Steam Boiler Sizing

ttekushan

The 1000 sf boiler is the one to use. Its very little oversize percentage-wise. Many suggest going a little over this as well if there are any unusual piping arragements (meaning long lengths, etc.). but from what you describe the 1000 should be OK.

The smaller one would be a huge mistake. Steam is a different animal from water or air. You need the headroom to heat the piping and purge the air. The undersizing results in very excessive warm-up times and poor heat distribution. Think about this: if you downsized a steam boiler to the point where there was no headroom for purging and distribution, the boiler would fire indefinitely without heating up a single radiator. Efficiency=0 in this instance. I've seen undersized boilers and they cost a fortune to run.

[Deleted User]

Steam Boiler Sizing

I am currently looking at a house that has 960 sq ft of connected cast iron radiation. The old boiler, circa 1959, has 325,000 BTU input. A new 350,000 btu input boiler covers 875 sq ft cast iron radiation, a 400,000 btu input boiler covers 1,000 sq ft cast iron radiation. The 3" steam min pipes are insulated. What input size would people recommend?