Add connected steam radiator EDR or enter a verified total to find the minimum Net AHRI Steam boiler rating needed.
Table of contents
How to use our Steam Radiator EDR and Boiler Size Calculator
- Choose how you will enter connected radiation. Use the known-total path only when a prior survey gives a verified EDR total.
- For the list path, add every connected radiator and record its room, identifying measurements, section count, and either a matching chart factor or known radiator EDR.
- Enter a proposed boiler rating only if you have its published steam rating, then choose the exact Candidate rating unit printed in the literature.
- Click Calculate and select a boiler whose Minimum Net AHRI Steam rating needed is at least the connected EDR shown.
- Sanity-check the result by confirming that every installed radiator is included once and that a candidate at 100 percent or more coverage is being compared in matching net steam units.
Definitions
EDR: Equivalent Direct Radiation. For this calculator, it is the connected radiator capacity stated in sq ft steam.
Connected radiation: All steam radiators, convectors, or other radiation connected to the steam system and included in the boiler-sizing survey.
Net AHRI Steam rating: The published net steam capacity used to compare a boiler with connected radiation for replacement sizing. [1]
MBH: One thousand BTU per hour. A net steam rating of 72 MBH equals 72,000 BTU/hr.
Candidate steam-rating margin: Candidate net steam capacity minus connected EDR. A negative number means the candidate is undersized for the entered radiation.
Common mistakes and quick fixes
Mistake: Entering house floor area instead of Known connected EDR.
Fix: Copy a connected-radiation total in sq ft steam from a radiator survey or list the installed radiators.
Mistake: Adding a known total and also using Radiators.
Fix: Choose one entry path so the same radiation is not counted twice.
Mistake: Using a chart factor that does not match the radiator's height, construction, or section details in Radiators.
Fix: Measure from the floor to the top, count columns or tubes in one section, and use the matching chart factor.
Mistake: Choosing MBH while the proposed number is actually sq ft steam under Candidate rating unit.
Fix: Copy both the rating number and its printed unit from the boiler literature before comparing it.
Mistake: Treating a negative Candidate steam-rating margin as acceptable.
Fix: A negative margin means the candidate is short of the connected radiation by that many sq ft steam.
Mistake: Comparing the Minimum Net AHRI Steam rating needed with boiler input capacity or DOE heating capacity.
Fix: Compare it only with the boiler's published Net AHRI Steam rating in equivalent steam units.
Limitations & Key Assumptions / Boundary Conditions
- The calculator sizes from connected radiation, not heat loss, fuel use, insulation, climate, or house floor area.
- Room-by-room results are only as accurate as the entered section count, known radiator EDR, or chart factor. A factor must match the radiator's construction and height.
- The 240 BTU/hr per sq ft steam conversion is used to show the connected steam radiation load. It is not boiler fuel input, DOE heating capacity, AFUE, or an operating-cost estimate.
- Compare the result with a published Net AHRI Steam rating. Do not substitute Input MBH or DOE heating capacity.
- No additional pickup multiplier is added when selecting against a published Net AHRI Steam rating. Unusual piping, uninsulated mains, system defects, and installation requirements may need a qualified steam contractor's review.
- A candidate rating that meets the connected EDR does not confirm correct piping, venting, pressure control, combustion setup, or safe installation.
Methodology
Calculation method
The calculator uses one of two separate paths. A verified total is used directly, or it adds the EDR from each visible radiator row. A factor-based row equals its section count times the EDR per section from the matching radiator chart. A row with known radiator EDR uses that entered value directly.
Total connected EDR = known connected EDR
Total connected EDR = sum of visible radiator-row EDR values
The minimum published net steam rating is the connected EDR itself. Boiler replacement sizing compares connected radiation with the boiler's Net AHRI Steam rating. [1]
Minimum Net AHRI Steam rating needed = total connected EDR
To display the equivalent radiation load, the calculator uses 240 BTU/hr for each sq ft steam.
Connected steam radiation load = total connected EDR x 240 BTU/hr per sq ft steam
If a candidate is entered in Net Steam MBH, it is multiplied by 1,000 to convert to BTU/hr and then divided by 240. If entered in Net Steam BTU/hr, it is divided by 240. The comparison keeps a negative margin visible.
Candidate steam-rating margin = candidate rating in sq ft steam - total connected EDR
Candidate coverage of connected radiation = candidate rating in sq ft steam / total connected EDR x 100
Worked example
For 300 sq ft steam of connected EDR, the minimum Net AHRI Steam rating needed is 300 sq ft steam. The radiation-load equivalent is 300 x 240 = 72,000 BTU/hr. A candidate rated 325 sq ft steam has a margin of 25 sq ft steam and covers 108.3 percent of the entered radiation.
Calculation choices
The calculator does not add a separate pickup multiplier after finding connected EDR because the selection target is the published Net AHRI Steam rating. It also does not estimate fuel input, seasonal efficiency, or operating cost, because those values do not determine the connected-radiation match.