TryBuildCalcCondensing Boiler Radiator Sizing Calculator (Low-Temperature Output Check)
Check your radiator output at low temperature instantly.
🕒 Last updated: September 19, 2026
Inputs
ℹ️The AVERAGE of supply and return, not the supply temperature alone — e.g. a 170°F supply with a 150°F return averages to 160°F. Using supply temperature by itself overstates the radiator's real output. Output also rises steeply, not linearly, with this temperature — a modern condensing/modulating boiler run at a low average can cut output well below the classic 170-180°F design rating.
ℹ️Typical planning values at a common ~26 in height — actual EDR varies by manufacturer. Use Custom if you know your radiator's real EDR per section.
ℹ️The overall height, floor to top — including the legs/base, matching how EDR reference charts and manufacturers specify nominal size. EDR per section is scaled from a 26 in reference — an approximation, not a verified per-height table.
ℹ️Count the individual cast-iron slices — each looks like one slice of a loaf of bread. Whole sections only.
ℹ️A room's required heat output, from a load calculation — enter this to check whether the radiator above meets it, and how many sections would.
Heat Output
1,804 BTU/hr
30 sq ft EDR × 60.1 BTU/hr per sq ft (Hot Water at 120°F)
Target: 6,000 BTU/hr — this radiator does NOT meet that target. Required EDR is 99.79 sq ft — at 4-tube, 26 in, that's at least 34 sections.
Radiator
Type: 4-Tube, 26 in
Sections: 10
EDR per section: 3 sq ft
Total EDR: 30 sq ft
System
Type: Hot Water
Average water temperature: 120°F
Output rate: 60.1 BTU/hr per sq ft EDR
Assumptions Used
EDR (Equivalent Direct Radiation) is the historic cast-iron radiator sizing standard: 1 sq ft EDR liberates 240 BTU/hr at ~215°F/~1 psi steam and 70°F room air (EDR's own defining condition, confirmed consistently across every source checked). For hot water, output per sq ft EDR follows a power-law curve — BTU/hr = C × (average water temperature − 70°F)^1.3 — using the EN 442 "characteristic curve" exponent industry-standard for panel/cast-iron radiator output (typical range 1.2-1.35), with the constant C calibrated so the curve passes exactly through the universally- agreed 240 BTU/hr steam condition. This average water temperature is the mean of supply and return, not the supply temperature alone. When EDR is determined by tube count and sections, the EDR-per-section figures are typical planning values at a common ~26 in height — real EDR varies meaningfully by manufacturer and casting design (confirmed by professional discussion of this exact issue, since the old IBR independent-testing program was discontinued and manufacturer ratings diverged afterward) — height scaling from the 26 in reference is an approximation, not a verified per-height table. A known actual EDR (nameplate, catalog, or documentation) is always more accurate than these planning defaults. Required EDR and required sections (when a target heat output is entered) are rounded UP, never down, so the displayed figure is never below the true requirement. This is a planning estimate, not a substitute for a licensed heating contractor's design or a specific manufacturer's published rating.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Cast-Iron Radiator EDR Sizing Calculator.
Condensing boiler retrofit sizing check
Running a condensing or modulating boiler at a low average water temperature (to maximize condensing efficiency) can cut an old cast-iron radiator's real output to well under half its classic 170-180°F design rating — this page helps check that real, lower output against your room's actual required heat.
Enter your system's AVERAGE water temperature, not its supply temperature alone — roughly the midpoint between supply and return. This page defaults to a 4-tube, 26 in, 10-section radiator at a 120°F average (e.g. a 130°F supply with a 110°F return) against a 6,000 BTU/hr target — edit the inputs above to match your actual retrofit plan.
EDR Formula: How Is It Determined?
Every calculation starts from the radiator's total EDR, then applies a system-specific BTU/hr rate.
Total EDR (By Tube Count & Sections)
EDR per Section (sq ft) = Typical Value at 26 in × (Actual Height ÷ 26)
Total EDR (sq ft) = EDR per Section × Number of Sections
The typical EDR-per-section value depends on tube/column count — a higher tube count packs more radiating surface into the same section. Height scaling from a 26 in reference is an approximation, not a verified per-height table. A known actual EDR (from a nameplate, catalog, or documentation) is always more accurate.
Heat Output — Steam
BTU/hr = Total EDR × 240
Steam radiators run at a fixed, well-defined temperature (~215°F at typical low residential pressure), giving one universal rate.
Heat Output — Hot Water
BTU/hr per Sq Ft EDR = C × (Average Water Temperature − 70°F) ^ 1.3
Output rises with a power (not a straight line) of the temperature difference above room temperature — the 1.3 exponent is the EN 442 "characteristic curve" value used industry-wide for panel/cast-iron radiator output (typical range 1.2-1.35), with the constant C calibrated so this curve passes exactly through the universally-agreed 240 BTU/hr steam condition. Average water temperature is the mean of supply and return — using supply temperature alone overstates the result.
Required Sections for a Target Output (Optional)
Required EDR (sq ft) = Target BTU/hr ÷ BTU/hr per sq ft EDR
Required Sections = ROUND UP(Required EDR ÷ EDR per Section)
Rounds up, never down — a partial section isn't purchasable, and rounding down would undersize the radiator relative to the target.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value |
|---|---|
| System type | Hot Water at 120°F |
| Tube/column type | 4-Tube |
| Height | 26 in |
| Sections | 10 |
Step-by-Step Calculation
| Step | Calculation | Result |
|---|---|---|
| EDR per section | typical × (26 ÷ 26) | 3 sq ft |
| Total EDR | 3 × 10 | 30 sq ft |
| Heat output | 30 × 60.1 | 1,804 BTU/hr |
| Required EDR for target | 6000 ÷ 60.1 | 99.79 sq ft |
| Required sections | round up(99.79 ÷ 3) | 34 sections |
Therefore, this radiator has 30 sq ft EDR and delivers 1,804 BTU/hr at the specified conditions. Against your 6,000 BTU/hr target, this radiator falls short — at least 34 sections would be needed to meet it.
Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.