Estimating a new steam boiler

Ted Robinson

My old Am Std A-3 boiler is rated at 650 sq ft.
I have calculated the actual radiator sizes as totaling 420 sq ft (100,800 equiv BTU). Does this sound about right for a 2100 sq ft detached house on LI? Last year I abandoned the hot water coil - it was leaking.
I am considering a new oil boiler, partially on the basis of lower total oil useage. What size should the new boiler be? What additional factors should be added for a hot water coil?
Am I missing anything?

Steamhead (in transit)

Size the boiler

to the connected load- 420 square feet.

If your boiler still has an old-style burner in it, and you're not ready to replace it, consider upgrading the burner and firebox. I did this on an A-34 last fall and along with some other upgrades it saved the lady 36% on her oil, plus less electricity from not having to use electric room heaters.

Ted Robinson

Old boiler

The boiler has a 'modern' Carlin burner, electronic controls and igniter. It is underfired with a 1.35 nozzle.

Steamhead (in transit)

Did they replace

the firebox too? Some of those A-3 boilers had stainless steel fireboxes which don't work well with flame-retention burners like your Carlin. The one in the A-34 that I upgraded was falling apart, even with an old-style burner.

Your burner isn't "down-fired" as much as you think. With a rating of 650 square feet your boiler model is the A-36. With an old-style burner it had a firing rate of 2 GPH. Since a flame-retention burner makes a hotter flame, it produces more BTUs from a given amount of fuel. This allows the firing rate to be reduced about 20% while still providing the same heat to the boiler. In your case, the 20% reduction from 2 GPH results in 1.6 GPH.

Your current rate of 1.35 GPH would be a 20% reduction from about 1.7 GPH. The next smaller boiler size, the A-35, had just this same firing rate, and an output of 541 square feet EDR. So your current output is somewhere around 541 square feet. Thus, your boiler is still oversized.

You can probably tell that I happen to like the A-3 boilers. They're very well made, and I've been able to get better efficiencies from them by putting in flame-retention burners and new fireboxes. Nonetheless, if you replace your oversized A-36 with a properly-sized boiler you will see some savings in fuel consumption.