Steam boiler size close...

Paul Mitchell_2

Figuring out a job for a new steam boiler. The EDR is 419. My manufacturer has one model that does from 283 sq.ft.steam to 446 sq ft steam. The 446 is the max on the firng rate..oil. Next size up gives me 471 on the lowest firng rate and three more gallons of H2O water content.
Any ideas would be appreciated.

Paul Mitchell_2

Also

Book on one calls for a 2" header the other 2 1/2" I do have a three inch main off, not very long, and a 2". Do you think I would be good with a 2" header?

Robert O'Connor_6

I'd

go with the one that meets the load. The house has probably been tightened up some too since the boiler was put in.

Regards,

Robert

ME

Constantin

Second the notion

...even though I'm just a homeowner. Here is why: From what I understand, the boiler is currently being spec'ed for the worst case scenario, a situation where all zones are calling for heat at once. This is unlikely to happen in real life. The last thing you need is a boiler that short cycles because it's way oversized.

I would make a serious effort to estimate the heat loss properly and size according to that, not the radiator ratings. You can always run a radiator at a lower temperature than it was intended to originally when the house was still drafty, uninsulated, etc.

John Mills_3

If you undersize

a steam boiler, you won't properly heat the house. Gotta size to the connected radiation.

scrook_2

steam boiler size selection

EDR is 419 sq. ft. The 283 to 446 sq ft boiler ratings are the net I=B=R steam outputs, right? Those would include the 33% steam pickup factor, so at max fire (about 1.20 GPH?) it would be 6% over requirement -- in theory ideal, but it has been suggested that the actual pickup factor for a steam system *may* often actually be less that 33% so you *may* be able to fire it at less than max. Min firing rate would be about 63% of the max so perhaps 0.75 GPH, definitely too low though.

A boiler w/ net I=B=R outputs of 283-446 sq ft would have a gross outputs (in sq ft) 33% higher or about 377-593 sq ft. Lets suppose it turns out the particular system has a pickup factor (once you get the near boiler piping reinsulated) of 15% (the nominal factor for hot water, BTW, and a little less than half the nominal 33% for steam), the net output for this assumption would be (gross)/1.15 or about 328-516 sq. ft. since you have 419 sq ft radiation then if it is fired at 1.20 GPH max than you could fire it at (419/516)*1.20 = 1.00 GPH for example.

You might find a little more or a little less works best but somewhere between max firing rate and perhaps 0.90 GPH or 1.00 GPH may be where you end up.

OTOH if you were to install the next larger size and min fire it (perhaps 1.25 GPH?) it would already be oversized by at least 12% (not too bad perhaps, but headed the wrong way), so unless you are doubtful about the accuracy of the 419 EDR number, or expect to add a significant forced hot water loop (or more steam radiation) to it for an addition, or need to heat a large indirect hot water tank, I'd go w/ the smaller and run near max as req'd.

Alternately if you have a ssecond brand you like you might look and see if they have a model that is in between the two you site in firing range.

scrook_2

for hot water...

yes, but for steam no. You have to supply enough to satisify the connected radiation! It is a subtle point though, easily overlooked so don't feel bad.

Paul Mitchell_2

Sorry...

It is steam and not hot water....gotta go by steel.

Paul Mitchell_2

Thanks...

appreciate your input. I was thinknig along those lines. The factor is in the figures and mains are insulated...not totally great but much better than most. Leaning towards the 3 section boiler.