Room-by-Room Radiator Schedule
| Room | Heat Loss | Required EDR | Sections (calc) | Sections (round up) | Actual Output |
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📏 Correction Factor Reference - Your Radiator Type
How many sections are needed for 1,000 W (3,412 BTU/h) of heat loss at different mean water temperatures. Room at 20°C.
| Mean Water Temp | CF | W per section | Sections per kW |
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How to Use the Radiator Sizing Calculator
Choose the cast iron or steel radiator type you're working with. Each type has a standard EDR value per section - the output at rated conditions. If you have manufacturer data with a specific EDR or watt output per section, use the Custom option and enter the exact value.
Enter the supply and return water temperatures for your hydronic system, and the design room temperature. The correction factor calculates automatically and updates the CF display. For condensing boiler systems running at 60°C supply / 50°C return, the correction factor will be around 0.60–0.65, meaning you need about 55% more sections than a standard calculation.
Add one entry per room. Enter the room name and the design heat loss in watts or BTU/h. If you haven't calculated room heat losses yet, use the heat load calculator first. The required number of sections calculates automatically for each room using the corrected EDR output per section.
The results table shows the required sections calculated, rounded up to the next whole section, and the actual output at that section count. Always round up - you can't install half a section, and a slight over-supply is fine in a hydronic system with a thermostatic radiator valve. The total EDR below the table feeds directly into the steam boiler calculator if you're sizing a steam replacement boiler.
The reference table below the main results shows how the section count per kilowatt changes at different mean water temperatures. Use this to understand the sensitivity of your design to temperature changes - useful if you're planning future boiler temperature setbacks or connecting to a heat pump in a hybrid system.
Radiator Sizing and Temperature Correction for Canadian Hydronic Systems
Cast iron radiators are still found in millions of Canadian homes, particularly in pre-war housing in Montreal, Toronto, Ottawa, and Winnipeg. When these buildings get new condensing boilers or heat pump upgrades, the existing radiators must be checked against the new lower operating temperatures. In most cases, they need more sections or supplemental emitters.
How EDR Ratings Work
Every cast iron radiator section is rated in EDR - Equivalent Direct Radiation. One EDR delivers 240 BTU/h (70.3 W) under standard conditions: 215°F (102°C) steam or 180°F (82°C) hot water supply, and 70°F (21°C) room air. These conditions produce a mean water temperature of about 180°F for steam or 170°F for hot water at standard 180/160°F supply/return. The EDR value is stamped on most older cast iron radiators and published in manufacturer catalogues for new units.
The Temperature Correction Factor
When a radiator operates below its standard conditions - as it does in any condensing boiler system - its actual output is less than the rated EDR suggests. The correction factor (CF) is calculated using the exponent method: CF = (Mean Water Temperature − Room Temperature) ÷ (Standard MWT − 70°F), raised to the power of 1.3. The exponent of 1.3 reflects the combined convective and radiant heat transfer characteristics of cast iron. At 70°C supply, 60°C return, and 20°C room, the mean water temperature is 65°C and the CF is approximately 0.73. A 10-section radiator rated at 2.0 EDR per section delivers not 4,800 BTU/h but 3,504 BTU/h at these temperatures.
Low-Temperature Systems and Radiator Count
A condensing boiler typically runs at 55–70°C supply and 45–55°C return. At 60°C mean water temperature and 20°C room temperature, the CF drops to about 0.64. This means a room that needed 10 sections at standard conditions needs 10 ÷ 0.64 = 15.6 sections, rounded up to 16. For an entire building, this can mean 50–70% more radiator sections than the original count. In practice, building owners have three options: add sections, replace smaller radiators with larger models, or supplement with other heat emitters such as baseboard in high-loss areas like bathrooms and under large windows.
Steam Systems vs. Hot Water Systems
The standard conditions differ slightly between steam and hot water systems. Steam radiators are rated at 215°F (102°C) steam temperature. Hot water radiators use a 180°F (82°C) supply and 160°F (71°C) return as the standard, giving a mean water temperature of 170°F (77°C). In practice, cast iron sections produced before the 1960s were usually designed for steam, and the same section works in a hot water system with the correction factor applied. Converting a building from steam to hot water typically requires replacing the one-pipe steam boiler and adding return piping, but the radiators themselves can stay - subject to the correction factor calculation this tool performs.
Thermostatic Radiator Valves
Modern Canadian hydronic systems pair cast iron radiators with thermostatic radiator valves (TRVs). A TRV senses room temperature and throttles flow to the radiator to maintain setpoint. This allows individual room temperature control and is mandatory in some provincial building codes for new systems. When sizing radiators with TRVs, size to the full heat loss - the TRV will reduce output when the room is warm, but the radiator must be capable of delivering full output on a design-temperature day in Calgary (-29°C) or Winnipeg (-33°C) when all rooms call for heat simultaneously.
Sizing Radiators for Heat Pump Hybrid Systems
Hybrid heat pump systems in Canada typically supply water at 45–55°C when the heat pump operates, switching to the gas boiler backup at colder temperatures when the heat pump loses efficiency. Sizing radiators for 50°C mean water temperature ensures adequate output in heat pump mode. The correction factor at 50°C mean / 20°C room is about 0.50 - meaning roughly twice as many sections as a standard high-temperature calculation. The payoff is year-round heat pump operation at the temperatures where heat pumps are most efficient, with the gas backup only kicking in below about -10°C to -15°C depending on the equipment. Use the heat emitter calculator to compare cast iron radiators against panel radiators and baseboard for hybrid system design.
Frequently Asked Questions
Start with the room heat loss in BTU/h or watts from a proper heat loss calculation. Calculate the mean water temperature (supply plus return divided by 2) and the temperature correction factor using CF = ((MWT − room temp) ÷ (standard MWT − room temp))^1.3. For hot water systems, the standard MWT is about 77°C (171°F). Divide the room heat loss by the corrected output per section (standard output × CF) and round up to the nearest whole section. At 60°C supply / 50°C return and 20°C room, expect to need about 35–40% more sections than a standard-temperature calculation. Use this calculator to handle all the correction factor arithmetic automatically.
The correction factor adjusts the radiator's rated EDR output for non-standard operating temperatures. CF = ((MWT − Troom) ÷ (Std MWT − Troom))^1.3. At 70°C MWT and 20°C room: CF ≈ 0.73. At 60°C MWT: CF ≈ 0.61. At 50°C MWT: CF ≈ 0.48. These lower factors mean radiators operating in condensing boiler or heat pump hybrid systems must be significantly larger than those sized for traditional high-temperature operation. The reference table in the results section of this calculator shows section counts per kW at a range of mean water temperatures for the radiator type you selected.