Sizing Boiler to Radiators

Gilbert Buras

My contractor and I need experienced advice for one of the very few residential steam system in New Orleans.

EDR chart says the 9 radiators will produce apprx 102,000 btu/hr. Single pipe, gravity steam system.

Old boiler (1923 Redflash) has 300,000 btu nat gas input. Distributor wants to size new boiler by gas input. This is dead wrong, correct?

For reference, my contractor says 125,000 btu/hr forced air furnace would be adequate for house (1,800 s ft).

From reading this site I understand it is better to slightly undersize rather than oversize a boiler.

Questions: Is there a rule of thumb to use to account for heat loss from (uninsulated) supply pipes? 10%/20%/30%?

Is 157,000 btu/hr way too big or would a 120,000 btu/hr be about right? Consider that New Orleans never has more than about 3 consecutive days with below freezing weather.

Is comparison to a forced air system a legitmate way to guess at the boiler necessary?

Thanks for whatever help you experts can provide. I have learned a lot on this site.

John S.

A steam boiler

should only be sized according to it's connected load. This would be all the radiators in the house. The pipes should be insulated, however, if they're not, a greater piping "pickup" factor could be added to the load to accomodate them. Pick up a copy of the "Lost Art of Steam Heating". It will provide you and your contactor with the knowledge and confidence to properly size a replacement boiler.

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thfurnitureguy_4

Gilbert, jus fix yo sef som o dat good boudan wit the hot sauce and save the steam for the craw fish! Naturally that was a poor imatation. This system will drive you nuts if you don't have it balanced, insulated and sized right. Most of your days will heat part of the radiators and your run times will be short. Insulate the mains and risers. Do the calc for your main vents (This is vital) and size to the radiation. The new boiler should have a Net I=B=R rating. This will give you the size of the radiation it will handle, including a normal pick up load. They assume covered pipes of normal lengths. Don't undersize and don't go way over either. look up the EDR of each of your radiators and know the actual load. This is the only safe way to do this. Best wishes to all in the Southland

Gilbert Buras

Boiler Size

We berl dem crawfish, too - that's why nobody down here knows anything about steam. I calculated the total surface area of the uninsulated pipes (100 sq ft - my uncle tells me that my grandfather removed the insulation in 1940 to heat the basement!) and that added 24000 btu/hr which makes the boiler sized around 125,000 btu/hr with a 200,000 btu/hr gas input, which considering the 1923 boiler was being fired with 300,000 btu/hr burner sounds just about right given the higher efficiency of a modern boiler.

I also understand from reading this site that these systems were almost always oversized to account for open windows.

If I'm wrong about the boiler size, I don't think I'm off by much.

I intend to change all of the radiator air vents (they're the originals) but how to I size the main vents? If I understand the principles right, getting air out of the system and steam into it is the key to quick warm-up and efficiency.

All of the shut off valves work freely so I assume there is no profound rust and sediment problem, but while I have the air vents off and the boiler out, is there some recommended cleaning method?

Next problem is getting a unit down here. The distributor down here says 14 days.