OVERSIZED OR NOT

FLYFISHER · July 2010

I am in the process of estimating a replacement steam boiler in a big old house in NH. I have been doing this for thirty years but this one has me doubting myself.

The existing boiler is a Weil Mclain Model B773-W The rating plate indicates the capacity of the boiler is 1570 sq ft and 436.5MBTU. The boiler has been converted from oil to gas with a midco gun burner.

My dilemma is the radiatiors connected to the system total 1008 sq ft or 241,920 btu orcalculated using the Sterling Cast Iron Radiator conversion reference book.

In the forty years I have been installinh I have seen boilers oversized before but not to this extent.

There was an old oil burner cleaning tag from 1968 that noted a 3GPH nozzle

I think I should be safe with a new Weil Mclain EGH-125 which would still be slightly oversized.

Suggestions or moral support would be appreciated

Robert O'Brien · July 2010

The only

The only piece of information you need is the sq/ft of radiation,the existing boiler,firing rate etc. is meaningless. If you aren't sure measure the radiation again and then put the right size boiler in. 50 % oversized is not at all unusual

Steamhead · July 2010

With that amount of radiation

and using a Weil-McLain, I'd go with the 4-80 and a power gas burner.

The 80 series W-M is a wet-base boiler that can capture more heat from the flame, unlike the EGH series which, like all atmospheric gas boilers, loses a lot of heat out of the base. Also the power burner needs less excess air than the atmospheric for a clean flame. Together these add up to a better thermal efficiency.

Piping up a 4-80 is also easier than the EGH. You only need one riser to the header rather than two. Sure, it's a 4-inch riser and header, but it's relatively easy to build one from pre-cut nipples.

The 4-80 has a rating of 1238 square feet, still a bit bigger but you can drop the firing rate a bit to compensate.

Now, how did that 7-73 boiler get so big?

When these systems were first installed, the lady of the house tended the boiler. Since she didn't want to shovel coal so often, the Dead Men installed boilers with larger fire-pots- and larger outputs. This also led to much air pollution from those slow-burning fires.

When it came time to replace these boilers, some contractors simply installed the same size boiler without checking the actual load. Dan calls this the "label method". So a boiler 50% oversized is really no surprise.

OVERSIZED OR NOT

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