Hydronic Boiler Sizing & EDR Calculator — Free Online Calculator
Equivalent Direct Radiation (EDR) is the traditional unit for sizing steam and hot-water radiators: one square foot EDR equals 240 BTU/h at standard conditions. When sizing a hydronic boiler, the connected radiation load must be converted to a net boiler output requirement — accounting for actual supply water temperature and the I=B=R heat-up allowance. Enter the radiator EDR and system details below to get the minimum boiler output and oversizing risk flags.
Enter your system details
Read directly from the I=B=R EDR tables. One section of typical cast-iron column radiator ≈ 3–6 sq ft EDR.
From a Manual J load calculation or your heat loss estimate. Used to check oversizing.
Net Boiler Output
Delta T
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IBR Gross Output
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Oversizing ratio
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Warning
See the breakdown
Boiler output calculated using the I=B=R method (Net IBR = EDR × 240; with temperature correction). IBR Gross = Net IBR × 1.15 (15% piping and pick-up allowance). Select a boiler with a gross output rating equal to or greater than IBR Gross.
I=B=R EDR standard ratings (standard conditions 215°F steam / 170°F water)
| Radiator type | Typical EDR per section |
|---|---|
| 4-column cast-iron (tall) | 4.5–6 sq ft EDR |
| 4-column cast-iron (short) | 2.5–4 sq ft EDR |
| Flat panel cast-iron | 1.0–2.5 sq ft EDR |
| Baseboard convector (per ft) | 0.75–1.5 sq ft EDR |
| Fin-tube baseboard (per ft) | 0.5–1.0 sq ft EDR |
The I=B=R boiler sizing formula
The Institute of Boiler and Radiator Manufacturers (I=B=R) developed the EDR method in the early twentieth century and it remains the standard for sizing cast-iron and steam radiation systems. At standard conditions (170°F supply water, 70°F room), one square foot EDR outputs 170 BTU/h for hot water and 240 BTU/h for steam. When supply temperatures differ from standard, the output must be corrected using the temperature-differential exponent (1.3 for water radiators).
What is EDR?
Equivalent Direct Radiation is a unit of radiator surface area normalised to a standard 240 BTU/h per square foot at standard steam conditions. It allows direct comparison of different radiator types and is listed in I=B=R publications and manufacturers' catalogs.
Temperature correction
Modern low-temperature hydronic systems (140–160°F) output significantly less than the standard 170 BTU/sq ft EDR rating. The exponent of 1.3 accounts for the non-linear relationship between water-to-air ΔT and natural convection output. Always derate EDR when designing for low-temperature systems.
Piping pick-up factor
The 15% piping and pick-up allowance (1.15 gross factor) accounts for heat lost from uninsulated supply and return mains during system warm-up and steady-state distribution. I=B=R recommends this for all hot-water residential systems.
Oversizing and short-cycling
Selecting a boiler much larger than the radiation load causes short-cycling — the boiler fires briefly, satisfies the thermostat, shuts down, and repeats. This degrades efficiency and stresses the heat exchanger. The 140% threshold is the traditional I=B=R limit; modern modulating boilers can tolerate larger ratios but still benefit from correct sizing.
Worked examples
Three scenarios showing standard sizing, low-temperature correction, and an oversizing risk case.
Older Victorian home — 800 sq ft EDR, 180°F supply, 70°F room
Result: Size boiler to 156,400 BTU/h gross output minimum.
Low-temperature modern system — 600 sq ft EDR, 150°F supply, 68°F room
Result: Low-temperature design delivers only 71% of standard EDR output — verify this meets the calculated heat loss.
Oversizing risk — 1,000 sq ft EDR system, building heat loss only 80,000 BTU/h
Warning: Exceeds 140% — short-cycling risk; consider modulating boiler or TRVs.
Frequently asked questions
Common questions about EDR, IBR boiler sizing, and hydronic system design.
What is EDR in hydronic heating?
EDR — Equivalent Direct Radiation — is the traditional unit for rating steam and hot-water radiators. One square foot EDR is defined as the surface area that outputs 240 BTU/h at standard steam conditions (215°F steam, 70°F room) or 170 BTU/h at standard hot-water conditions (170°F supply, 70°F room). EDR values for every cast-iron and baseboard radiator type are tabulated in I=B=R publications.
What is IBR Net vs. IBR Gross boiler output?
IBR Net output is the heat delivered to the building's radiation system after boiler jacket losses. IBR Gross output adds a 15% piping and pick-up allowance to cover heat lost in distribution piping and the energy needed to warm the system from cold. When selecting a boiler, the nameplate IBR Gross rating must equal or exceed the calculated IBR Gross requirement.
Why does supply water temperature affect radiator output?
Radiators transfer heat by natural convection, which follows a non-linear relationship with the temperature differential between water and room air (exponent ≈ 1.3). A system running at 150°F supply instead of 170°F sees roughly 72% of standard EDR output — meaning the same radiators deliver significantly less heat. This is critical when upgrading to a condensing boiler designed to run at 140°F.
What causes hydronic boiler short-cycling?
Short-cycling occurs when the boiler output greatly exceeds the radiation's ability to absorb heat — the boiler fires for a very short period, satisfies the thermostat quickly, and shuts down. The I=B=R limit is 140% of building heat loss. Beyond that, short-cycle losses reduce seasonal efficiency and cause thermal stress on heat exchanger sections.
What is the piping and pick-up allowance?
The 15% piping and pick-up factor (IBR Gross = Net × 1.15) accounts for: (1) heat conducted from uninsulated supply and return piping to unheated spaces, and (2) the mass of water in the system that must be heated from cold at each start. For well-insulated modern systems this factor may be reduced to 1.10, but 1.15 is the conservative standard.
Can I size a boiler to the EDR alone without a heat loss calculation?
No. Sizing to EDR only tells you the minimum boiler output needed to keep the radiators hot — it does not verify that the radiator system is sized correctly for the building's actual heat loss. If the radiation is undersized for the building, no boiler size will correct that. Always run a Manual J or I=B=R design heat loss alongside the EDR calculation.
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