Undersized Boiler?

OutMart

Recently I had a new boiler, Weil-McLain 980, installed replacing a Smith Series 19A -08.
I was told by the Super that the boiler short cycles when heating water. He noted the times one afternoon, the boiler would go on and off for minute or two while heating the water this was repeated nine times in one two hour period that he observed.
I contacted the plumber who installed it. He went to look at and told the Super this was normal; the new boiler was more efficient. When I spoke to him, he told me maybe usage was high at the moment. This never happened with the old boiler as far as I know.

I found specs for Smith 19A- 08 and comparing those numbers with McLain 980:

Gross Output Smith: 1063MBH, Net Steam (Sq. Ft.): 3321, Net Steam: 797MBH, NetWater: 924MBH, Boiler Water/Steam Content: 91.5 gal(W), 72.6gal(S)

Gross Output McLain: 991mbh, Net Steam(Sq.Ft.): 3096, Net Steam: 743MBH, Net Water: 862MBH, Boiler Water/Steam Content: 106.5 gal(just this number given).

Looking at these numbers it seems as if the new one is undersized?

LRCCBJ

The new one is not undersized. Just forget about that. If it was undersized, it would run nonstop and never shutdown.

It appears that it is cycling on pressure. You need to confirm.

If it does cycle on pressure, either it is OVERSIZED (doubtful since is is smaller than the Smith) OR you do not have enough venting of the mains.

Look at the mains and advise what you have for main vents. I'll bet they are insufficient.

mattmia2

looking at those number it looks like they are very nearly the same size. short cycling would be caused by being oversized among other things like poor venting if it is steam

bburd

@OutMart The super reported that the boiler short cycles "when heating water". Assuming that the same boiler heats domestic hot water for the building and provides space heating as well, shorter cycles are normal because the new boiler probably has less metal mass and water content than the older one.

Short cycling while there is a call for space heating is a different issue. Most steam boilers are at least somewhat oversized for the load, especially in multi unit buildings where some radiators may be turned off at times. A certain amount of cycling on pressure is to be expected.