Radiator Sizing and EDR

How is radiator sizing measured?

Radiator sizing is measured in square feet of EDR, short for equivalent direct radiation. It is a heat rating printed in old radiator catalogs and charts, not the painted surface you could measure with a tape. Every radiator in a house can be given an EDR figure, and adding them up gives the connected load the boiler has to serve.

That total does two jobs. On steam, it is the number the boiler is sized to. On hot water, it tells a technician whether the radiators can still heat the house at the water temperature a new boiler would run. The DOE steam balancing study (2013) calls the room-by-room list a "radiator census" and uses it both for a boiler replacement and for sizing radiator air vents.

What is EDR?

EDR is the amount of heating surface that gives off 240 Btu per hour when filled with steam at 215°F and surrounded by 70°F air. That definition comes from Hoffman's two-pipe steam equipment bulletin (FHD-205, 1997), which adds that the rating "may have no direct relation to the actual surface area." McDonnell & Miller's catalog uses the same 240 figure.

On hot water, the same square foot gives less, because the water is cooler than steam. U.S. Boiler's radiator emission chart (2024, 70°F room) lists the output per square foot by average water temperature. These are general figures; one model's catalog table can differ by a few Btu/h, so a technician works from the standard chart.

How do you measure a cast-iron radiator's EDR?

Measure its height, count its sections and columns, and look the figure up in a radiation chart. U.S. Boiler's sizing page (2017) sets out the steps:

1. Decide whether the radiator is a column type (thick, separate columns) or a tube type (many slender tubes).
2. Measure the height from the floor to the top of the radiator.
3. Count the columns or tubes in one section.
4. Count the sections.
5. Find the square feet per section in a chart for that height and type, then multiply by the number of sections.
6. Repeat for every radiator and add the totals.

U.S. Boiler's worked example is a 22-inch, three-column radiator at 3 sq ft per section. Utica's radiation table, a maker sheet read from a mirror copy, gives 38-inch column radiators at 3, 4, 5 and 8 sq ft per section for one, two, three and four columns, and 38-inch tube radiators at 3-1/2, 4-1/4, 5 and 6 sq ft for three-, four-, five- and six-tube sections. Its example is a 38-inch, 5-tube radiator with 7 sections: 35 sq ft EDR.

Measuring and counting is safe for a homeowner: it needs only a tape measure and a notepad, and nothing on the radiator is touched. Steam pipes and radiators get hot, so do it with the heat off. If unsure about the type or the chart, a licensed heating technician makes the same survey before quoting a boiler.

EDR in numbers

• 240 Btu/hthe steam rating behind every square foot of EDRHoffman bulletin FHD-205 (1997)
• 35 sq ftEDR of a 38 in, 5-tube, 7-section radiatorUtica radiation table (maker sheet)
• 1.333piping and pick-up allowance built into steam ratingsWeil-McLain EG manual, 2025
• x 4pounds of condensate per hour to sq ft EDRHoffman FHD-205 (1997)

Why does EDR size a steam boiler?

A steam boiler has to fill every radiator and pipe with steam each cycle, so it is sized to what is connected, not to how much heat the house loses. U.S. Boiler's replacement tips (2018) are blunt: do not complete a heat-loss calculation for a steam boiler, and do not simply match the old boiler size. Its 2019 article on boiler size says the same: the boiler "must be sized for the volume of radiation in the home."

Makers' ratings already allow for the pipes. Weil-McLain's EG manual (2025) says steam ratings are based on a piping and pick-up allowance of 1.333, against 1.15 for water ratings, and U.S. Boiler says a boiler makes about 33% more steam than its listed square feet for that reason. Extra allowance is only for unusual piping. Uninsulated mains are the exception people miss: U.S. Boiler says an uninsulated steam main "is equal to a radiator" and must be added to the load. Its advice is not to oversize. The steam heat section explains why pipes, vents and pressure matter as much as the boiler.

How does EDR help with a hot-water boiler?

On hot water, the boiler is sized from a room-by-room heat-loss calculation (Manual J), and EDR tells a technician what water temperature the radiators need. U.S. Boiler's conversion article (2024) describes a connected-load survey, then the emission chart to find the supply temperature that meets the heat loss.

That matters for condensing boilers, which need cool return water to condense flue vapor. If the radiators are large compared with the heat loss, they can heat the house with cooler water. The "Cast Iron Radiators" preset in Weil-McLain's condensing ECO Tec control targets 180°F at most and 120°F at least (Weil-McLain, 2024). The guest author of U.S. Boiler's conversion article goes further, saying cast-iron radiators can run with water as low as 75°F; that is his view as a contractor, not a maker rating. Boiler choices are covered in the boilers section, and the air-to-water heat pump article explains why radiator size decides whether a heat pump can feed them.

Were old radiators oversized?

Often, yes. The guest author of U.S. Boiler's conversion article writes that after the 1918 flu, systems were sized to heat rooms with windows partly open. U.S. Boiler's own sizing article adds that "in a lot of cases, older boilers are oversized for the homes they serve."

A heat-loss calculation shows how the house compares with its radiators today. On hot water, spare radiation is useful: the emission chart shows a bigger radiator can deliver the same heat from cooler water, so a new boiler can run at lower temperatures. On steam, the boiler still has to match the radiators, because U.S. Boiler sizes it to connected radiation either way. The radiators section and the radiator covers page cover the other things that change how much heat a radiator delivers.

Questions from the boiler room