more information, here is something you may find helpful, from Mr. Holohan himself:


http://www.heatinghelp.com/newsletter.cfm?Id=64

Scott B.

Can someone explain the difference between the NET IBR and Heating Capacity ratings on a boiler. If i size a boiler based on the cast iron radiation and I need 86,000 btu's, which rating do i use and why?

Thank you Thank you Thank you,

Scott