Boiler size

T. Riley

Here is the situation:

The installed furnace(from the early '70's), has an A.G.A. input rating of 525 MBH, an output of 420 MBH and a net I=B=R rating of 315.1 MBH. (Calculates to a 1.33 pick-up factor). There are dual gas inlets. One of the inlets has been shut off for years. The pressuretrol is set for a cut-out of about 1 lb. and a cut-in of 1/2 lb.

Calculation of EDR load from the radiator sizes gives 1,156 sq. ft. On the coldest days of the year, the house heats easily. The inlet valves on the radiators are adjusted to keep most radiators heating no more than halfway across during those cold days. All of the most distant radiators heat well. The firing rate (with only the one inlet in use) checks out to approx. 267,000 btu/hr over a six minute period. All this tells me that the true EDR is a lot less than the 1,156 sq. ft. and that the old boiler is very oversized.

As I see it, the boiler, operatiing at an INPUT of 267 MBH
(the measured firing rate), heats all that piping and radiators just fine. In my mind that equates to a boiler with a net I=B=R rating of 160 MBH.

Am I wrong?