Do you know how to properly size replacement space-heating steam boilers? They are not sized like other space-heating equipment, to the heat load of the building. They need to be sized to the attached radiation in the system. If the boiler is too big, it will short-cycle, operate inefficiently, and aid in causing an early failure of the boiler. If the boiler is too small, it will fail to generate sufficient steam pressure to carry heat to the radiator and will cause the building to not heat.
In the early days of heating with steam, the units used to measure heat output were “square feet of equivalent direct radiation” or “EDR”. All radiators were rated this way, as well as the boilers. One square foot of EDR equals 240 BTUs at 215 degrees Fahrenheit.
Here is how to properly size a replacement boiler:
First, make a list of all the radiators in the building.
Looking at the end of the radiator, count the number of “tubes” or “columns” in a section.
A cross-section of a vertical tube in a tube-type radiator would be about 1” in diameter.
A cross-section of a column in a column-type radiator would be oval-shaped and about 1” x 2”.
Count the number of tubes or columns, and write it down.
Next count the number of sections, left to right, and write that down.
Finally, measure the height of the radiator from the floor up, and write it down.
Next, go to this link and check out the radiator chart, then do a little math to find the total square feet EDR for each radiator in the building. Total the EDR for all radiators, then find the boiler with the same or next larger EDR size. This will give you the properly sized replacement steam boiler.
Old steam boilers were huge and had a large internal space to dry out the steam before it made its trip through the system to the radiators. New steam boilers aren’t that large, the industry has trended towards smaller and lighter. Since the newer boilers are so small, the manufacturers are depending on the nearby piping to slow down and dry out the steam. So don’t get creative with your piping. Pipe it exactly like the manual says! If it says to use two risers, use two risers. If it says to use a close or shoulder nipple when you connect the return to the Hartford connection on the equalizer, use a close or shoulder nipple. Variations in your piping can lead to problems.
Finally, these systems were designed to operate at low pressures, with a maximum of two psi. Usually, ½ to 1.5 psi will work perfectly. Raising the pressure over 2 psi to correct a problem will almost always create another problem.
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